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1. To create this layout drag two comboboxes to the central GUI layout The Combo Box option can be found by scrolling in the Widget Box section In the same way we ll add two labels on the top of each combobox To edit the labels just double click on them to enter into the label edit mode This action is equivalent to changing the Text property in the Property Editor section After creating the layout we can associate each combobox with an object name that will be used to distinguish the function of each combobox To do this we will change the objectName property in the Property Editor section For example we can set the name of the raster combobox as rastersCombo In the same way we rename the vector combobox as vectorsCombo The string used as objectName will be the name of the Python variable that refers to the graphical element It will be used in the Python code when we want to get or set some property of the graphical element Modifying the GUI logic Our plugin doesn t need modification in the dialog code test_plugin_dialog py because all the GUI updates will be guided by the test_plugin py code directly populating the rasterCombo and vectorsCombo elements This is a design decision in other cases it could be better or cleaner to have the logic inside the dialog code to hide combo names using the dedicated function added to the dialog 321 Creating QGIS Plugins with PyQGIS and Problem Solving Modifying t
2. Simplified interface E E E e The Processing Toolbox showing the Models category with the Landcover group and the Riparian trees model The simplified interface is shown on the left and the advanced interface is on the right Adding inputs To begin a model you will need to define the inputs The graphical modeler will accept the following e Boolean e Extent e File e Number e Raster layer e String e Table e Table field e Vector layer i 76 Chapter 8 To add an input either double click on the appropriate category from the Inputs tab or drag the input onto the modeler canvas The Parameter definition dialog will open Give the parameter a name and fill in any other details which change depending on the input that is chosen When an input parameter is defined and added to the model it is essentially a conceptual parameter It will not actually be connected to a GIS data layer until you are ready to run the model For this example we will add a vector layer We will specify the geometry of the vector data and classify it as a required parameter Parameter definition Parameter name Rivers shape type Once you click on OK the input object is added to the modeler canvas All the objects in the modeler canvas can be selected with a mouse click and dragged to reposition Clicking on the pencil icon of an input will open the Parameter definition dialog so that changes can be mad
3. Buffers Input vector layer Portland_FireStations Use only selected features Segments to approximate Butter distance Buffer distance field STATION Dissolve buffer results Output shapefile and Output Chapter4 Portand_FireStations_buffer shp Browse Add result to canvas _ The Buffer distance field will accept both positive and negative values If a negative value is entered the buffer will be extended j inside the polygon boundary If a positive value is entered the buffer will be extended outside the polygon boundary 3 QGIS cannot create true curves but it provides an option to set Segments to approximate The higher the number the smoother the output will be because QGIS will use more segments to approximate the curve In this example it has been set to 20 instead of the default of 5 4 Checking Dissolve buffer results will merge buffer polygons if they overlap 124 Chapter 4 4 QGIS f2b7cOf Buffer Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help ODARA FOS PS SUPA A J m Ps o ar P 5 2 4 i z 4 Q amp Q Ve 2a B coordinate 7628940 734887__ Scale 1 177 340 Render EPsG 2913 J A _ When you navigate to Processing Toolbox Geoalgorithms Vector gt Geometry Operations you will find the Variable distance buffer tool Q This tool creates a multiple ring buffer where t
4. Output raster D PACKT Data Chapter 6 zone_map_modified tif Target SRS EPS6 2903 Generate pdf map D PACKT Data Chapter 6 7oningGeoRefMap pdf Generate pdf report D PACKT Data Chapter 6 ZoningGeoRefReport pdf Set Target Resolution Horizontal Vertical Use 0 for transparency when needed Load in QGIS when done Cancel Help Advanced Data Creation and Editing 6 Click on the Start Georeferencing button to complete the operation The georeferenced raster will be written out and added to the QGIS Desktop map canvas Checking the topology of vector data In GIS there are two main data models vector and raster They are called models because they are not real but are representations of the real world It is important that we ensure our data is modeling the world as accurately as possible Vector datasets often have hundreds or thousands of features making it nearly impossible to verify each feature However using topology rules we can let QGIS evaluate the geometry of our datasets and ensure that they are well constructed Topology is the relationship between contiguous or connected features in a GIS Here you will be introduced to the Topology Checker plugin This plugin allows you to test topological relationships in your data and ensure that they are modeling the real world accurately An example of a topological relationship rule is polygons must not overlap Imagine a country boundaries dataset It is no
5. VERA M leT RS Go beyond the basics and unleash the full power of QGIS with practical step by step examples Foreword by Phillip Davis Director National Information Security amp Geospatial Technology Consortium PACKT Mastering QGIS Go beyond the basics and unleash the full power of QGIS with practical step by step examples Kurt Menke GISP Dr Richard Smith Jr GISP Dr Luigi Pirelli Dr John Van Hoesen GISP PACKT open source PUBLISHING BIRMINGHAM MUMBAI www allitebooks com Mastering QGIS Copyright 2015 Packt Publishing All rights reserved No part of this book may be reproduced stored in a retrieval system or transmitted in any form or by any means without the prior written permission of the publisher except in the case of brief quotations embedded in critical articles or reviews Every effort has been made in the preparation of this book to ensure the accuracy of the information presented However the information contained in this book is sold without warranty either express or implied Neither the authors nor Packt Publishing and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals However Packt Publishing cannot guarantee the accuracy of
6. Chapter 5 To build a raster overview in QGIS click on Build Overviews Pyramids by navigating to Raster Miscellaneous This will open the Build overviews Pyramids tool window as shown in the following screenshot Build overviews Pyramids Batch mode for processing whole directory Input file Select Clean Overview format External GTiff ovr Resampling method Average Levels 2 Custom levels Create Options Profile Default gdaladdo AVERAGE ro The Build overviews tool provides a few options as well as the ability to edit the GDAL command that will build the overviews By selecting Batch mode an entire directory of rasters will be processed instead of the default of a single raster In either case the following options are available Input file The raster or directory of rasters that will have overviews built e Clean If this is selected any previously built overviews will be deleted 159 Preparing Raster Data for Processing e Overview format The format of the built overviews The options are as follows O External GTiff ovr In this format the overviews are stored in the external ovr file The file will have the same base name as the original input file Internal if possible In this format the overviews are stored within the raster file Note that the Clean option will not remove previously built internal overviews External Erdas Imagine aux In this format the o
7. Layer Properties Counties Joins e General Join layer Join field Target field Memory cache Style f 4 Add vector join o lpn Se x Labels Join la Coun ttributes E Fields ver tya Join field ADMIN_FIPS Rendering Target field ADMIN_FIPS gd Display Cache join layer in virtual memory O _ Create attribute index on join field i Actions Choose which fields are joined L_ ADMIN_FIPS L POP IEE Diagrams C SQ MILES i Metadata Restore Default Style ox Ji cn Joins only exist in virtual memory within the QGIS Desktop gt document To preserve the join outside the map document click Q on Save as under Layer and save a new copy of the layer The new layer will include the attributes appended via the join Editing data Vector data layers can be edited within QGIS Desktop Editing allows you to add delete and modify features in vector datasets The first step is to put the dataset into edit mode Select the layer in the Layers panel and click on Toggle Editing under Layer Alternatively you can right click on a layer in the Layers panel and choose Toggle Editing from the context menu Multiple layers can be edited at a time The layer currently being edited is the one selected in the Layers panel Once you are in the edit mode the digitizing toolbar shown in the following screenshot can be used to add delete and modify features Chapter 1 From left to
8. O By lightness In this method an average of the lightness value of multiple raster band values will be applied to the gray color ramp with the saturation set as 0 If the raster only has one band then each cell s lightness value will be used The lightness value is calculated using the formula 0 5 max R G B min R G B By luminosity In this method a weighted average of multiple raster band values will be applied to the gray color ramp Luminosity approximates how you perceive brightness from colors The weighted average is calculated using the formula 0 21 red 0 72 green 0 07 blue By average In this method the average of the raster band values for each cell will be applied to the color ramp If the raster only has one band this selection will have no effect For example if the raster had three bands with cell values of 25 50 and 75 then 50 would be applied as the cell value for the gray color ramp The average is calculated using the formula R G B 3 Hue This parameter adds a hue to each cell of the raster To apply a hue check the Colorize box and then select a color using the color picker The Strength parameter linearly scales the application of the selected Colorize color to the existing raster colors 78 Chapter 3 Raster resampling Raster resampling prepares the raster for display when not every raster cell can be mapped to its own pixel on the display If each raster cell is mapped to it
9. The color ramps displayed in the Color ramp tab are available for quick access from drop down selection boxes when you style the data For example the following screenshot shows the quick access color ramps in a drop down box when we specify a color ramp to apply a pseudocolor to a single band raster Generate new color map Oranges PRGn 58 Chapter 3 Six operations are available to manage color ramps in the Style Manager rename remove export import add and edit Each of these operations will be explained now Renaming a color ramp To rename a color ramp click once on the color ramp to select it pause and then click on it a second time this is a slow double click to make the name editable Type in the new color ramp name then press Enter to save it Removing a color ramp To remove a color ramp select the color ramp then click on the Remove item button 2 The color ramp will no longer be available Exporting a color ramp To export a color ramp navigate to Share Export to open the Export style s window shown in the following screenshot 4 Export style s Select symbols to export GnBu Greens Greys RdYlGn Reds Select the symbols that you wish to export and then click on Export to export the selected symbols to an XML file The exported symbols can later be imported to the Style Manager using the Import function 59 Styling Raster and Vector Data Importing a color ramp
10. Repairing topological geometry errors via topological editing Creating points from coordinate data There is a lot of data with spatial components stored in spreadsheets and tables One of the most common forms of tabular spatial data are x and y coordinates that are stored in a delimited text file The data may have been collected with a GPS receiver it may have been generated by a surveyor or it may have been transcribed off topographic maps Regardless QGIS can map these coordinates as points by using the Add Delimited Text Layer tool 9 This tool can be found by navigating to Layer Add Layer Add Delimited Text Layer or on the Manage Layers toolbar 169 Advanced Data Creation and Editing Delimited text data is simply a table with column breaks that are identified by a specific character such as a comma With this tool QGIS can accept either x and y coordinates or Well Known Text WKT representations of geometry WKT can contain point line or polygon geometry The following is a sample data cougar sightings csv viewed in a text editor This is a comma delimited file with x and y coordinate values SSAMPID C 20 SEX C 10 UTM N 19 11 0TM Y N 19 11 883304 PA087 F 115556 PA097 F 116870 PA098 M 116148 PNOO1 M 482000 PNO002 M 296192 PNOO3 M 347990 044021 3486272 343644 3469102 694117 34863420 016751 3700996 720405 4053069 948523 3990302 55056 50411 34463 38808 265
11. The Processing Toolbox Finding proximity to surface water using GDAL Proximity To accomplish this we need need to create a layer representing proximity to surface water and query that layer for areas within 1 000 meters of any surface water Our first step is to execute the GDAL Proximity raster distance tool in the processing toolbox We need to select the binary true or false layer representing surface water lulc_sw tif set the Values field to 1 leave the Dist units field as GEO change the Output raster type to Int32 leave all the other defaults as they are and name the output layer as illustrated in the next screenshot Q Proximity raster distance Parameters Log Help Input layer Surface Water raster FP5G 26710 Values 1 Dist units GEO Max dist negative value to ignore 1 No data negative value to ignore L Fixed buf val negative value to ignore 1 Output raster type Int32 Output layer C Mastering_OGIS GRASS orox_water tif 2 Open output file after running algorithm 248 Chapter 7 The rationale for setting Values to 1 and Dist units to GEO is that we are asking the algorithm to assume that the distance is measured in increments of 1 based on the geographic distance in this case meters and not on the number of pixels We can now query the resulting grid for the area that is within 1 000 meters of surface water but it is important to recognize that we want to identify the ar
12. This path can be added in the session PYTHONPATH or directly in the QGIS environment by modifying PYTHONPATH by navigating to Options System in the Environment section as shown in the following screenshot Options System General Path s to search for additional C plugins libraries Add Remove amp System Data Sources Rendering Colors i Canvas amp Legend E Map Tools v QSettings E Composer i Reset the User Interface settings to reset QGIS to it s default view W Digitizing GDAL w Environment CRS V Use custom variables restart required include separators Add Remove m Locale Apply Variable s Network Append c PYTHONPATH Users ginetto Aptana Studio_3 6 plugins org python pydev_3 8 0 20140925123 v Current environment variables read only bold indicates modified at startup Variable v Value LOGNAME ginetto This way it will be possible to import the Python module pydeva in the QGIS Python Console and set the connection to the PyDev debug server as described in the next paragraph To test if the path is set correctly try to type the following code in the QGIS Python Console import pydevd If it generates an error it means that PYTHONPATH is not set correctly with the path of the pydevd module Starting the Pydevd server The first step to connect to PyDev server is to start the server in the Aptana environment This can be achieved by opening the Debug perspective o
13. Toolbox ft Graphical modeler a History and log X Options and configuration 2 Results viewer 2 Commander 217 The Processing Toolbox To get started you need to make sure that each of the providers that you intend to use are activated and depending on your operating system and installation approach you may need to specify the necessary folders to run TauDEM LAStools and R The next screenshot illustrates that each of the algorithms are activated and the necessary folders are specified i Processing options Search E General A fi Models i Providers i DAR E y GRASS GIS 7 commands EF y GRASS commands g x oY GRASS folder C OSGEO4 1 apps grass grass 6 4 3 W Log console output y Log execution commands y Msys folder C OSGE04 1 apps msys s ai Modeler only tools z E Orfeo Toolbox Image analysis 5 QGIS geoalgorithms i R scripts SAGA x amp Enable SAGA Import Export optimizations amp oa Log console output z g Log execution commands 3 TauDEM hydrologic analysis ff Activate x MPICH2 OpenMPI bin directory C Program Files MPICH2 bin Number of MPI parallel processes to use 1 TauDEM command line tools folder C Program Files 86 Taudem TauDEM5Exe 3 Tools for LIDAR data 5 x r Fusion folder X LAStools folder C OSGeo4W64 LAStools i W Wine folder E Scripts Once you click on OK QGIS will update the list of algorithms accessible through the p
14. 177 RIGHTHIGH 14302 00000000 14324 00000000 14308 00000000 14340 00000000 14320 00000000 14320 00000000 14334 00000000 14330 00000000 14318 00000000 14314 00000000 14232 00000000 COPPER MEL SMITH BAUER ENCANTADO PIEDRAS SKYLINE OAKWOOD ARCADIA WINDSOR GRAND Advanced Data Creation and Editing In the following example we are looking at Easy Street On the odd side of the street the addresses range from 101 to 199 On the even side they range from 102 to 200 If you want to map 150 Easy Street QGIS would assume that the address is located halfway down the even side of that block Similarly 175 Easy Street would be on the odd side of the street roughly three quarters of the way down the block Address geocoding assumes that the addresses are evenly spaced along a linear network QGIS should place the address point very close to its actual position but due to variability in lot sizes not every address point will be perfectly positioned Right From 101 Right To 199 Easy Street Left From 102 Left To 200 Now that you ve learned the basics you ll see how each MMQGIS geocoding tool works Here both tools will be demonstrated against the Addresses csv sample data The first example will use web services The second example will use the Streets shp sample data In both cases the output will be a point shapefile containing all the attribute f
15. QgsFeatureRegquest request setSubsetOfAttributes 0 2 request setFilterRect rect for index feature in enumerate myVector getFeatures request i print The record d has ID d index feature id This will produce the following list of only eight records The record 0 has ID 223 The record 1 has ID 593 The record 2 has ID 596 The record 3 has ID 599 The record 4 has ID 626 The record 5 has ID 627 The record 6 has ID 630 The record 7 has ID 636 In the preceding code the first line creates a QgsRectangle method that is used in the setFilterRect method to get only features that are within the rectangle Then only the values of columns 0 and 2 are fetched setting the filter with setSubsetOfAttributes It should be noted that the QgsFeatureRequest class has the setFilterExpression method that is useful to select only features that are bigger than 1000 but it can t be used in the upper case The OgsFeatureRequest code forces to exclusively use a spatial filter or an expression filter For this reason to filter at the same time by expression and by bound box it s necessary to set the expression at a layer level with the setSubsetString method The enumerate statement is a Python instruction to get something that can enumerate and return pairs of elements with the first element as the index and the second as the enumerated element 351 PyQGIS Scripting Describing the iterators The preceding c
16. The raster band that is being styled If a multiband raster is being used then the combobox will be populated with all raster bands e Color gradient Black to white The gradient to apply to the selected gray band The choices are Black to white and White to black e Min 105 The minimum cell value found in the gray band e Max 207 The maximum cell value found in the gray band e Contrast enhancement Stretch to MinMax The method used to stretch the color gradient to the gray band with respect to the Min and Max values wY Band rendering Render type Singleband gray Gray band Band 1 Gray Load min max values Cumulative count cut 2 0 Color gradient Black to white 105 Min max 207 Mean Contrast standard deviation x Stretch to MinMax enhancement Extent Accuracy Full Estimate faster Current Actual slower Load 67 Styling Raster and Vector Data The Min Max and Contrast enhancement parameters work together to determine how to stretch the color gradient to the gray band To understand how these parameters work together we need to first discuss how the Min and Max values are derived which draws our attention to the Load min max values section of the Band rendering options The Load min max values section contains parameters that are used to calculate which Min and Max values should be set Three sets of parameters must be set before you click on the Load button they are as f
17. do not appear in the Processing Toolbox They are tools that only make sense when they are used in the context of the graphical modeler Processing modeler wy GRASS commands a Modeler only tools E Modeler only tools Calculator Raster layer bounds Vector layer bounds H at Models The Calculator tool is perhaps the most commonly used of the three tools It allows you to perform arithmetic calculations on numeric outputs from other algorithms For example if you use one of the statistical output tools such as Raster layer statistics in your model or if you have numeric inputs the associated numeric values will be available to the Calculator algorithm The calculator lists the available numeric values within the model They are labeled from a to x with the description to the right Below this is a text box for entering a formula For example the formula given in the following screenshot divides the Canopy density value by the Owl Habitat Acres value and multiplies the result with the standard deviation from the Raster layer statistics algorithm The output from the Calculator algorithm can be fed into other algorithms Calculator You can refer to model values in your formula using singletetter variables as follows a gt Canopy density b gt Owl Habitat Acres c gt Minimum value from algorithm Raster layer statistics d gt Maximum value from algorithm Raster layer statistics e gt Sum from algorithm
18. gt 5000000 THEN Result ELSE STATE NAME END Any command line input or output is written as follows sudo apt get install qgis plugin grass New terms and important words are shown in bold Words that you see on the screen in menus or dialog boxes for example appear in the text like this You can explore the QGIS plugin ecosystem by navigating to Plugins Manage and Install Plugins Warnings or important notes appear in a box like this M Q Tips and tricks appear like this xii Preface Reader feedback Feedback from our readers is always welcome Let us know what you think about this book what you liked or disliked Reader feedback is important for us as it helps us develop titles that you will really get the most out of To send us general feedback simply e mail feedback packtpub com and mention the book s title in the subject of your message If there is a topic that you have expertise in and you are interested in either writing or contributing to a book see our author guide at www packtpub com authors Customer support Now that you are the proud owner of a Packt book we have a number of things to help you to get the most from your purchase Downloading the example code You can download the example code files from your account at http www packtpub com for all the Packt Publishing books you have purchased If you purchased this book elsewhere you can visit http www packtpub com su
19. showCoordinates from PyQt4 QtCore import Qt def manageCLick currentPos clickedButton if clickedButton Qt LeftButton print Clicked on d d currentPos x current if clickedButton Qt RightButton iface mapCanvas setMapTooL previousMapTool myMapTool deleteLater myMapTooL canvasClicked connect manageClick 1face mapCanvas setMapTooLl myMapTool 30610574 1943864 Scale 2 694 119 v 964 A Right click on the canvas to return to the previous Map Tool but you still need to remove the canvas event handler As explained in the previous paragraph removing the xyCoordinates handler is done by executing the following code iface mapCanvas xyCoordinates disconnect showCoordinates 372 Chapter 10 Using point click values The previous paragraph explained how to get a point click coordinate This coordinate can be used to get the correspondent value in a raster layer or to get the underlying feature of a vector layer To identify a feature for a vector layer it s better to use the dedicated Map Tool QgsMapToolidentifyFeature directly but for a raster layer that has point coordinates we can use the QgsRasterDataProvider identify method to get raster values at a specified point By loading and selecting the landcover raster from qgis_sample_data raster we can modify the manageClick method of the previous example in the following way def manageC
20. Add geometry column 43 Creating Spatial Databases The Columns tab lists all the fields their type whether they allow null values and their default values Below the field list there are four buttons The Add column button opens a window and allows you to create a new field and specify its properties The Add geometry column button opens a window and allows you to create a new geometry field and specify its properties The Edit column button opens a window and lets you change the selected field s properties The Delete column button deletes the selected field A SpatiaLite does not support table altering commands such as Q editing and deleting existing fields therefore these options will be disabled The Constraints tab lists all the constraints on the table their name their type and the column s that are affected by the constraints The Add primary key unique button opens a window and allows you to create a new primary key constraint The Delete constraint button deletes the selected constraint A SpatiaLite does not support adding or removing a constraint Q from an existing table therefore these options will be disabled The constraints can be managed using other SQLite clients The Indexes tab lists all the indexes on the table their name and the column s that are a part of the index The Add index button opens a window that allows you to create a new index by selecting the field to index and provides an index
21. Creating Spatial Databases 2 Navigate to SpatiaLite GiffordPinochet sqlite to see a tree listing of all tables views and general information about the database as shown in in the following screenshot Database Table 1 al e Toolbar Tree a El Postals T El a Spatialite LTCC GiffordPinochet sqlite LELI Ss EFT EI geometry_co geometry_colurnns_field_intos geometry_colurnmns_statistics geometry_columns_time a wii spatialite_history Lil gd j sqlite sequence vector_layers El vector_layers_auth vector_layers_field_infos vwector_layers_statistics views_geometry_colurmns views_geometry_columns_a views_geometry_colurmns_fi views_geometry_colurmms_st virts_qeometry_colurmns virts_qeometry_colurnns_auth virts_geometry_colurnns_fiel virts_qeometry_colurmms_stat Menu Bar DB Manager Info Table Preview GiffordPinochet sqlite Connection details Filename DB GiffordPinochet sglite General info Information Panel SOLite version 3 7 17 SpatiaLite Library 4 1 1 GEOS 3 4 2 API 1 8 2 r3921 Proj Rel 4 8 0 6 March 2012 When a new SpatiaLite database is created it is automatically populated with multiple tables and views These tables and views hold records used by the DBMS to manage the structure and operation of the database You should not modify or delete these tables or views unless you are absolutely sure of what you ar
22. EPSG 26710 Additional layers optional 2 elements selected Formula athe Result C Mastering_QGIS SAGA habitat tf X Open output file after running algorithm Water Reclass EPSG 26710 Proximity to Water EPSG 26710 Land Use water EPSG 26710 Elevation Recass EPSG 26710 LULC Redass EPSG 26710 Land Use Land Class FPSG 26710 Park Elevation EPSG 26710 Elevation EPSG 26710 hillshade EPSG 26710 Q Multiple selection 252 Chapter 7 We could have used the native GDAL Raster calculator tool or the GRASS r mapcalculator tool but once again this demonstrates how easy it is to switch between the various toolbox options Similar to the GRASS syntax the SAGA algorithm identifies the inputs in the order they are selected as a b and c To ensure that we understand the values reported in the resulting output from this calculation we need to remember the reclassified water and elevation layers are binary so it will have the values of 0 and 1 while the reclassified land use layer contains the values from 0 to 3 Therefore the new layer can only contain values of from 0 to 3 where 0 indicates no habitat 1 indicates poor habitat potential 2 indicates moderate habitat potential and 3 indicates good habitat potential as illustrated in the next screenshot H X Gi Vector Layers fi Analysis layers E 3 Final Product _ x BP Habitat potential No Habitat 0 Poor Potential
23. For example one could test whether a bus stop s layer point is within 50 feet of the centerline of a road line by setting a tolerance of 50 feet and using the must be covered by rule 205 Advanced Data Creation and Editing 13 Once the topology rules have been set you can choose to validate the topology for the entire layer Validate All or just within the current map extent Validate Extent 14 For this example choose Validate All 15 The topology errors are displayed in the Topology Checker panel In this case 17 errors are found including 6 gaps 9 overlaps and 2 duplicate geometries as shown in the following screenshot pao Chake Ex Error Layer Feature ID gaps parcels parcels parcels wf r H o parcels parcels 2 gaps parcels overlaps parcels overlaps parcels overlaps parcels overlaps parcels 10 overlaps parcels 11 overlaps parcels 12 overlaps parcels 13 overlaps parcels 14 overlaps parcels luplicate geometry parcels 15 duplicate geometry parcels 2 E 16 The Show errors box can be checked to see where in the dataset the errors occur Errors will be highlighted in red as shown in the following figure 206 Chapter 6 D i E E F S l Ei l _ _ Ea TUT tae a Repairing topological errors via topological editing Once the geometry errors have been identified the work of repairing the layer beg
24. Loading databases 8 Web services 9 Working with coordinate reference systems 10 Working with tables 11 Table joins 12 Editing data 14 Snapping 15 Styling vector data 16 Styling raster data 18 Contrast enhancement 19 Blending modes 20 Composing maps 23 Adding functionality with plugins 24 Summary 26 i Table of Contents Chapter 2 Creating Spatial Databases 27 Fundamental database concepts 27 Database tables 28 Table relationships 29 Structured Query Language 29 Creating a spatial database 30 Importing data into a SpatiaLite database 33 Importing KML into SpatiaLite 33 Importing a shapefile into SpatiaLite 35 Importing tables into SpatiaLite 36 Exporting tables out of SpatiaLite as a shapefile 40 Managing tables 41 Creating a new table 41 Renaming a table 43 Editing table properties 43 Deleting a table 44 Emptying a table 45 Creating queries and views 45 Creating a SQL query 45 Creating a spatial view 46 Dropping a spatial view 48 Summary 49 Chapter 3 Styling Raster and Vector Data 51 Choosing and managing colors 52 Always available color picker components 53 Changeable panels in the color picker 54 Color ramp 54 Color wheel 55 Color swatches 55 Color sampler 57 Managing color ramps of Managing the QGIS color ramp collection 58 Renaming a color ramp 59 Removing a color ramp 59 Exporting a color ramp 59 Importing a color ramp 60 Adding a color ramp 60 Editing a color ramp 65 Styling single band rast
25. This can be simply opened as a file but it s better to learn how to have a view of the entire plugin as a PyDev project This allows us to use Aptana as a debug and develop environment This is done in two steps 1 Creating a PyDev project in Aptana Studio 3 2 Linking the source code to the project Creating a PyDev project called Test Plugin is done by navigating to File New Project This will open a wizard where we ll have to look for a PyDev Project entry Select it and click on the Next button at the bottom Here the wizard will pass to the phase to insert the project name Test Plugin and then click on the Finish button at the bottom A new project called Test Plugin will be shown in the PyDev Package Explorer Aptana section The next step is to add the folder of our Test Plugin code inside the project To add it select the TestPlugin PyDev project right click on it to add a new folder as shown in this following screenshot File Edit Navigate Search Project Pydev Commands Run Window Help a S e a o n y e ee a Me ba Project New From Template gt B File v TestPlugin Go Into Colder ee CE Eee EE Publish gt PHP Project mm CSIWMSLegend i E ae E Copy vain ice Jock flirrorMap Paste Ruby Project i EQCAP components Delete amp Web Project GDAL tools oes l _ Move Link to Existin r gem mt EQCAP 1 8 F a Rename Source Folder i gem mt EQCAP QGIS Plugin f geoportale queries
26. counties include multiple islands To illustrate this point the following figure shows a single record selected in the attribute table that selects multiple polygons that are tied to the single record 4 Attribute table Hawaii_counties Features total 5 filtere o 8 amp amp amp 5 GEOID10 NAME10 NAMELSAD10 o 15003 Honolulu Honolulu County 4 15001 Hawaii Hawaii County 2 15007 Kauai Kauai County 3 15005 Kalawao Kalawao County a Naca d Show All Features 115 Preparing Vector Data for Processing Using the Multipart to Singleparts tool which can be found by navigating to Vector Geometry Tools with Hawaii_counties shp as the input we generate a singlepart shapefile with 32 features The following figure shows that a single polygon is now tied to a single record in the attribute table 4 Attribute table hawaiimultipart Features total 32 filtere GEOID10 NAME10 NAMELSAD10 15 15009 Maui Maui County a ECan 17 15009 Maui Maui County 1g 15009 Maui Maui County 19 15009 Maui Maui County d Show All Features _ The Singleparts to Multipart tool which can be found by navigating to Vector Geometry Tools generates a multipart layer based on an attribute you specify in the Unique ID field input The options of the Singleparts to Multipart tool are shown in the following screenshot 116 Chapter 4 W Singleparts to multipart Input line or
27. creating a new directory in the selected path In our case the new directory will have the name TestPlugin At the end of the code generation there will appear a dialog with a message explaining the steps to complete plugin creation The generated plugin is not available yet it s necessary to restart QGIS and activate it in the Plugin Manager interface The plugin is now fully functional but after the first activation its button in the QGIS toolbar will be without an icon Compiling the icon resource To make the icon visible it s necessary to compile the icon resource so as to have it available in Python Resource compilation is a process to render an icon s platform independent of the Qt framework which is the graphical infrastructure on which OGIS is built To compile the icon resource it s necessary to have installed the GNU make and the pyrcc4 command In Windows using the OSGeo4W QGIS installation this command will be automatically installed and can be addressed only inside the OSGeo4W shell On other platforms they have to be installed using another command The make command is usually available in every Linux operating system distribution 315 Creating QGIS Plugins with PyQGIS and Problem Solving The pyrcc4 command is the Qt resource compiler for Ot4 and it s available in the pyqt4 dev tools qt4 designer packages After the make and pyrcc4 commands are installed make has to be called inside the directory of t
28. e Nearest neighbour In this method each raster cell is assigned the value of the nearest cell measure between cell centers This is a great method to choose when the raster represents discrete categorical data as no new values are created e Bilinear In this method each raster cell is assigned an average value based on the four closest cells with original values This method will smooth the data and may flatten peaks and fill valleys e Average In this method each raster cell is assigned an average value based on surrounding cells with original values This method will smooth the data and may flatten peaks and fill valleys 79 Styling Raster and Vector Data e Cubic In this method each raster cell is assigned an interpolated value based on the surrounding cells with original values Unlike the bilinear method this method will not smooth the peaks or valleys as much and it tends to maintain local averages and variability This is the most computationally intensive method Styling vectors In this section the six different vector styling types will be covered The six types are single symbol categorized graduated rule based point displacement and inverted polygons Note that even though layer rendering is part of vector style properties it will be discussed separately in the next section as it is common to all vector styling types Single symbol vector styling The single symbol vector style applies the same symbol
29. f geometry column The name of the geometry field in the table the view is selecting from read_only In this field enter 1 for the spatial view to be read only and enter 0 for the spatial view to be read write Note that as of version 2 6 0 of QGIS views set as read write cannot be edited in QGIS Desktop However views may be editable in some plugins or with SQL queries 8 Click on the Execute button to create the view The following screenshot displays a successfully written and executed view of the Waterfalls table SQL window GiffordPinochet sglite SpatiaLite SOL query T Store Delete 1 INSERT INTO views_geometry_columns view_name view_geometry view_rowid f_table_name f_qeometry_column read_only 2 VALUES mowich lake waterfalls geom rowid waterfalls geom 1 Execute F5 1rows 0 0 seconds The view is now registered as spatial and can be added to the QGIS Desktop canvas like any other SpatiaLite spatial table Dropping a spatial view Dropping a spatial view requires that you drop the spatial view table and delete the relating entry in the view_geometry_columns table To drop the spatial view table use the SQL DROP VIEW command For example to drop the mowich_lake_ waterfalls view you will need to execute the following SOL command DROP VIEW mowich lake waterfalls 48 Chapter 2 With the view dropped the final step is to delete the related entry in the view_ geometry_co
30. green G and blue B values for the currently selected color are displayed Each of these color parameters can be individually modified by either using the slider controls or by changing the numeric values Red green and blue values must be specified between 0 and 255 where 0 represents no color and 255 represents full color each degree represents a different location and color on the color wheel Saturation and value are specified using percentages and range between 0 representing no saturation or value and 100 representing full saturation or value Q Hue is specified in degrees ranging from 0 to 359 where Below the color parameters is the Opacity setting The right half of the Current and Old fields display the color with the applied opacity level For example in the following screenshot the current color is shown with no opacity 100 on the left and with the currently selected opacity of 50 on the right The old color is shown with no opacity on the left and with the previously selected opacity of 100 on the right in this case both sides are the same See 53 Styling Raster and Vector Data The HTML notation textbox displays the HTML color notation of the current color The color notation can be changed to one of the four different formats by clicking on the down arrow in the HTML notation textbox Lastly the Reset button resets the current color to match the old color Changeable panels in
31. icon png B Makefile B metadata txt plugin_upload py B pylintrc i README html B README txt P resources_rc py 9 resources qrc 9 test_plugin_dialog base ui test_plugin_dialog py b python usr bin python2 7 i CSIWMSLegend Chapter 9 e test_plugin fF al 177 t LLUM self iface removeToolBarIconlaction def run self from qgis core import QgsMapLayerRegistry layersDict QgsMapLayerRegistry instance mapLayers dict items ayer VectorLayer a s pLayer RasterLayer rasters append map name else print self dlg rastersCombo insertItems rasters self dlg vectorsCombo insertItems vectors self dlg show result self dlg exec_ if result pass Adding breakpoints Breakpoints are debugger instructions to stop execution at a specified line to allow users to investigate variable values and eventually change their values manually Our scope is to add a simple breakpoint and check that the plugin execution stops exactly at that point passing the control to the remote debugger 333 Creating QGIS Plugins with PyQGIS and Problem Solving To add a breakpoint open the Debug perspective and double click on the left hand side of the line number for example the line with the for id map code in layersDict items Aptana will add a breakpoint as shown in this screenshot S eterminat d gt Debug Server r Python Server A lt terminated exit value 0 gt Debug Server Il a TestP
32. is because SpatiaLite treats the input as a nonspatial table even though it has coordinates stored in the table We will add the spatial component to the table in a later step 36 Chapter 2 5 Click on the OK button to import the file z Import vector layer Input ns Mastering QGIS Chapter 2 GST103_Lab2_Data Data Waterfalls xls Update options Output table Schema Table Waterfalls Options Primary key Source SRID Encoding Drop existing table reat sngie pa art QEOMETIES In Fat ii Create spata index Cancel 6 After a few moments you will be notified that the import is complete To view the newly created table you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then click on Refresh under Database or press the F5 key on your keyboard The waterfalls table should now appear and have the table icon next to it 7 Select the Waterfalls table Click on the Info tab on the information panel Note the Northing and Easting fields These fields contain the coordinates of the waterfalls in NAD 83 UTM Zone 10 North EPSG 26910 Click on the Table tab on the information panel to view the entries in the table Note that the Preview tab is not selectable because the selected table does not have any geometry field At this point the table import is complete However since the waterfalls table has coordinate pairs a point geometry column can be added to the
33. name The Add spatial index button adds a spatial index to the table This option is only available if the table is a geometry field The Delete index button deletes the currently selected index Deleting a table There are two ways to delete a table from a database within QGIS by using the Browser panel in QGIS Desktop or by using the DB Manager To delete a table using the Browser panel in QGIS Desktop expand the database from which you wish to delete a table then right click on the table and choose Delete layer 44 Chapter 2 To delete a table using DB Manager open DB Manager by clicking on DB Manager under Database In the Tree panel expand the tree and select the database that contains the table that you wish to delete In the Tree panel select the table that you wish to delete Then click on Delete table view under Table You can also right click on the table in the Tree panel and choose Delete from the contextual menu Emptying a table To remove every record from a table without deleting the table open DB Manager by clicking on DB Manager under Database In the Tree panel expand the tree and select the database that contains the table that you wish to empty In the Tree panel select the table you wish to empty Then click on Empty table under Table Creating queries and views DB Manager has a SQL window that allows SQL queries to be executed against the database This section will explain how to use the SQL window
34. qgis org api classQgsVectorLayer html As for rasters this class is a specialization of the generic QgsMapLayer class To get the extent of the layer it s necessary to use the extent method of the OQgsMapLayer class print myVector extent toString Executing the preceding code will generate the following result 7115212 9837922714650631 1368239 6063178631011397 4895579 8114661639556289 7805331 2230994049459696 This shows the corner coordinates in the format xmin ymin xmax ymax To know how many records or features contain the vector use the following code myVector featurecout This will produce the result of 653L records In the result L means that it is a Python long integer which is an integer limited only by the available memory 345 PyQGIS Scripting As we Saw earlier vectors can be sourced from different providers each one with its proper capabilities and limitations To discover the capability of myVector use the following method myVector capabilitiesString This will give the following result u Add Features Delete Features Change Attribute Values Add Attributes Delete Attributes Create Spatial Index Fast Access to Features at ID Change Geometries Simplify Geometries with topological validation A Unicode string describes all the possible actions available on the vector Describing the vector structure Compared to rasters vectors are more complex A vector involves a
35. save as button We will name this new model file as TWI_ for watersheds model The TWI From DEM model is located in the Algorithms tab and is added as an algorithm to our new model Remember that models need to be saved to the Models folder to appear as algorithms You will notice that the model icon in the modeler canvas for the TWI From DEM algorithm identifies the algorithm as a model 4 Processing modeler TWI for Individual Watersheds Hydrology y GRASS commands Modeler only tools Boundaries Example models Hydrology amp DEMs Clipped to Watersheds In E 8 TWI for Individual Watersheds TWI From DEM Clip grid with polygon Landcover P Orfeo Toolbox Image analysis E QGIS geoalgorithms Out H Q R scripts S SAGA t TauDEM hydrologic analysis In XU Tools for LIDAR data TWI From DEM Out E The DEM will be clipped to the watersheds layer and the clipped DEM will be the input to the TWI From DEM algorithm This will create one output a TWI raster covering the watersheds However if the Iterate over this layer setting is used the DEM will be clipped to each of the 21 watersheds and the TWI will be calculated for each This will use both a nested model and the iterate feature in the same model 296 Chapter 8 W TI for Individual Watersheds Parameters Log Help DEM elevation EPSG 26915 Save to temporary file Open output file
36. value of 0 The next step is to clip the land use layer using the SAGA Clip grid with polygon tool If you remember we clipped a raster layer in a previous exercise using the native GDAL Clipper tool so again this is merely demonstrating the number of options we have to perform spatial operations 246 Chapter 7 We need to select land use lulc_clnp as our input s raster layer the park boundary as our polygon s layer name the output file as lulc_clip tif and click on OK Remember from an earlier exercise that you can load the lulc_palette qml file if you would like to properly symbolize the land use layer but this step isn t necessary Querying land use for only surface water using the SAGA Raster calculator Now we can query this layer for the areas that represent surface water We can again use the SAGA Raster calculator tool and enter a 11 in the Formula box as illustrated in the next screenshot In this example we are stating that if the land use layer that is a is equal to 11 the resulting output value will be 1 otherwise it will be 0 j Raster calculator Parameters Log Help Main input layer Land Use clip EPSG 26710 Additional layers optional 0 elements selected Formula fa 11 Result C Mastering_QGIS GRASS ulc_sw tif Open output file after running algorithm Now we have a raster layer that we can use to identify potential habitat within 1 000 meters of surface water 247
37. which are as follows Select the attribute s from the Assigned attributes list then click on the Add attribute button Click on the Add expression button and then create an expression that will be added as a single entry Once an attribute has been added the Assigned attributes colors can be changed by double clicking on the color patches in the Color column 95 H oN AN Styling Raster and Vector Data The following figure shows two examples The top example shows a single attribute that has been added to the Assigned attributes list and the bottom one is an example of a single attribute and an attribute that was calculated with an expressions Attributes Available attributes Assigned attributes STATEFP10 POPWHITE COUNTYFP10 POPBLACK COUNTYNS10 POPAIAN GEOID10 POPASIAN NAME10 POPNHOPI a LSAD10 POPTWO Attributes Available attributes Assigned attributes STATEFP10 POPHISP COUNTYFP10 POPWHI K COUNTYNS10 GEOID10 Expression NAME10 POPWHITE2 POPBLACK2 NAMELSAD10 CANIN POPAIAN2 POPASTAN2 POPNHOPI2 POPOTH2 POPTWO2 Creating a pie chart diagram A pie chart diagram displays attribute s in a round pie chart where each attribute occupies a pie slice proportional to the percentage that the attribute represents from the total of all attributes added to the pie chart As an example the
38. 04 fi Metadata t Stretch to MinMax Estimate faster Actual slower Restore Default Style 18 Chapter 1 Contrast enhancement Another important consideration with raster styling is the settings that are used for contrast enhancement when rendering the data Let s start by loading the Jemez_ dem img image and opening the Style menu under Layer Properties shown in the figure below This is an elevation layer and the data is being stretched on a black to white color ramp from the Min and Max values listed under Band rendering By default these values only include those that are from 2 percent to 98 percent of the estimation of the full range of values in the dataset and cut out the outlying values This makes rendering faster but it is not necessarily the most accurate 4 Layer Properties Jemez_dem Style e l General W Band rendering Render type Singleband gray asl Transparency Gray band Band 1 Min 1797 95 Histogram Max 2279 04 fi Metadata t Stretch to MinMax Accuracy Estimate faster Actual slower Restore Default Style Next we will change these settings to get a full stretch across all the data values in the raster To do this perform the following steps 1 Under the Load min max section choose Min max and under Accuracy choose Actual slower 2 Click on Load 19 A Refreshing Look at QGIS 3 You will notice that th
39. Advanced settings are available by clicking on the Advanced button 84 Chapter 3 Rule based vector styling The rule based vector style applies one symbol per created rule and can apply maximum and minimum scales to toggle symbol visibility This vector style is the best when you want a different symbol that is based on different expressions or when you want to display different symbols for the same layer at different map scales For example if you are styling roads a rule could be set to make roads appear as thin lines when zoomed out but when zoomed in the thin lines will disappear and will be replaced by thicker lines that are more scale appropriate There are no default values for rule based styling however if a style was previously set using a different styling type the style will be converted to be rule based when this style type is selected The following screenshot shows the Categorized style type from the previous section that is converted to the Rule based style type parameters for polygon vector data of the populations of the countries Its properties will be very similar to that of point and line vector data Rule based Label Min scale Max scale Coun Duplicate count X 1671599 postesti gt 16715999 000000 AND postesti lt 4905 X E 4905248 postesti gt 49052489 000000 AND postesti lt 1112 X E 1112117 postesti gt 111211789 000000 AND postesti lt 313 X E 3139730 postesti
40. Click on Close to close the Select by expression window On the schools attribute table in the lower left corner click on Show all Features button and choose Show Selected Features Only the selected records will now display in the attribute table 18B Preparing Vector Data for Processing Summary Data is rarely in the form needed to perform processing and analysis Often data needs to be merged checked for validity converted calculated projected and so on to make it ready for use This chapter covered many common preparation tasks to convert raw data into a more useable form In the next chapter this theme of data preparation will continue but it will be applied to raster data You will learn how to mosaic reclassify resample interpolate and convert raster data to make it more meaningful as an input to processing tasks 144 Preparing Raster Data for Processing This chapter covers how to prepare raster data for further processing with the GDAL menu tools and Processing Toolbox algorithms There are specific considerations and tools for managing raster data The topics that you will cover in this chapter are as follows e Reclassification e Resampling e Rescaling e Raster mosaics e Generating overviews pyramids e Data format conversions e Interpolation Reclassifying rasters Raster data sets often have hundreds or thousands of values For an analysis you may need to synthesize the data into m
41. Encoding LTF 8 File format CSV comma separated values Custom delimiters Regular expression delimiter Record options Number of header lines to discard 1 First record has field names Field options Trim fields Discard empty fields Decimal separator is comma Geometry definition Point coordinates Well known text WET No geometry attribute only table Geometry field field_1 Geometry type Detect Layer settings Use spatial index Use subset index field_1 1 POLYGON 1530937 96480 34601 1484805 234476 1293 1530800 63548094595 1484862 7901427944 1530800 666476 1242 POLYGON 1530783 4314761279 1484862 9744761295 1530783 1664761244 14848 12 6844761225 1530646 8214761249 175 Advanced Data Creation and Editing The data layer will be added to the Layers list and will behave like any other vector layer The following figure shows the resulting parcel boundaries Well known text representations of parcels mapped via the Add Delimited Text Layer tool An easy way to explore WKT geometries is to use the getWKT plugin This allows you to click on a selected Q feature in the QGIS map canvas and see the WKT for that feature The WKT can be copied to the clipboard Geocoding address based data Another useful and common tabular spatial data source is a street address Geocoding addresses has many applications such as mapping the customer base for a store members of an organization public health records or i
42. NGS Here you will learn how to load a precompiled set of ground control points to georeference the image 1 i Load the image into the Georeferencer window Choose EPSG 2903 NAD83 HARN New Mexico Central ftUS for the CRS Click on the Load GCP Points tool gg and choose the zone_map points file The destination coordinates for the locations in this file were obtained from the NGS website http www ngs noaa gov cgi bin datasheet pr1 From the website the Station Name link under DATASHEETS was used and a station name search conducted for each The destination coordinates are in EPSG 2903 NAD83 HARN New Mexico Central ftUS 197 Advanced Data Creation and Editing The following figure shows the image in the Georeferencer window with 5 GCP s entered based off of datums whose destination coordinates were looked up online Their location is identified by numbered boxes within the image window The To and From coordinates are displayed in the GCP table window along with the RMSE values E Georeferencer zone_map bmp File Edit View Settings Help PS a a F QQ ta 2 gt ere o FG mo UAL LR wS ZONING MAP dX map units dY map units residual map units F592975 55 1935414 15 1524608 32 1484404 47 0 09 4 49 6274098 49 8399918 00 1523925 76 1480815 95 1 39 0 03 wk 5780754 77 5490891 27 1523645 13 1482436 21 E 1 46 7 EFJ 3 10850286 74 6171365 35 152644
43. Operating Systen ca Data D IE File name County_attributes xls hi All files e Once the data is loaded a join can be completed by following these steps 1 Select the GIS layer in the Layers panel that will receive the new data from the join Navigate to Layer Properties and choose the Joins menu Click on the add join button the one with green plus sign Choose the Join Layer Join Field and Target Field values The Join Layer and Join Field values represent the standalone table The Target Field value is the column in the attribute table on which the join will be based Although in this example the join field and the target field NS have the same name this is not a requirement The two fields merely need to hold the same unique ID At this point you can choose Cache the join in virtual memory Create attribute index on the join field and Choose which fields are joined The last option lets you to choose which fields from the join layer to append to the attribute table At this point the Add vector join window will look like the following screenshot Once created the join will be listed on the Joins tab The extra attribute columns from the join layer will be appended to the attribute table where the value in the join field matched the value in the target field 13 A Refreshing Look at QGIS 7 Joins can be removed by clicking on the remove join button the one with red minus sign
44. QAction class e The unload self method is used to unroll all the QGIS GUI elements added with the previous add_action method This method is always called when the plugin is unloaded in QGIS using the Plugin Manager Setting up a debugging environment Software development is a complex task and there s no software without bugs Debugging is the process to remove software failures Debugging is a task that can involve some other software to facilitate the debugging process A plugin can become complex requiring debugging tools to discover problems The complexity of the debugging process can start by inserting some prints inside the code or adding log messages to finish controlling the execution instructions by instructing how to find execution problems Inserting a breakpoint to stop the execution at a certain point of the code of a third party QGIS plugin can be useful to discover how it works What is a debugger There is a set of possible tools to debug the Python code but we ll focus only on PyDev which reduces the number of installation steps and allows remote debugging without modification of the plugin code PyDev is an Eclipse plugin where Eclipse is a free software programmable framework used to develop almost everything PyDev can be added to a local installation of Eclipse adding it from the marketplace but to reduce the number of installation steps it s suggested that you install Aptana Studio 3 an Eclipse c
45. QGIS Desktop provides powerful field calculation functionality In the field calculator advanced mathematical geometry string date and time type conversion and conditional functions are available for use Leveraging these advanced functions along with standard operators allows for some powerful field calculations This section will explain the field calculator interface in detail followed by multiple examples of advanced field calculations from a variety of functional areas It is assumed that you know the basics of field calculations and common operators 131 Preparing Vector Data for Processing Exploring the field calculator interface The field calculator can be opened in three ways which are as follows e Open the attribute table of the layer whose details you wish to calculate and then click on the field calculator button 4 on the attribute table toolbar e Open the attribute table of the layer whose details you wish to calculate and then press Ctrl I on your keyboard e Select the layer whose details you wish to calculate in the Layers panel and then click on the field calculator button E23 on the attributes toolbar The Field calculator window shown in the following figure has five sections e Field designation This determines which field will hold the output of the expression You can use Create a new field or Update existing field by selecting the desired option and setting the relevant option s A virtual fie
46. QGIS Plugins with PyQGIS and Problem Solving The Python Console The Python Console is a wonderful instrument to explore and learn PyQGIS It s available in every QGIS installation and can be opened by selecting the Python Console voice in the Plugins menu The Python Console is a dockable interface and like all dockable interfaces you can change its position inside QGIS or separate it You can try moving the console by dragging and dropping it The console is shown in this screenshot Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help B BBQs c5 Kk PPPHBPLPABR b MyFirstRaster Python Console 2Use iface to access QGIS API interface or Type help iface for mo re info 3 gt gt gt myRaster QgsRasterLayer mnt data PROGRAMMING GIS GISDATA qgis_sample_data raster landcover img MyFirstRaster 4 gt gt gt QgsMapLayerRegistry instance addMapLayers myRaster 5 lt qgis _core QgsRasterLayer object at 0x955f3b0 gt Coordinate 2794653 5450800 Scale 0 557 437 v M Render EPSG 2964 A As you can see the console is composed of a button toolbar marked by the red box The bigger Python Console is marked by the upper right red box and is where all the command results are shown and finally a bottom command line marked by the bottom right red box is where you can edit commands 306 Chapter 9 Here we describe how to test code interactively and explore th
47. Raster layer statistics f gt Mean value from algorithm Raster layer statistics g gt valid cells count from algorithm Raster layer statistics h gt valid cells count from algorithm Raster layer statistics i gt Standard deviation from algorithm Raster layer statistics 280 Chapter 8 The algorithm dialog will look very similar to how it would if you were running it from the Processing Toolbox There are inputs tool parameters and outputs However there are some important differences because the graphical modeler is a self contained universe of data inputs The differences are as follows Input layers are limited to those that have been added to the model Output can be left blank if it is an intermediate result that will be used as an input for another algorithm If the output is a layer that needs to be saved enter the name of this layer in the text box When naming an output layer you won t actually need to provide an output filename This will be done when the tool is run Instead you just need to enter the name of the layer for example stream buffer Numerical values or string value parameters can be entered as numbers or strings They can also be chosen from other inputs of the Number or String type The fields of an attribute table or other standalone table can be specified by typing the field name or by using the Table field input These fields will be chosen
48. SAGA installing on 153 SAGA troubleshooting on 153 well known text WKT representations of geometry about 170 mapping 174 176 Subversion 314 views creating 45 viewshed combining with r mapcalculator 240 242 388 PACKT open source PUBLISHING Thank you for buying Mastering QGIS About Packt Publishing Packt pronounced packed published its first book Mastering phpMyAdmin for Effective MySQL Management in April 2004 and subsequently continued to specialize in publishing highly focused books on specific technologies and solutions Our books and publications share the experiences of your fellow IT professionals in adapting and customizing today s systems applications and frameworks Our solution based books give you the knowledge and power to customize the software and technologies you re using to get the job done Packt books are more specific and less general than the IT books you have seen in the past Our unique business model allows us to bring you more focused information giving you more of what you need to know and less of what you don t Packt is a modern yet unique publishing company that focuses on producing quality cutting edge books for communities of developers administrators and newbies alike For more information please visit our website at www packtpub com About Packt Open Source In 2010 Packt launched two new brands Packt Open Source and Packt Enterprise in order to continue it
49. Summary This chapter provided an overview of the structure within the processing toolbox and an introduction to the variety of advanced spatial analyses tools than can be accessed through the toolbox You specifically learned how to create a shaded relief map calculate the least cost path evaluate a viewshed reclassify raster layers query and combine raster layers and calculate raster statistics using GRASS algorithms You then learned how to crop raster layers using a polygon mask and reclassify query and combine raster layers using SAGA algorithms You learned how to delineate a watershed and extract a vector stream network from a DEM using TauDEM algorithms And in our last exercise we explored the integration of spatial statistics using R packages to identify characteristics of landscape features Perhaps most importantly we saw how interoperable the native QGIS tools are with the tools executed from within the processing toolbox Although we explored these tools through hypothetical scenarios to illustrate how these analyses might be applied to real world questions it is important to clarify that this chapter is by no means exhaustive in its coverage of the complete suite of tools or their various applications There are additional powerful algorithms provided by the R and LAStools environments In the next chapter you will learn how to automate geospatial workflows using the graphical modeler within the processing toolbox We will ex
50. We can then sum the area for each combination to calculate the total visual impact of these three towers in Crater Lake National Park as follows r stats 0 623402400 000000 1 7756000 000000 10 128996100000000 11 31945300000000 100 85009000 000000 101 33947 700 000000 110 8058800 000000 111 63800700 000000 242 Chapter 7 We can also provide a more informative visual depiction of the impact as demonstrated in the next screenshot using this approach rather than the traditional binary visible not visible viewshed maps In this exercise we used a variety of GRASS algorithms to explore the analytical power of the processing toolbox We performed both common geoprocessing and advanced spatial analyses to arrive at hypothetical scenarios that would be time consuming to address without the support of a GIS these analyses included the following e Creating a shaded relief map using r shaded relief e Calculating slope using r slope e Reclassifying raster data using r recode e Creating a cost grid using r cost 243 The Processing Toolbox e Calculating a least cost path using least cost paths e Calculating viewshed using r viewshed e Utilizing raster calculation functions within r mapcalculator e Summarizing raster attributes using r stats In the next section we will continue exploring the types of analyses that are possible using the SAGA algorithms that are available through the toolbox SAGA The SAGA
51. _ multipart features which have multiple geometries linked to a single record in the attribute table Standalone tables are y not associated with GIS data layers However they may have data of a spatial nature from which a spatial data layer can be generated for more information see Chapter 6 Advanced Data Creation and Editing They may also contain data that you wish to join to an existing attribute table with a table join Attribute tables are associated with the features of a GIS to Z Attribute table Counties Features total 3118 filtered 3118 selected 0 lt Table Information J E S PB Table Tools ADMIN_NAME ADMIN_FIPS STATE STATE_FIPS NAME SUFFIX PRIM_MILES COUNTYPO10 055 Multnomah County 41051 41 Multnomah County 153 18450113700 243 00000000000 056 Polk County 41053 41 Polk County 121 81613491000 299 00000000000 057 Sherman County 41055 41 Sherman County 188 32807527400 239 00000000000 058 Umatilla County 41059 atilla County 298 69500416300 213 00000000000 __ Table Body 059 Union County 41061 on County 287 03910718000 228 00000000000 060 Wallowa County 41063 41 Wallowa County 260 73823956900 214 00000000000 2061 Wasco County 41065 41 Wasco County 298 49648113800 245 00000000000 FJ Show All Features _ Table Menu Table View gt Sle Table joins Let s say that you need to make a map of the total population by county However the counties GIS layer does not have pop
52. added to each rule by clicking on the Refine current rules button To calculate the number of features included in each rule and to calculate the duplicate feature count click on the Count features button When a rule is added or edited the Rule properties window which is shown in the following screenshot displays five rule parameters which are as follows Label This should have the rule label that will be displayed in the Layers panel Filter This will have the expression that will select a subset of features to include in the rule Click on the ellipsis button to open the Expression string builder window Then click on the Test button to check the validity of the expression Description This has a user friendly description of the rule Scale range This has the Minimum exclusive and Maximum inclusive scales between which the rule will be visible Symbol This has the symbol that will be used to symbolize features included in the rule 86 Chapter 3 None of the parameters are required Label Filter and Description could be left blank to exclude Scale range and Symbol from the rule uncheck the boxes next to these parameters Label Capital Cities Filter admOcap 1 Description Capital cities of countries X Scale range Minimum 1 20 000 000 exclusive xX Symbol Marker e Simple marker Simple marker ally Rule properties Maximum f 1 1 inclusive Unit
53. after running algorithm The following figure shows the output of the TWI for Individual Watersheds nested model using the iterator feature Automating Workflows with the Graphical Modeler As of version QGIS 2 6 you can have as many levels of _ nested models as you wish There is no limit Using batch processing with models Models can also be used in batch mode just like other processing algorithms To do this simply locate the model in the Processing Toolbox panel right click on it and choose Execute as batch process as shown in the following screenshot Processing Toolbox Recently used algorithms GD GDAL OGR 34 geoalgorithms n GRASS commands 168 geoalgorithms ait Models 7 geoalgorithms Boundaries Example models EF Hydrology Landcover Execute as batch process Edit rendering styles for outputs Edit model Delete mana E Ei 4 E Ei KT Tools for LIDAR data 58 genari Advanced interface 298 Chapter 8 The tool will operate like any other geoalgorithm in QGIS You can click on Add rows Delete rows and Run when ready With this method the model can be utilized on datasets from different geographies This technique is also useful in cases where you have to repeat several geoprocessing steps on a collection of files 4 Batch Processing Riparian trees Parameters Buffer distance Riparian Trees Load in QGIS i 100 AK Trees iii AK Riparian Trees WA Rivers
54. all the four band rendering options it is time to shift our focus to the bottom two sections of the raster style window color rendering and resampling Creating a raster composite To combine many separate raster files each representing one raster band into a raster composite we can use the r composite tool in the Processing Toolbox The processing toolbox which is covered in detail in Chapter 7 The Processing Toolbox allows us to access tools from GRASS GIS which contains the r composite tool The r composite tool assigns three singleband rasters to a red green and blue raster band and produces one multiband raster To open the r composite tool open the processing toolbox first by navigating to Processing Toolbox In the search box at the top of the processing toolbox type r composite to find the tool and then double click on the tool to open it This will open the r composite tool window as shown in the following screenshot 74 Chapter 3 Z r composite Combines red green and blue raster ma Parameters Log Help Red C Users Rick OneDrive Publications Mastering QGIS Chapter 3 Green C Users Rick OneDrive Publications Mastering QGIS Chapter 3 Blue C Users Rick OneDrive Publications Mastering QGIS Chapter 3 Number of levels to be used for lt red gt 32 Number of levels to be used for lt green gt 32 Number of levels to be used for lt blue gt 32 Dither Use closest color GRASS region
55. and your e mail address Setting optional plugin parameters There are also optional parameters that are really useful if your plugin would be available for other users The parameters are as follows Repository A repository can be added later it is the location where the plugin code is located Its common to use a Version Control System VCS repository to maintain your code Some popular VCSs are Git or Subversion and some related to Git There are famous online services available at http www github comor https bitbucket org where you can upload your project and maintain modifications For example the repository of the code of the Plugin Builder is https github com g sherman Qgis Plugin Builder For our plugin we can leave this blank for now Bug tracker It s good practice to maintain a service to track the bugs of the plugin Plugin users can file issues by preparing test cases that help to reproduce the bug Tracking traces of the bugs and their solutions help us to know the evolution of the plugin Usually the use of a VCS web service as shown previously provides a bug tracking service For example the bug tracker for the plugin called Plugin Builder is provided by the QGIS infrastructure and can be found at the http hub aqgis org projects plugin builder issues For our plugin we can leave this blank at the beginning Home page If the plugin has a web page where it is described its good practice to add a plugin
56. are a series of tools on the Composer Items toolbar The lower portion of the toolbar contains buttons for adding map elements to your map These include the map body images text a legend a scale bar graphic shapes arrows attribute tables and HTML frames Map elements become graphics on the composition canvas By selecting a map element graphic handles will appear around the perimeter These can be used to move and resize the element The upper portion of the Composer Items toolbar contains tools for panning the map data moving other graphic content and zooming and panning on the map composition The majority of the map customization options can be found in the composer tabs To specify the sheet size and orientation use the Composition tab Once map elements have been added to the map they can be customized with the Item properties tab The options available on the Item properties tab change according to the type of map element that is selected The Atlas generation tab allows you to generate a map book For example a municipality could generate an atlas by using a map sheet GIS layer and specifying which attribute column contains the map sheet number for each polygon The Items tab allows you to toggle individual map elements on and off 23 A Refreshing Look at QGIS The toolbars across the top contain tools for aligning graphics the Composer Item Actions toolbar navigating around the map composition the Paper Navigation toolb
57. book To get the most from this book it is recommended that you install QGIS and follow the explanations If you choose to do so you will need a reasonably modern computer with access to the Internet to download and install QGIS read documentation and install plugins QGIS can run on Windows Mac OS X and many Linux distributions Who this book is for This book is for intermediate to advanced GIS users developers and consultants who are familiar with QGIS but want to look deeper into the software to unleash its full potential The reader is expected to be comfortable with common GIS functions and concepts as possession of this knowledge is assumed throughout the book This book focuses on how to use QGIS and its functions beyond the basics xi Preface Conventions In this book you will find a number of styles of text that distinguish between different kinds of information Here are some examples of these styles and an explanation of their meaning Code words in text database table names folder names filenames file extensions pathnames dummy URLs and user input are shown as follows Type a comma after Snow and enter dd MM yyyy followed by a close parenthesis A block of code is set as follows CASE WHEN POP1996 gt 5000000 THEN Result ELSE STATE NAME END When we wish to draw your attention to a particular part of a code block the relevant lines or items are set in bold CASE WHEN POP1996
58. by checking the Invert parameter The color ramp can be applied to the raster cell values in a Continuous or Equal interval classification mode terminating at the Min and Max values O Continuous This stretches the color ramp between the Min and Max values with each unique value being assigned a unique color Equal interval This assigns a number of colors designated by the Classes parameter across groups of values For instance if five classes are specified then no matter how many unique values exist in the raster five colors will be applied to the raster where each color will be applied to groups of values with group value ranges of Max Min Classes Set the mode to Continuous the Min value to 105 and Max value to 207 Click on the Classify button to apply the color ramp to the values The classification list on the left will populate with values colors and labels The last step is to choose the Color interpolation method The following three methods are available and they have a significant effect on how the raster will be rendered O Discrete Assigns only and exactly the colors chosen in the classification list Values between values listed in the Value column are assigned the color assigned to the next highest listed value In other words if there are say 164 unique values in the raster and 15 colors listed in the classification list the raster will be rendered with exactly the 15 listed colors This method i
59. can be opened within Desktop by selecting the layer table in the Layers panel and then by either clicking on Open Attribute Table under Layer or by right clicking on the data layer and choosing Open Attribute Table from the context menu They can also be previewed in Browser by choosing the Attribute tab The table opens in a new window that displays the number of table rows and selected records in the title bar Below the title bar are a series of buttons that allow you to toggle between editing managing selections and adding and deleting columns Most of the window is filled with the table body The table can be sorted by clicking on the column names An arrow will appear in the column header indicating either an ascending or a descending sort Rows can be selected by clicking on the row number on the left hand side In the lower left corner is a Tables menu that allows you to manage what portions of the table should be displayed You can choose Show All Features default setting Show Selected Features Show Features Visible on Map only available when you view an attribute table Show Edited and New Features create column filters and advanced filters expression The lower right corner has a toggle between the default table view and a forms view of the table 11 www allitebooks com A Refreshing Look at QGIS layer Typically one record in the attribute table corresponds to one feature in the GIS layer The exception to this is
60. can be used by other QGIS tools Loading a layer means loading a reference to the layer and its metadata The layer is not necessarily loaded in memory but is usually fetched only when data is accessed to be processed or visualized Loading and registering a layer are separate steps A layer can be loaded processed and modified before it is visualized or it can be loaded as temporary data for an algorithm In this case it s not necessary that the QGIS framework would be aware of the layer A The iface object has shortcuts to load raster and vector Q layers in a single step instead of loading and registering them via separated steps Every layer type is managed by a provider manager QGIS has some internal implemented providers but most of them are external libraries The list of available providers depends on the QGIS installation This list can be obtained by typing the following code snippet in the QGIS Python console gt gt gt QgsProviderRegistry instance providerList u WFS u delimitedtext u gdal u gpx u grass u grassraster u memory u mssgl u ogr u ows u postgres u spatialite u wcs u wms 338 Chapter 10 The preceding result shows a Python list of strings that have to be used when a PyQGIS command needs the provider parameter Managing rasters Like most free software projects the QGIS community doesn t want to reinvent the wheel if it s not strictly necessary For this reason mo
61. classifying the loaded layers we set the values of the combobox of the plugin interface This can be done with the following code self dlg rastersCombo insertItems 0 rasters self dlg vectorsCombo insertItems 0 vectors We named our comboboxes with object names rastersCombo and vectorsCombo in the two code lines earlier comboboxes are populated with a standard QComboBox with the insertItems call passing the two lists of layers The 0 parameter is the index where we start to add new elements 323 Creating QGIS Plugins with PyQGIS and Problem Solving Understanding self In the two preceding lines there is the keyword self that may confuse everyone when approaching object oriented programming for the first time To explain it try to follow where the Test PluginDialog interface is created and saved At the beginning of the test_plugin py code the function init self iface is where the Plugin GUI is created for the first time with the instruction self dlg TestPluginDialog This means that the result of the creation of the dialog the Test PlugiDialog constructor call is saved in the dig variable that belongs to the current instance of the plugin In this case dlg is called an object or instance variable where the instance of the plugin is referred to with the self variable The variable self is almost always available in every Python function this allows us to access the dlg variable everywhere in the c
62. click on Get models from on line scripts collection The Get scripts and models window will open Choose the models to load and click on OK The new models will be loaded in the Models section of the Processing Toolbox panel under the Example models group This collection of online models will be continually expanded by QGIS users Get scripts and models H a Updatable E 7 Notinstalled Processing resources manager alt Multiply 2 layers af Rasterize and fil Check uncheck algorithms in the tree to amp Installed select the ones that you want to install or remove Algorithms are divided in 3 groups Installed Algorithms already in your system with the latest version Upgradable Algorithms already in your system but with a newer version available in the server Not mstalled Algorithms not installed in your system 291 Automating Workflows with the Graphical Modeler Executing model algorithms iteratively Models like all QGIS algorithms can be executed iteratively Here we will demonstrate this feature with one of the QGIS example models DEMs_Clipped_to_ Watersheds model We will use two inputs a DEM covering Taos New Mexico and a watersheds polygon layer for the area The elevation tif and watersheds shp sample data will be used Input data watersheds and a DEM The model has just one algorithm It uses the Clip grid with polygon tool to clip the DEM to watersheds There are 21 watersheds coverin
63. contains tools to add vector raster database web service text layers and create new layers The Map Navigation toolbar contains tools that are useful for panning zooming and refreshing the map display The Attributes toolbar provides access to information selection field calculator measuring bookmarking and annotation tools QGIS Desktop offers a number of customization options You can toggle the visibility of toolbars by navigating to View Toolbars or by right clicking on the menu bar or the enabled toolbar button which will open a context menu allowing you to toggle the toolbar and panel visibility You can assign shortcut keys to operations by navigating to Settings Configure shortcuts You can also change application options such as interface language and rendering options by navigating to Settings Options QGIS Browser The QGIS Browser interface shown in the following screenshot is composed of three parts toolbar data tree view and information panel 4 QGIS Browser Ro Refresh EF Manage WMS V New Shapefile set layer CRS Toolbar tb Home Param Metadata Previe Attributes 3 Favourites p e Data Tree S F By Tabs Oracle postais f SpatiaLite ows amp wcs VO WES WMS 5 A Refreshing Look at QGIS The data tree view is an expandable tree listing of all geospatial data files on your computer and through connections The information display which takes mo
64. conversion may be desirable In the following example we are provided with a slope layer in a polygon format Here we will convert it to a line geometry so that it can be styled as contour lines All the attributes will be maintained during either conversion Conversions from line to polygon are necessary if the area needs to be calculated An example of this is parcel data from a CAD program that is provided to you as a line layer In a GIS environment parcel data is better represented as a polygon layer than as a line layer 110 Chapter 4 Polygon centroids Input polygon vector layer Neighborhoods pdx Output point shapefile 2ortand Output Chapter4 Neighborhood_centroids shp Add result to canvas l 0 Creating polygons surrounding individual points There are two tools for generating polygons around individual points in a layer Voronoi and Delaunay triangulation Voronoi polygons represent the area of influence around each point These are named after the Russian mathematician Georgy Voronoy who invented the algorithm They are also referred to as Thiessen polygons and are named after Alfred Thiessen who independently created the same algorithm You can use the Voronoi polygon in QGIS by navigating to Vector Geometry Tools Voronoi Polygons The resulting polygon represents the areas closer to the point used as the input than any other points in the layer The Delaunay triangulation tool can be found by navigatin
65. corridors Morato et al 2014 plan recreation and transportation networks Gurrutxaga and Saura 2014 and maximize safe backcountry travel in avalanche prone areas Balstrom 2002 to name just a few applications 235 The Processing Toolbox For more information please see the following references e Howey M 2007 Using multi criteria cost surface analysis to explore past regional landscapes a case study of ritual activity and social interaction in Michigan AD 1200 1600 Journal of Archaeological Science 34 11 1830 1846 e Morato R G Ferraz K B de Paula R C amp Campos C d 2014 Identification of Priority Conservation Areas and Potential Corridors for Jaguars in the Caatinga Biome Brazil Plos ONE 9 4 1 11 doi 10 1371 ournal pone 0092950 e Gurrutxaga M and Saura S 2014 Prioritizing highway defragmentation locations for restoring landscape connectivity Environmental Conservation 41 2 157 164 doi 10 1017 S0376892913000325 e Balstrom T 2002 On identifying the most time saving walking route in a trackless mountainous terrain 102 1 51 58 10 1080 00167223 2002 10649465 Evaluating a viewshed Another advanced spatial analysis technique involves evaluating viewsheds to address the intervisibility between features or the potential visual impact of vertical structures such as wind turbines and radio or cell towers This type of analysis is often incorporated into an environmental impact evalu
66. elevation layer to the park boundary so that we save processing time by only evaluating the viewshed within the park We can find this tool by typing clip in the search bar The following screenshot illustrates how to set the parameters for this tool Q Clip raster by mask layer Parameters Log Help Input layer Elevation EPSG 26710 Mask layer Boundary EPSG 26710 Nodata value leave as none to take the nodata value from input none Create and output alpha band Keep resolution of output raster Additional creation parameters optional Output layer C Mastering_OGIS GRASS View felev_clip tif x Open output file after running algorithm booo M O Run Close 237 The Processing Toolbox Calculating viewsheds for towers using r viewshed Once we have a clipped elevation layer we can set the transparency of the value 0 to 100 percent in the transparency tab of the Layer Properties interface and begin the process of calculating the viewshed using the r viewshed tool If you open this tool using the processing commander or double click on the entry within the toolbox you will be presented with a dialog box that contains the option to enter a coordinate identifying the viewing position However we have three towers of interest and although we could manually execute this tool three different times most of the algorithms in the toolbox have the option to execute as batch process By right clicking on the tool we can sele
67. entering text at the bottom right corner To edit an element select it in the Select element to edit box Once it is selected use the Element description box to type a description or necessary documentation Click on OK when finished 288 Chapter 8 Help editor Algorithm description This is a simple model that identifies riparian woodlands It uses streams and tree data as inputs The rivers are buffered and the buffer data is used to clip the trees to create the riparian trees data Input parameters Rivers Perennial streams of Alaska Buffer distance Distance in meters to butter streams Trees Polygonal tree data for Alaska Outputs Riparian Trees Trees within 100 meters of perennial streams Select element to edit Element description This is a simple model that identifies riparian El Input parameters woodlands It uses streams and tree data as inputs Rivers The rivers are buffered and the buffer data is used Buffer distance to dip the trees to create the riparian trees data Trees El Outputs gt Riparian Trees Algorithm created by Algorithm help written by Algorithm version 289 Automating Workflows with the Graphical Modeler This help information will then be available on the Help tab when the tool is in the execution mode Riparian trees Saving loading and exporting models Models can be saved anytime by clicking on the Save button in the Processing modeler window It is best to
68. file for polygons shapefile Name and extension of the output file that will hold the resulting polygons Note that dozens of common vector formats are supported so be sure to specify the extension for the format that you wish to use Field name The field name that will hold the cell values Use mask If selected the specified file will be used to mask the input Load into canvas when finished Loads the output file into the QGIS canvas The Polygonize tool is essentially a frontend for GDAL So at the bottom of the tool window the gdal_polygonize GDAL command is displayed and modified when the options are selected To manually change the GDAL command press the Edit button to make the GDAL command editable a The gdal_polygonize GDAL command produces polygon E features from raster data For more detailed information about Q this command and its parameters visit the documentation page athttp gdal org gdal_polygonize html There is a similar GRASS tool in the Processing Toolbox named r to vect that converts a raster to a polygon line or a point vector format Complete documentation for this tool can be found on the GRASS GIS help pages at http grass osgeo org grass65 manuals r to vect html Converting from vector to raster rasterize To convert a vector to a raster format QGIS provides the Rasterize tool This tool converts a shapefile to a raster and applies the values in a specified attribute field to the ce
69. geometries For this option you will also need to choose the field containing the WKT geometry definitions If the file does not contain any spatial information choose No geometry and the table will be loaded simply as a table 8 Additionally you can choose to enable the following options O O O Use spatial index Creates a spatial index Use subset index Creates a subset index Watch file This setting watches for changes to the file by other applications while QGIS is running 171 Advanced Data Creation and Editing 9 After you click on OK the Coordinate Reference System Selector dialog box will open Use this dialog box to identify the coordinate reference system of the data It is very important to correctly identify the CRS of the input data There is a setting that can affect the behavior of the Coordinate Reference System Selector for both new layers and layers that are loaded into QGIS without a defined CRS By navigating to RS Settings Options CRS you can choose how these situations are handled The choices are Prompt for CRS Use Project CRS or Use default CRS displayed below The default setting is Prompt for CRS However if you have this set to Use project CRS or Use default CRS displayed below then you will not be prompted to define the CRS as described earlier The following screenshot shows an example of a completed Create a Layer from a Delimited Text File tool re Create a Layer from a De
70. get a specific field using its index with the following code field 0 header 0 Usually it is more useful to manage all the column data as a list For this reason there is a specific method for example to iterate on all column fields use this snippet for column in header toList lt do something with the variable column that is a QgsField gt Each QgsField has its methods to obtain the column name with name get its type and typeName methods and also get its precision method if it is numeric In the following code we can explore the characteristics of the header of myvector header myVector pendingFields print How many columns header count print does the column 4 exist header exists 4 print does the column named value exist header indexFromName value print Column 0 has name header 0 name for column in header toList print name column name print type column typeName print precision column precision 347 PyQGIS Scripting This will produce the following output How many columns 3 does the column 4 exist False does the column named value exist 1 Column 0 has name cat name cat type Real precision 0 name NAME type String precision 0 name AREA MI type Real precision 15 Describing the rows Each row of myVector is a QgsFeature instance The feature contains all attribute values and a geometry There are also as many val
71. gt 313973000 000000 AND postesti lt 133 Refine current rules y Count features Rendering order The Rule based style properties window shows a list of current rules with the following columns e Label The symbol and label that will be visible in the Layers panel are displayed here The checkbox toggles rule activation unchecked rules will not be displayed e Rule This displays the filter applied to the vector dataset to select a subset of records e Min Scale This displays the smallest zoomed out scale at which the rule will be visible 85 Styling Raster and Vector Data Max Scale This displays the largest zoomed in scale at which the rule will be visible Count This displays the number of features that are included in this rule This is calculated when the Count features button is clicked Duplicate count This displays the number of features that are included in the current and other rules This is helpful when you are trying to achieve mutually exclusive rules and need to determine where duplicates exist This is calculated when the Count features button is clicked To add a new rule click on the Add rule button to open the Rule properties window To edit a rule select the rule and then click on the Edit rule button to open the Rule properties window To remove a rule select the rule and then click on the Remove rule button Additional scales categories and ranges can be
72. home page where you can leave usage instructions and the usages of the plugin We can leave this blank at the start Tags This field is really important to allow QGIS users to find the plugin It s used by the plugin manager to look for plugin keywords For example if the plugin is managing GPS data its tags could be gps gpx satellite and so on Try to find the tags that best describe the plugin and edit them separated with commas 314 Chapter 9 e The last checkbox of the Plugin Builder interface is checked if the plugin is in the experimental stage By default the Plugin Manager shows only plugins that are not experimental To list the experimental plugins it s necessary to tick the relative checkbox option in the Plugin Manager configuration During the first developmental stage of the plugin it s good practice to set it as experimental Generating the plugin code After setting all the necessary plugin parameters it s time to generate the code by clicking on the OK button which will open a path selection dialog which will select the location of the new plugin Selecting the same directory that contains the plugin Plugin Builder allows QGIS to find the new plugin The default path should be is lt your home path gt qgis2 python plugins However you can create your plugin anywhere Just remember to link or deploy it in the plugin directory to allow QGIS to load it After selecting a path the code will be generated
73. info craterlake data html e http www mrlc gov Performing raster analyses with GRASS The GRASS short form for Geographical Resources Analysis Support System environment represents one of the first available open source GIS options It has a long history of providing powerful geospatial tools that were often overlooked because the GRASS interface and data organization requirements weren t as intuitive as other often proprietary options The integration of GRASS algorithms within the processing toolbox provides access to these powerful tools within an intuitive GUl based interface 223 The Processing Toolbox To explore the types of GRASS algorithms available through the toolbox we will work through a series of hypothetical situations and perform the following analyses e Calculating a least cost path across a landscape e Evaluating a viewshed Please make sure that you have downloaded unzipped and added the necessary data to QGIS and set the project CRS value to EPSG 26710 We need to organize this data so that the elevation layer is at the bottom of the data layer panel as illustrated in the next screenshot ne rr nn ee re oe a 224 Chapter 7 The ZIP folder contains the following files e Elevation file dems _10m dem e Boundary file crlabndyp shp e Surface water file hydp shp e Land use file lulc_clnp tif e Search and rescue office file Start shp Injured hiker file End shp
74. is automatically installed with QGIS and enabled by default The tools can be found in the Geoprocessing Tools menu under Vector The icons next to each tool in the menu give a good indication of what each tool does We will look at some commonly used spatial overlay tools such as clip buffer and dissolve In the case of a simple analysis these tools may serve to gather all the information that you need In more complex scenarios they may be part of a larger workflow Vector Raster Database Web Processing Help OpenStreetMap 4 EJ Analysis Tools pu la Research Tools me cabel cal ex Hull s Geometry Tools Buffer s a Data Management Tools Sey Geoprocessing Tools Conv ma Intersect gt Union Symetrical Difference i Difference 2 Dissolve Ic Eliminate Sliver Polygons 118 Chapter 4 The tools covered in this chapter are also available via the Processing plugin which is installed by default with QGIS Desktop When enabled this plugin turns on the Processing menu from which you can open the Processing Toolbox The toolbox is a panel that docks to the right side of QGIS Desktop The tools are organized in a hierarchical fashion The toolbox contains tools from different software components of QGIS such as GRASS the Orfeo toolbox SAGA and GDAL OGER as well as the core QGIS tools covered in this chapter Some tool
75. it is more complete and is the simplest to use Its version should be greater than version 2 0 3 To find the experimental version of the Plugin Builder in Plugin Manager it s necessary to configure the manager to show the experimental plugin by ticking the checkbox when you navigate to Settings Show also experimental plugins Locating plugins The Plugin Builder as every third party Python plugin is by default is installed in your home directory at the following path lt your home path gt qgis2 python plugins 311 Creating QGIS Plugins with PyQGIS and Problem Solving Over here you ll find your Plugin Builder code at the pluginbuilder directory You will notice that each installed plugin has a proper code directory We ll create a new plugin that to be loaded by default by QGIS has to be created in the Python plugin directory It s possible to change the default plugin directory path but this is outside the scope of this topic Creating my first Python plugin TestPlugin Starting the Plugin Builder will open a GUI to insert the basic parameters to set up the generation of your first QGIS plugin The interface is shown in the following screenshot QGIS Plugin Builder Class name e g PhotoLinke Plugin name e g Photo Linke Description a g This plugin Module name Version number Winimum QGIS version Text for the menu item Author Company Email address Optional Items Bug tracker H
76. mailman listinfo qgis user These lists can be read and searched for using an online service at http osgeo org 1560 x6 nabble com Quantum GIS 4099105 html where you can also find lists of other QGIS sub projects or the mailing lists of several other local QGIS user groups 303 Creating QGIS Plugins with PyQGIS and Problem Solving If you don t find your nearest local QGIS user group first ask in the user or developer mailing lists and depending on the answer try to create a new QGIS local group and announce it to the community IRC channel Internet Relay Chat IRC is a fantastic way to get real time support from users and developers Remember that this help is always voluntary and the answers depend on your politeness and the available time of the connected users You can connect to the qgis channel at the http www freenode net server with your preferred IRC client via http webchat freenode net channels qgis The philosophy of IRC problem solving is condensed in the first chat message sent to you by the qgis channel it will be Don t ask to ask just ask and hang around a while to see if someone answers Please refer to http osgeo pastebin com instead of pasting more than five lines The StackExchange community Technical social networks like StackExchange have a GIS sub project that can be accessed from http gis stackexchange com Here you can look for problems reported by other users about QGIS and
77. matplotlib and QGIS are available at http www kyngchaos com software qgis Installing QGIS on Ubuntu Linux There are two options when installing QGIS on Ubuntu installing QGIS only or installing QGIS as well as other FOSSGIS packages Either of these methods requires the use of the command line sudo rights and the apt get package manager Installing QGIS only Depending on whether you want to install a stable release or an experimental release you will need to add the appropriate repository to the etc apt sources list file With sudo access edit etc apt sources 1list and add the following line to install the current stable release or current release s source code respectively deb http qgis org debian trusty main deb src http qgis org debian trusty main 2 Chapter 1 Depending on the release version of Ubuntu you are using you will need to specify the release name as trusty saucy or precise For the latest list of QGIS releases for Ubuntu versions visit download qgis org With the appropriate repository added you can proceed with the QGIS installation by running the following commands sudo apt get update sudo apt get install qgis python qgis To install the GRASS plugin recommended install the optional package by running this command sudo apt get install qgis plugin grass Installing QGIS and other FOSSGIS Packages The ubuntugis project installs QGIS and other FOSSGIS packages such as GRASS on Ub
78. n 10 localhoste There are different ways to run a system command in Python The preceding method using the subprocess module allows for a non blocking run and interaction with the program using the stdin pipe 366 Chapter 10 After creating a subprocess Popen pipe the code snippet starts a for loop to read the standard output of the stdout program printing the messages in the console J If your command is stuck in the QGIS interface try to wrap it in a Python script and run the wrapper with python i lt your Q command wrapper gt where your command wrapper could be simply a one line code like import os os system lt your command gt Run the runping script in the processing toolbox GUI by double clicking on it Its execution will produce the following screenshot lugins Vector Raster Database Web Pi Parameters Log Help PPPN Jd Jd me i Algorithm runping starting Python Console Python Console 2Use iface to access QGIS API int 3PING localhost 127 0 0 1 56 84 4 564 bytes from localhost 127 0 0 bytes from localhost 127 0 0 bytes from localhost 127 0 0 bytes from localhost 127 0 0 Processing algorithm a a a bytes from localhost bytes from localhost 127 0 0 bytes from localhost 127 0 0 1 1cmp_req 7 ttl 64 time 0 046 ms Se E runthreademptyloop k gt Utils gt Vector b gt Vector laver Advanced interface This shows progres
79. number of the plugin It can be any number Generally the versioning has this format lt MAJOR gt lt MINOR gt lt PATC H gt where A MAJOR version will specify that there are incompatible API changes from the previous majors A MINOR version will specify that there are new functionalities in a backwards compatible manner A PATCH version will specify that there are backwards compatible bug fixes At the moment we can leave the default value set at 0 1 Minimum QGIS version This refers to the minimum QGIS version in which the plugin will run Each QGIS version has its own API set the plugin can be compatible with a specified newer version but not with older ones if it s not programmed to be compatible The minimum QGIS version is used by the QGIS plugin manager to show only plugins that are compatible with the running QGIS This means that in QGIS 2 0 it s not possible to see the plugin for 1 8 or plugins that are designed to work only with 2 4 or newer versions We can leave the default value 2 0 313 Creating QGIS Plugins with PyQGIS and Problem Solving Text for the menu item This refers to the text of the submenu opened under the voice Plugin Name described previously We can insert the Test Plugin starter value Author Company and Email address The parameters are obvious and are used to contact the developer if a user finds problems in the plugin For example you can set your name surname or company name
80. open a connection information window Depending on the database type you are connecting to different connection options will be shown Once you have created a database connection select it from the drop down list and click on Connect you will see a list of all layers contained within the database display If there are a large number of tables you can select Search options and perform a search on the database To load a layer select it in the list and click on Add If you only wish to load a portion of the layer select the layer and then click on Set Filter to open the query builder If you set a query and then add the layer only the filtered features will be added Web services QGIS supports the loading of OGC compliant web services such as WMS WMTS WCS and WFS Loading a web service is similar to loading a database service In general you will create a new server connection connect to the server to list the available services and add the service to the QGIS project 9 A Refreshing Look at QGIS Working with coordinate reference systems When working with spatial data it is important that a coordinate reference system CRS is assigned to the data and the QGIS project To view the CRS for the QGIS project click on Project Properties under Project and choose the CRS tab It is recommended that all data added to a QGIS project be projected into the same CRS as the QGIS project However if this is not possible or convenient QG
81. output raster size and resolution to match the existing output file Raster size in pixels This allows manual designation of raster width and height in pixels 163 Preparing Raster Data for Processing e Raster resolution in map units per pixel This allows manual designation of raster width and height in the units of the map e Load into canvas when finished Loads the output file in to the OGIS canvas The Rasterize tool is essentially a frontend for GDAL So at the bottom of the tool window the gdal_rasterize GDAL command is displayed and modified when the options are selected To manually change the GDAL command press the Edit button 7 to make the GDAL command editable a The gdal_rasterize GDAL command burns vector geometries E into the raster band s of a raster For more detailed information Q about this command and its parameters visit the documentation page athttp gdal org gdal_rasterize html There are two similar GRASS tools in the Processing Toolbox named v to rast attribute andv to rast value that convert a vector to the raster format The v to rast attribute tool assigns attribute values to the output raster cells fs Thev to rast value tool assigns a single value or calculated value to the output raster cells Complete documentation for these tools can be found on the GRASS GIS help pages at http grass osgeo org grass71 manuals v to rast html Creating raster surfaces via interp
82. parameter does not require a value to be set then it is treated as a flag As an example the following figure displays a Proj 4 definition for the USA Eckert IV CRS Notice that this CRS has seven parameters each parameter is prefaced with a character Also notice that six of the parameters have associated values and one parameter is a flag Each value is set after the character USA Eckert IV proj eck4 Hon_0 96 0 x_0 0 y_0 0 datum W6584 units m 4 no_defs 127 Preparing Vector Data for Processing The parameters displayed in the preceding figure show only subset parameters that Proj 4 contains If a CRS does not use a parameter it is simply omitted from the parameter line The following is a list and discussion of common parameters used when defining a CRS Projection proj This is always required It is the name of the cartographic projection to use The value provided is an abbreviated name of a supported projection Spheroid e11ps This is a model of the earth s shape that is used in transforming a projection The reference spheroid or ellipsoid is generated by choosing the lengths of the major and minor axes that best fit those of the real earth Many such models are appropriate for different locations on earth Datum datum This is the name of the spheroid to use Central meridian 1on_0 This is the longitude on which a map is centered x origin Latitude of origin 1at_0 This is the lati
83. parameters visit the Proj 4 project website at an https trac osgeo org proj Defining a new custom coordinate reference system There are two methods for creating a custom CRS write a Proj 4 definition from scratch or copy the Proj 4 definition from an existing CRS and modify it No matter which creation method you choose both are completed using the Custom Coordinate Reference System Definition window The following figure shows the New England shp sample data in its unprojected WGS 1984 form In this section we will create a custom CRS to display the New England states using an equal area map projection 129 Preparing Vector Data for Processing Open the Custom Coordinate Reference System Definition window by clicking on Custom CRS under Settings This window has two parts Define and Test We will not use discuss the Test part but instead we will focus on the Define part of the window to create our new CRS We will modify the USA_Contiguous_ Albers _ Equal _Area_Conic EPSG 102003 projection so that it focuses on New England 1 2 Click on the Add new CRS button to create a blank CRS entry Set the name of the new CRS to New England Albers Equal Area Conic At this point we have two options we can write the Proj 4 projection from scratch in the Parameters textbox or we can copy an existing CRS Proj 4 string from a projection that closely matches what we want and then modify it to our needs Let s elect to copy an e
84. perform the following steps 1 Open the attribute table of Neighborhoods _ pdx Click on the Select features using an expression button to open the Select by expression window This window is a subset of the Field calculator window that was explained in the Exploring the field calculator interface section earlier in this chapter If you are unfamiliar with the interface review the aforementioned section 140 Chapter 4 In Function list expand the Fields and Values node and double click on COALIT to add it to the Expression area In the Operators area click on the Equal operator button or press on your keyboard to add it to the Expression area With COALIT still selected in Function list click on the all unique button to load a list of all unique values found in the field Double click on SEUL to add it to the Expression area after the equal sign As we want to choose either SEUL or SWNI coalition neighborhoods we will use the Boolean OR operator to connect the two expressions together In Function list expand the Operators node and double click on OR to add it to the end of the expression Using what you learned so far add COALIT SWNI to the end of the expression The completed expression is shown in the following screenshot Expression COALIT SEUL OR COALIT SWHI Click on Select to perform the aspatial selection Close the Select by expression window and the attribute table
85. plugin using Plugin Builder e Analyze your first basic plugin to know its structure e Setting up a runtime debugging environment that can be useful in developing complex plugins Webography where to get API information and PyQGIS help One of the characteristics of most free software projects is that their documentation is freely available and can be used for learning QGIS is one of the best documented projects thanks to training material a coding cookbook and the automatic documentation of its Application Programming Interfaces APIs In this chapter we will focus on main resources that are available on the web to learn how to script QGIS and how to solve your scripting problems 301 Creating QGIS Plugins with PyQGIS and Problem Solving PyQGIS cookbook The main resource is a community content driven cookbook that gives a general introduction to scripting QGIS You can find this documentation at http www qgis org en docs index html You have to choose the QGIS target version of your plugin and then choose the PyQGIS Developer Cookbook link If you are interested in the latest QGIS APIs you have to choose the testing version or directly go to http docs qgis org testing en docs pyqgis developer cookbook If you need a copy on your system you can also download a PDF version of the cookbook from http docs qgis org lt qgis version or testing gt pdf For example if you need the documentation for the version 2
86. save early and often when working on a model As Parameters Log Help Algorithm description This is a simple model that identifies riparian woodlands It uses streams and tree data as imputs eet are buffered and the buffer data is used to clip the trees to create the riparian trees data Input parameters Rivers Perennial streams of Alaska we mentioned in the Configuring the modeler and naming a model section of this chapter the first time a model is saved you will be prompted to name the model file Subsequent saves update the existing model file There is also a Save as button that can be used to save a new version of a model Models that are not saved to the Modeler folder can be opened using either of the following two ways By enabling the Processing Toolbox panel navigating to Models Tools and then double clicking on Add model from file By using the Processing modeler window and clicking on the Open model button In either case navigate to the model file 290 Chapter 8 Models can also be exported as image files This is useful if the workflow needs to be presented or included in a report To export a model click on the Export as image button amp 4 in the Processing modeler window The model will be saved as a PNG file QGIS now has an online collection of models and scripts that can be loaded From the Processing Toolbox panel navigate to Models Tools and double
87. set of classes that are used to represent every piece of the vector from the header to the single attribute We can think of a vector as a table with rows and columns and one header that describes each column of the table Each row has its own geometry and attributes that is archived in the columns of the row Each vector could contain only a geometry type For example it can be composed of only points lines polygons or collections of these geometry types The structure of the classes involved in a vector table is shown in the following picture Head aed QgsField or QgsField QgsAttributes 0 na QgsAttributes n QgsGeometry Row fealureCount 1 FogSAmmONESO P QasAttributesin iggscssmeny A description of these classes is the subject of the following paragraphs 346 Chapter 10 Describing the header The container of the header information is the QgsFields class that contains methods to point to every column description A column description is abstracted by the QgsField class To get the header container for myVector we use the following code header myVector pendingFields Here header will store a QgsFields instance The pendingFields method is named pending because it always refers to the current state of the vector This is also the case for editing the vector and adding or removing columns We can use QgsFields methods to add or remove columns get the column index by name with indexFromName or
88. showCoordinates map canvas callback iface mapCanvas xyCoordinates disconnect showCoordinates reset to the previous mapTool iface mapCanvas setMapTool previousMapTool clean remove myMapTool and relative handlers myMapTool deleteLater remove the rubber band from the canvas iface mapCanvas scene removeItem myRubberBand myMapTool canvasClicked connect manageClick iface mapCanvas setMapTool myMapTool 8375 PyQGIS Scripting Executing the preceding code and clicking on the canvas with the left mouse button will produce results similar to the following screenshot Python Console 70 if c lickedButton Qt LeftButton myRubberBand addPoint currentPos terminate rubber band editing session if c JO aa 1 32 gt gt gt myMapToo 33 gt gt gt 1face ma 34 eo lickedButton Qt RightButton remove showCoordinates map canvas callback iface mapCanvas xyCoordinates disconnect showCoordinates reset to the previous mapTool 1face mapCanvas setMapTool previousMapTool clean remove myMapTool and relative handlers myMapTool deleteLater remove the rubber band from the canvas 1face mapCanvas scene removeltem myRubberBand L canvasClicked connect manageClick pCanvas setMapTooLl myMapTool The image in the preceding screenshot has the alaska shp layer as base map to show the evidence of the transparency of the drawn rubber band Summary This chapte
89. sqlite in the Tree panel 2 Navigate to Table Create Table to open the Create Table window Enter Peaks as the table name Click on the Add field button to add a new table field A new row will appear in the field list Set the Name field to Name and the Type field to character 20 from the list of field type options 41 www allitebooks com Creating Spatial Databases 5 Click on the Add field button to add a second field with the Name field set to Elevation and the Type field set to integer 6 Set the Primary key field to Name This will require the peak names to be unique 7 Select Create geometry column and choose the following options Create geometry column POINT O Name geom Dimensions 2 SRID 26910 Select Create spatial index to create a spatial index for the table Your dialog should look like the following screenshot If it does click on the Create button to create the new table Create Table schema Name Peaks E Name Type Null Add field held Name character 20 Delete F Delet 2 Elevation integer Primary key Name X Create geometry column POINT Name geom Dimensions 2 SRID 26910 w Create spatial index 10 If the table is created successfully a prompt will confirm that everything went fine Dismiss the dialog then click on the Close button to close the Create Table window 42 Chapter 2 11 To view the new Peaks tabl
90. still get the error identify the installation path for SAGA In QGIS navigate to Settings Options System and make sure that the PATH variable includes the value for the path to the SAGA binaries This can be found under Current environment variables as shown in the following screenshot Options System pe General v Current environment variables read only bold indicates modified at startup Variable Value GNOME_DESKTOP_SESSION_ID this is deprecated PATH usr local sbin usr local bin usr sbin usr Rendering GDM_LANG en_US XDG_SESSION_PATH org freedesktop DisplayManager SessionO f Data Sources Colors Canvas amp Show only QGIS specific variables Legend Map Tools Cancel Composer 154 Chapter 5 Rescaling rasters When an analysis calls for multiple rasters to be combined mathematically it is often desirable to have the values in each raster converted to a common scale For example in a site selection analysis you need the data values for the input rasters to be scaled from 0 to 100 This can be done with the advanced interface by navigating to Processing Toolbox GRASS commands Raster r rescale tool In the following example the RiparianSurface img raster with values ranging from 10 to 95 5 will be rescaled to a raster with values ranging from 0 to 100 To do this perform the following steps 1 Select the input raster layer by clicking on the down arrow to choose a raster lo
91. that this i chapter will cover is the relational database 27 Creating Spatial Databases Database tables A relational database stores data in tables A table is composed of rows and columns where each row is a single data record and each column stores a field value associated with each record A table can have any number of records however each field is uniquely named and stores a specific type of data A data type restricts the information that can be stored in a field and it is very important that an appropriate data type and its associated parameters be selected for each field in a table The common data types are as follows e Integer e Float Real Decimal e Text e Date Each of these data types can have additional constraints set such as setting a default value restricting the field size or prohibiting null values In addition to the common data types mentioned previously some databases support the geometry field type allowing the following geometry types to be stored e Point e Multi point e Line e Multi line e Polygon e Multi polygon The multi point line polygon types store multi part geometries so that one record has multiple geometry parts associated with it A ESRI shapefiles store geospatial data in multi type geometry Q so using multi type geometry is a good practice if you plan on converting between formats 28 Chapter 2 Table relationships A table relationship connects rec
92. the QGIS code and can be found at http qgis org api where you can look for the class that you need For example if you need to know public methods of the QgsVectorLayer class in QGIS 2 4 you can refer to http qgis org api 2 4 classQgsVectorLayer html or for the latest development version http qgis org api classQgsVectorLayer html It should be mentioned that QGIS is mainly written in C and its API documentation follows C notation Most of the methods of the QGIS classes are available as a Python bind The way to discover if a method is exported to Python is to test it in the QGIS Python Console or read about its class documentation in the QGIS Python Console as described in the next chapter The QGIS community mailing lists and IRC channel One of the advantages of the open source project is that you can talk directly to other developers and frequently with the core developers of the project QGIS has three official ways to support development and problem resolution Mailing lists All official mailing lists are listed at http qgis org en site getinvolved mailinglists html There are two extremely important lists from the user s point of view one for developers QGIS core and plugin developers and the other for users Depending on your profile choose one list e Developer list For this refer to http lists osgeo org mailman listinfo qgis developer e User list For this refer to http lists osgeo org
93. the answers that are given by other users or directly by QGIS core developers Messages relating to QGIS can be found looking for the qgis tag for example http gis stackexchange com tags qgis Sharing your knowledge and reporting issues In StackExchange the IRC channel and the mailing lists you can actively support other users who have problems that you have have already solved An important way to support a QGIS project other than funding it is by reporting bugs that are packaged with a detailed use case and data that allows others to replicate the problem This will speed up bug fixing There are two kinds of issues those that are related to QGIS or its core plugins such as Processing and those that are related to third party plugins that you can install with Plugin Manager 304 Chapter 9 To report a QGIS issue or a core plugin issue you need an OSGeo account created at https www osgeo org cgi bin ldap_create_user py using which you can log in to the QGIS Redmine bug tracker to report issues at http hub qgis org projects quantum gis issues al Beware Check to make sure your issue hasn t already been reported by looking in the issues list A good guide to reporting a QGIS issue is available at http qgis org en site getinvolved development index html bugs features and issues Reporting a third party plugin issue depends on the plugin developer and where he she decides to host the bug tracker Y
94. to every record in the vector dataset This vector style is best when you want a uniform look for a map layer such as when you style lake polygons or airport points The following screenshot shows the Single Symbol style type with default parameters for point vector data Its properties will be very similar to line and polygon vector data Single Symbol Parameters p rev i ew Unit Millimeter BW cie 200000 2 O Transparency 0 aoe AN Color B o Symbols in group Open Library Simple marker ae i 7 Symbol pui ci ibra r Symbols Component Tree city diamond ellipse pentagon o Ww ennuara etar cetar etar2 Advanced Let s take a quick tour of the four parts of the properties window for the Single Symbol style type that is shown in the previous screenshot e Symbol preview in the upper left corner shows a preview of a symbol with the current parameters 80 Chapter 3 e Symbol parameters in the upper right corner has the parameters for the symbol selected in the symbol component tree these will change slightly depending on geometry type of the vector data e Library symbols in the bottom right corner lists a group of symbols from the library which is also known as Style Manager Clicking on a symbol sets it as the current symbol design If symbol groups exist they can be selected for viewing in the Symbols in group drop down menu To open the Style Manager click on Open Library
95. to view the results on the map The following figure shows the selected neighborhoods Preparing Vector Data for Processing With the two coalitions selected we will next perform a spatial selection to determine which schools are within the selected neighborhoods To do this perform the following steps 1 Navigate to Vector Spatial Query Spatial Query from the QGIS menu bar to open the Spatial Query window This window allows features from one layer to be selected based on their spatial relationship with the features in a different layer Depending on the geometry of the two layers the spatial relationships will change to match the appropriate selections that are available In the Select source features from section choose schools Set Where the feature to Within from the drop down box In the Reference features of section choose Neighborhoods_pdx Keep 46 selected geometries selected With this selected only the currently selected neighborhoods will be used for the spatial query For the And use the result to field choose Create new selection Click on Apply to execute the query Once complete the Spatial Query window will expand to display the results The following screenshot shows the expanded Spatial Query window You can select individual result feature IDs and select Zoom to item to center the map on the selected item Note that the window still contains the original spatial query so it is possible to modify
96. tool Geocode from Street Layer Input CSV File UTF 8 BD PACKT Data Chapter 6 Addresses csv Street Name Field STREET street Layer Streets Street Name Attribute STREETNAME Bldg Setback map units 20 Output Shapefile r Number Field NUMBER From X Attribute street line start To X Attribute street line end Left From Number LEFTLOW Left To Number LEFTHIGH Left ZIF optional none BD PACKT Data Chapter 6 StreetAddresses shp Not Found Output List D PACKT Data Chapter 6 Streets_notfound csyv OK 182 From Attribute street line start To Y Attribute street line end Right From Number RIGHTLOW Right To Number RIGHTHIGH Right ZIP optional none Browse Browse rit Chapter 6 Geocoding is often an iterative process After the initial geocoding operation review the Not Found CSV file If it s empty then all the records were matched If it has records in it compare them with the attributes of the streets layer This will help you determine why the records were not mapped It may be due to inconsistencies in the spellings of street names It may also be due to a street centerline layer that is not as current as the addresses Once the errors have been identified they can be addressed by editing the data or obtaining a different street centerline dataset The geocoding operation can be rerun on the unmatched addresses This process can be repeat
97. visualize query and edit GIS data using the ArcGIS JavaScript APIs 3 Step by step tutorial that teaches you how to design and customize a GIS web application from scratch Please check www PacktPub com for information on our titles
98. when the model is run Parent algorithms is an additional parameter found only in tools that are run from graphical modeler It allows you to define the execution order of algorithms Setting an algorithm as a parent forces the graphical modeler to execute this parent algorithm before the current algorithm can be run When you set the output of one algorithm as the input for the next one you automatically sets the first algorithm as the parent However in complex models there may be several branches and it may be necessary for an operation in a separate branch of the model to be completed before another operation can run 281 Automating Workflows with the Graphical Modeler For this example we will be buffering streams by 100 meters and then clipping trees by that buffer layer The first algorithm that we will add is Fixed distance buffer Double click on the tool from the Algorithm tab and the tool dialog will open The tool will be filled like the following screenshot Notice that instead of setting an explicit buffer distance the Buffer distance input is being used Also note that no output is named since this output will be considered as an intermediate dataset Fixed distance buffer Parameters Help Description Fixed distance buffer Input layer Rivers Distance Buffer distance Segments Ot Dissolve result Yes Buffer lt OutputVector gt Enter name if this is a final result Parent algorithms 0 elements
99. with the dire need for professional training this book has been created to provide you with the concise technical expertise that will serve you well both now and in future versions of this powerful GIS software I have enjoyed the privilege of working closely with the contributing authors of this book for the past 2 years We have been engaged in an intense curriculum development process to create the first ever national GIS curriculum that is based around a national standard the U S Department of Labor s Geospatial Technology Competency Model GTCM This effort has resulted in a series of GIS courses all based around QGIS that provide a solid foundation upon which this book can be used to enhance your technical skills Each of the contributing authors is a very experienced GIS professional and many of them serve as instructors for highly respected academic GIS programs Dr Richard Rick Smith a certified GIS Professional GISP serves as an assistant professor for the Geographic Information Science program at Texas A amp M University Corpus Christi Texas USA Rick has been onboard the curriculum effort since day one where his expertise in GIS and cartography is highlighted Kurt Menke is a certified GIS Professional GISP and operates his own GIS consulting business Bird s Eye View GIS in New Mexico USA where he teaches open source GIS software at the local college and universities Kurt is well respected in both conservation
100. 0 rasters and vector data models converting 161 color rendering 76 78 converting to vector 161 floating point and integer rasters converting 148 layer visualizing 341 managing 339 QOgsRasterLayer exploring 340 reclassifying 145 147 resampling 79 149 152 rescaling 155 SAGA installing on different platforms 152 154 SAGA installing on Linux 153 SAGA installing on Mac OS X 153 SAGA installing on Windows 153 SAGA troubleshooting on different platforms 152 154 SAGA troubleshooting on Linux 153 154 SAGA troubleshooting on Mac OS X 153 SAGA troubleshooting on Windows 153 single band rasters styling 65 vector converting to 162 164 raster surfaces creating via interpolation 164 166 r cost tool used for calculating cumulative cost raster 233 Reclassify grid values tool used for reclassifying land use 251 Remove item button 59 Remove rule button 86 resampling method about 160 Average 79 Bilinear 79 Cubic 80 193 Cubic Spline 193 Lanczos 193 Linear 193 Nearest neighbour 193 resources qrc file URL 318 r mapcalculator used for combining viewsheds 240 242 root mean square error RMSE 195 r reclass about 147 URL 147 r slope tool used for calculating slope 229 r stats used for calculating raster statistics 242 244 rule based vector style 85 87 run function 322 385 r viewshed used for calculating viewsheds for towers 238 239 S SAGA about 244 habitat evaluating 245 installing on Linux 1
101. 01 102 Symmetrical Difference tool 121 122 System for Automated Geoscientific Automation See SAGA T table creating 41 42 deleting 44 emptying 45 exporting as shapefile 40 importing into SpatiaLite database 36 40 managing 41 properties editing 43 44 renaming 43 386 tables joins 12 14 working with 11 TauDEM about 215 pits removing 255 URL 222 used for exploring hydrologic analyses 254 261 Terrain Analysis Using Digital Elevation Models See TauDEM TestPlugin basic logic adding 319 breakpoints adding 333 334 creating 312 customizing 319 debugging 330 335 GUI logic modifying 321 icon resource compiling 315 316 layout modifying with Qt Designer 319 320 mandatory plugin parameters setting 313 314 optional plugin parameters setting 314 315 plugin code generating 315 plugin logic modifying 322 pull down menus adding 321 PyDev project creating 331 332 text chart diagram creating 98 99 thematic data displaying with diagrams 93 Topographic Wetness Index TWI 295 topological editing topological errors repairing via 207 topological errors duplicate geometries resolving 208 gap between polygons repairing 213 214 overlaps repairing 208 repairing via topological editing 207 214 Topology Checker installing 200 using 203 206 Triangular Interpolation TIN 164 U Ubuntu Linux QGIS installing on 2 Union tool used for overlaying polygon layers 123 V vector converting to raste
102. 1 be Moderate Potential 2 Good Potential 3 E 3 i Elevation 253 The Processing Toolbox Hopefully it is clear that this is a very simple model with many assumptions that any ornithologist who actually studies the Northern Spotted Owl would not actually use to evaluate habitat However the various tools and general approach that has been taken to evaluate this hypothetical scenario could be applied by paying more rigorous attention to the underlying assumptions about the variables that influence potential habitat The goals of working through this type of analysis were threefold to showcase a variety of useful SAGA algorithms to demonstrate that there are similar tools with subtle differences that are available through the toolbox and to illustrate how easy it is to switch between native QGIS tools and those found in the toolbox Exploring hydrologic analyses with TauDEM The TauDEM short form for Terrain Analysis Using Digital Elevation Models environment contains a suite of tools with a specific emphasis on hydrologic and surface flow analysis GRASS and SAGA also contain some algorithms that calculate similar parameters but TauDEM has a comprehensive suite of tools in a single location We are going to continue using data from the provided ZIP file and we will need the following files e Elevation file dems_10m dem available in the cRass data folder e Gauge shapefile gauge shp e Rivers file hyd sh
103. 1582 43105 0 03638900 3 81 19400 4 29708100 15 1785739 653 1000 2 00000000 q hd Show All Features v When you select the last record in the attribute table it will be highlighted Editing using QgsVectorLayer Editing using QgsVectorLayer gives you much more power to interact with the QGIS interface and to control the editing flow The QgsVectorLayer methods to modify the attributes or geometry of a feature are slightly different than the method used in QgsVectorDataProvider but the main characteristic is that all these methods work only if the layer is in the editing mode otherwise they return False to notify failure An editing session on the myVector vector follows the steps described in the following pseudocode myVector startEditing lt do vector modifications saved in myVector editBuffer gt if lt all ok gt myVector commitChanges else myVector rollback Each step generates events that can be cached if they are useful for our processing scopes The list of generated events can be read in the QgsVectorLayer documentation For example startEditing will generate the editingStarted event adding a feature will generate the featureAdded event committing changes will emit the beforeCommitChanges event before applying changes and then the editingStopped event will be generated eS YY PyQGIS Scripting A useful exercise for you is to create a simple script to conne
104. 170 Chapter 6 4 Use the File format radio boxes to specify the format of the delimited text file You will see how QGIS is parsing the file by the example at the bottom of the Create a Layer from a Delimited Text File window The following are the options for File format O O Choose CSV if it is a standard comma delimited file Custom delimiters can be checked to identify the specific character used The choices are Comma Tab Space Colon Semicolon or Other delimiters Choose the Regular expression delimiter option if you wish to enter the regular expression for the delimiter For example t is the regular expression for the tab character 5 The Record options section allows you to specify the number of header lines to discard In most cases this option will be set to First record has field names 6 The Field options option allows you to control some field parameters O Check Trim fields if you need to trim leading or trailing spaces from your data Check Discard empty fields to prevent empty fields from being put into the output If commas are also the separators for decimal place values check Decimal separator is a comma 7 Once the file has been parsed choose an appropriate value from the Geometry definition option O If your file contains x and y coordinates choose Point coordinates and identify the fields containing the x and y coordinates Choose Well known text WKT if your file contains WKT
105. 2 you have to direct your browser to http docs qgis org 2 2 pdf However if you need documentation of a version under development you can visit http docs qgis org testing pdf Once you open the PDF documentation page you can choose your preferred translation _ QGIS even versions for example 2 2 2 4 2 6 and so CaN on are always stable versions Odd versions are always y developing versions for example 2 3 or 2 5 Odd versions are generally known as testing versions API documentation APIs are the doors that use the components of a software program As a program evolves the APIs can change affecting in our case all the plugins that directly use them The biggest QGIS API changes append jumping from version 1 8 to 2 0 but moving from one version to another there are new APIs added and others are deprecated because of the normal development life cycle of a complex software Generally deprecation and new APIs are added to satisfy new features for example multithreading visualization added in version 2 4 or the newer legend engine added in version 2 6 or due to code refactoring refer to http en wikipedia org wiki Code _ refactoring 302 Chapter 9 So depending on API changes and on what QGIS version you want to integrate your code into you should use one API set or another or better write code that can be executed in multiple QGIS versions API documentation is automatically generated from
106. 4 Preparing Vector Data for Processing covers techniques useful for turning raw vector data into a more usable form The chapter will start with data massaging and modification techniques such as merging creating indices checking for geometry errors and basic geoprocessing tools Next advanced field calculations will be covered followed by complex spatial and aspatial queries The chapter will end by defining new or editing existing coordinate reference systems Chapter 5 Preparing Raster Data for Processing covers the preparation of raster data for further processing using the GDAL menu tools and the Processing Toolbox algorithms Specifically these include reclassification resampling rescaling mosaics generating pyramids and interpolation The chapter will conclude by converting raster to vector Chapter 6 Advanced Data Creation and Editing provides advanced ways to create vector data As there is a great deal of data in tabular format this chapter will cover mapping coordinates and addresses from tables Next georeferencing of imagery into a target coordinate reference system will be covered The final portion of the chapter will cover testing topological relationships in vector data and correcting any errors via topological editing Chapter 7 The Processing Toolbox begins with an explanation and exploration of the QGIS Processing Toolbox Various algorithms and tools available in the toolbox will be used to complete common s
107. 53 154 installing on Mac OS X 153 installing on Windows 153 troubleshooting on different platforms 152 154 troubleshooting on Linux 154 troubleshooting on Mac OS X 153 troubleshooting on Windows 153 URL 222 SAGA Raster calculator used for calculating elevation ranges 245 used for combining raster layers 252 253 used for querying land use 247 SAGA Reclassify grid values tool using 251 second dataset used for georeferencing 188 settrace method 330 shaded relief calculating 225 227 URL 227 shapefile importing into SpatiaLite database 35 36 merging 106 tables exporting as 40 41 singleband gray band renderer 67 69 singleband pseudocolor band renderer 70 73 single band rasters paletted raster band renderer 66 singleband gray band renderer 67 69 singleband pseudocolor band renderer 70 73 styling 65 single symbol vector style 80 81 Solar Position Calculator URL 227 spatial database creating 30 32 spatial indices creating 107 SpatiaLite database data importing 33 KML importing 33 34 shapefile importing 35 36 tables importing 36 40 spatial overlay tools about 119 Clip tool 120 Difference tool 120 Intersect tool 121 122 Symmetrical Difference tool 121 122 Union tool 123 spatial view creating 46 48 dropping 48 SQL about 29 30 query creating 45 46 URL 45 StackExchange community about 304 URL 304 Structured Query Language See SQL styles loading 101 102 restoring 102 saving 101 102 setting 1
108. 6 S 57 Base file 17 Keyhole Markup Language As you can see these are the same parameters that were accepted in the algorithm GUI There is also the list of format accepted values Running algorithms from the console Running a processing algorithm can be done by using the runalg method or the runandload method The first one generates the output without visualizing it and is useful when you want to generate temporary data The second method loads and visualizes the output layer in QGIS The parameters accepted in this method are the processing command name strings followed by all the parameters depending on the command used We will perform an exercise and convert the alaska shp shapefile that is available in qgis_ sample data shapefiles into the SpatiaLite format This format is a very powerful SQLite spatial database that creates a self contained spatial database in a single file The SQLite format is referred to with the value 3 as specified in the algorithm help shown before To export algorithms into this format we ll follow these steps e Load the Alaska shapefile in QGIS e Run the convert algorithms e Load the result in QGIS Assuming that the alaska shp shapefile has been loaded in QGIS we will run the algorithm based on the parameters shown in the previous section this is done using the following snippet processing runalg gdalogr ogr2o0gr alaska 3 None F temp alaska In the preceding code it should
109. 6 This is the EPSG code for all KML datasets Select Target SRID and enter 26910 This is the EPSG code for NAD 83 UTM Zone 10 North Select Create spatial index 33 Creating Spatial Databases 6 Refer the following screenshot to make sure your settings match If so click on the OK button to import the file By setting the target SRID to a different value than the source oN SRID the data will be projected to the new coordinate system during the import process saving you a step Import vector layer Input ions Mastering QGIS Chapter 2 GST103_Lab2_Data Data streams kml Update options Output table Schema Table streams Options Primary key Geometry column Source SRID 4325 X TargetSRID 276910 Encoding Drop existing table Po a ba a Rart msanmmoetrioce motas rea snge part geome mes msta OT Create spatial index 7 After afew moments you will be notified that the import is complete To view the newly created table you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then click on Refresh under Database or press the F5 key on your keyboard The streams table should now appear and have the polyline icon next to it 8 To preview the attribute table click on the Table tab on the information panel To preview the geometry click on the Preview tab on the information panel To view the newly created SpatiaLite layer in QGIS Desktop right click o
110. 9 40 1482056 68 0 86 11700879 35 10356281 00 1526925 10 1479718 37 0 88 Transform Polynomial 1 Mean error 6 68678 9006038 94503 EPSG 2903 a 198 Chapter 6 4 The zone map points file consists of five columns in a comma delimited text file The columns MapX MapY are the destination coordinates Columns pixelX pixelY are the source coordinates and enable column has a value of 1 if the GCP is to be used in the transformation and a value of 0 if it is not to be used The following are the contents of this points file mapX mapY pixelX pixelY enable 1524608 32 1484404 47 7532975 55 1935414 15 1 1523925 76 1480815 95 6274098 49 8399918 00 1 1523645 13 1482436 21 5780754 77 5490891 27 1 1526449 40 1482056 68 10850286 74 6171365 35 1 1526925 10 1479718 37 11700879 35 10356281 00 1 5 Click on Transformation Settings under Settings and set the following parameters O Choose Polynomial 1 as the Transformation type Choose Nearest neighbour as the Resampling method Choose a Compression of None Name the output raster Choose EPSG 2903 as the Target SRS Complete the Generate pdf map and Generate report pdf fields Check Load in QGIS when done Click on OK The following figure shows the completed Transformation settings W Transformation settings Transformation type Polynomial 1 Resampling method Nearest neighbour Compression NONE Create Worl
111. 93 PNoo4 F 431049 PNOOS F 498461 PNOO6 F 319083 74714 39968099 74491 933615 4013066 46126 D06347 39665685 77826 In this example the first row contains the column names and definitions for the data type in each column The column names and definitions are enclosed in quotes and are separated by commas The first column reads SAMPID C 20 In this case the field name is SAMPTID It is a text field signified by the letter c which stands for character with a width of 20 characters The final two columns contain the coordinates These are numeric fields signified by the N character They have a precision of 19 and a scale of 11 QGIS has three requirements for the delimited text file to be mapped e The first row must be a delimited header row of field names e The header row must contain field type definitions Ifthe geometry values are stored as x and y coordinate values they must be stored as numeric fields The Create a Layer from a Delimited Text File tool is simple but robust enough to handle many file format contingencies The following is the workflow for mapping data held in such a file 1 Navigate to Layer Add Layer Add Delimited Text Layer 2 Select the file name by clicking on Browse and locate the delimited text file on your system QGIS will attempt to parse the file with the most recently used delimiter 3 Select Layer name By default this will be the prefix of the delimited text file
112. Attribute POP10 E Find maximum value 0000 Size 40 t Scale Area X Increase size of small diagrams Minimum size 17 00 lt Attributes Available attributes Assigned attributes Attribute Attribute Color STATEFP10 H POPHISP O COUNTYFP10 POPWHI i COUNTYNS10 cs GEOID10 NAME10 NAMELSAD10 LSAD10 CLASSFP10 Ma eee ese nann 93 Styling Raster and Vector Data Parameters common to all diagram types Since the Size Position and Attributes sections are common to all diagrams the next section will first cover these parameters The following three sections will cover the parameters that are unique to each type of diagram Diagram size parameters The Size tab shown in the following screenshot provides the following parameters e Fixed size If this is checked all charts will have the specified area or diameter for a pie and text chart or length for a bar chart e Size units This contains the unit of the Fixed size parameter Scale linearly between 0 and and the following attribute value diagram size If Fixed size is not checked then the length for a bar chart area or diameter for a pie and text chart set by the Scale parameter of the charts will be scaled down linearly from the selected Size value The selected Size value represents the maximum or set Attribute value To enable QGIS to determine the maximum attribute value click on the Fin
113. Clip Data LAStools X lasclip Running a model The model can be run either from the Processing modeler window or from the Processing Toolbox panel To run a model from the Processing modeler window click on the Run model button To run a model from the Processing Toolbox panel first save and close the model Then find the model by navigating to Processing Toolbox Models right click on it and choose Execute from the context menu In our example the model will be found in the Landcover group 284 Chapter 8 Processing Toolbox Recently used algorithms do GDAL OGR 37 geoalgorithms y GRASS commands 168 geoalgorithms aki Models 8 geoalgorithms Example models Hydrology E Landco Ver Execute Execute as batch process Edit rendering styles for outputs SAGA 227 g Edit model Scripts 0 get Delete model TauDEM ih droog anase e geua naE W Tools for LIDAR data 82 geoalgorithms E E E E E Advanced interface The model dialog will open with the listed inputs For the data layer input you can choose data loaded into QGIS by using the drop down arrow or you can use the browse button j to locate the data on disk For this example we are using the AKrivers shp and the trees shp sample data The Buffer distance field is set to 100 since this was the default value set for the number input For the output you can choose to have the layer as a temporary one or choose a locati
114. Geo charter member and a member of the pyCSW and GeoNode Project Steering committees He is the coauthor of PostGIS Cookbook by Packt Publishing and he writes a popular blog on open source geospatial technologies at http www paolocorti net Abdelghaffar Khorchani has a license degree in geographic information systems a fundamental license in natural science applied in biology and geology and a master s degree in geomatics and planning He is also a computer engineer Currently he is pursuing his master s degree in planning and regional development University of Laval Canada and his PhD in marine sciences University of Milano Biccoca Italy He has prepared courses in Japan on fishery resource management approaches for young leaders and in Spain in the field use of geographic information systems for scheduling and management He has also prepared other training modules in Tunisia on urban administration He has 8 years of experience in the geomatics field and has worked on several projects in the agriculture sector environment transport and mapping Currently he is in the Ministry of Agriculture in Tunisia and is responsible for the mapping service for project VMS short for Vessel Monitoring System He is also a trainer in the mapping field of Geographic Information System GPS and CAD He is particularly interested in the development of decision support tools A special thanks to Packt Publishing for this opportunity to p
115. IS can project layers on the fly to the project s CRS If you want to quickly search for a CRS you can enter the EPSG code to quickly filter through the CRS list An EPSG code refers gt to a specific CRS stored in the EPSG Geodetic Parameter Dataset online registry that contains numerous global regional and local CRS An example of a commonly used EPSG code is 4326 that refers to WGS 84 The EPSG online registry is available at http www epsg registry org To enable the on the fly projection perform the following steps L Click on Project Properties under Project Choose the CRS tab and Enable on the fly CRS transformation Set the CRS that you wish to apply to the project and make all layers that are not set to the project s CRS transform on the fly To view the CRS for a layer perform the following steps 1 Open the layer s properties by either navigating to Layer Properties or by right clicking on the layer in the Layers panel Choose Properties from the context menu and then choose the General tab If the layer s CRS is not set or is incorrect click on Specify to open the CRS selector window and select the correct CRS To project a layer to a different CRS perform the following steps 1 2 Right click on the layer in the Layers panel and then choose Save As from the context menu In the Save vector layer as dialog set the file format and filename then set CRS to Selected CRS and click on C
116. If a GCP has not been precisely placed the Move GCP Point tool fs can be used to adjust the position of the control point in either the Georeferencer image window or the QGIS Desktop map canvas Q GCPs can be deleted in two ways as follows Using the Delete point tool and clicking on the point in the Georeferencer image window e By right clicking on the point in the GCP table and choosing Remove 189 Advanced Data Creation and Editing The following figure QGIS Desktop and the Georeferencer window with a single GCP entered Z QGIS 2 6 0 Brighton Project Edit View Layer Settings Plugins Vector Raster Database Web MMQGIS Processing Help Da BAQR AMF SPeRBPL Aas 4 P I BRRL AIEA ICSW 4 IY amp BBIYNARS Cy Coordinate Z Georeferencer Scanned 1990 tif File Edit View Settings Help P P rp P f yf gt 5 676 a atog Car onoff id sex ser dsx dse cxfpixels ripixets resicualfpinels x o 2132 244 85 474793 69 441059769 000 o 000 GCP 1 Source and Destination Coordinates T 7 76 2 194 6 None A 190 Chapter 6 Repeat steps 6 to 11 for the remaining ground control points until you have entered eight GCPs Use the pan and zoom controls to navigate around each image as needed Once all the points have been entered the Save GCP Points As button can be used to save the points to a text file with a points extension Th
117. Import PyDev Module geosisma ctrlU py Export PyDev Packag i Geosisma prototype Refresh p5 Ci Other Geosisma prototvoe saveas Close Project 2 1 item selected 331 Creating QGIS Plugins with PyQGIS and Problem Solving Here we can see the contextual menu to add a new folder for the selected project This action will open a GUI where we can create or link a new folder In our case it s useful to link to the existing plugin code which can be done using the Advanced features of the GUI as shown here Folder Create a new folder resource A Enter or select the parent folder TestPlugin TestPlugin Folder name TestPlugin lt lt Advanced O amp Use default location O amp Folder is not located in the file system Virtual Folder Ca Link to alternate location Linked Folder me ginetto qgis2 python plugins TestPlugin Browse Variables Resolved location file home ginetto qgis2 python plugins TestPlugin Resource Filters Cancel Finish After linking the folder it will appear under the Test Plugin PyDev project where we can look for the test_plugin py code Double click on the file it will be opened on the right hand side of Aptana as shown in the following screenshot 332 a a ie aida gt Mille airs Al PyDev Package Explorer v g TestPlugin v Eg TestPlugin f help f amp i18n amp scripts se test E _init_ py amp
118. L YALE BLVD 6 Notice that all the attributes are identical It is wise to verify this to ensure you do not delete any unique data 7 Select the feature with the higher row number and click on the Delete selected button E 8 Toggle off editing for this layer and save the changes Example 2 repairing overlaps To repair overlaps there are some editing parameters with which you should familiarize yourself and set Setting the editing parameters 1 Click on Snapping Options under Settings to check the snapping tolerances 2 Click the checkbox to the left of the parcels layer to select it Choose to vertex and segment in the Mode field and a value of 10 map units in the Tolerance field 208 Chapter 6 3 Besides layer based snapping options you can also enable topological editing from the Snapping Options dialog box Click on the Enable topological editing checkbox Checking this option allows you to edit common boundaries in adjacent polygons QGIS will detect shared polygon boundaries and vertices on these shared boundaries they will only have to be moved once and both polygons will be edited together 4 There are two other editing options available here Avoid intersections This can be checked for a particular layer to avoid creating overlaps when digitizing new polygons With this option checked you can digitize a new polygon adjacent to an existing one so that the new polygon intersects the exist
119. Layer dialog window 180 Soe oS 11 12 13 14 15 Chapter 6 Select the Addresses csv sample data as the Input CSV File UTF 8 field by clicking on Browse and locating the delimited text file on your system Select the street name field from the Street Name Field drop down menu Select the number field from the Number Field drop down menu Select the zip field from the ZIP Field drop down menu Select the street GIS layer loaded in QGIS from the Street Layer drop down menu Select the street name field of the street layer from the Street Name Attribute drop down menu This tool allows geocoding from street address ranges or via From X Attribute and To X Attribute coordinates RS The latter assumes that you have attribute columns with Q the To and From coordinates for each street segment To geocode via To and From coordinates select the From X Attribute To X Attribute From Y Attribute and To Y Attribute fields from the drop down menu In this example only address ranges will be used Populate the From X Attribute To X Attribute From Y Attribute and To Y Attribute dropdown menus with the street line start street line end street line start and street line end option Since address ranges will be used for geocoding select the Left From Number Left To Number Right From Number and Right To Number attributes from the drop down menu If the street data has left and right zip code attrib
120. Millimeter Transparency 0 Color v Size 4 00000 Rotation 0 00 Symbols in group Open Library OQ t o Circlew airport arrow capital aik A N capital 87 Styling Raster and Vector Data As an example of use using the Populated Places shp sample data capital cities megacities and all other places can be styled differently by using rule based styling Additionally each rule is visible to the minimum scale of 1 1 although they become invisible at different maximum scales The following screenshot shows the rules created and a sample map of selected populated places in the country of Nigeria Rule based X 0 0 X Not capitals admOcap 0 AND megacity 0 1 9 000 000 1 14 X Admin 0 admOcap 1 1 20 000 000 1 11 1 18 X Megacity megacity 1 AND admOcap 0 1 15 000 000 0 lt gt Refine current rules y Count features Rendering order Point displacement vector styling The point displacement vector style radially displaces points that lie within a set distance from each other so that they can be individually visualized This vector style works best on data where points may be stacked on top of each other thereby making it hard to see each point individually This vector style only works with the point vector geometry type 88 Chapter 3 The following screenshot shows how the Point displacement style works by using the Single Symbol renderer wh
121. QGIS Python console myVector QgsVectorLayer qgis sample data shapfiles alaska shp MyFirstVector ogr This way the layer is loaded and referred by the myVector variable After adding the layer to QgsMapLayerRegistry it will be visualized in the legend with the name MyFirstVector In the QgsVectorLayer constructor call is possible to find a third string parameter ogr that specifies to use the OGR library to load the alaska shp file As usual we will check whether the loaded layer is valid using the following code myVector isValid Managing database vectors If vector data is hosted in a spatial database it can be loaded by specifying the location and connection information using the QgsDataSourceURI class A URI short form for Uniform Resource Identifier is how a resource can be identified on a network like the World Wide Web The following code snippet shows how to fill the URI with the necessary information to connect to a remote spatial database as PostGIS uri QgsDataSourceURI uri setConnection localhost 5432 myDb myUserName myPassword uri setDataSource public myTable the geom mywWwhere print ricu ri The first line creates an instance of QgsDataSourceURI that is filled with other information in the next lines The setConnection method accepts the IP or the symbolic name of the database engine the connection port the database name the username and the passwo
122. Render EPSG 25829 A In this case the progress bar will be opened in QgsMessageBar in the QGIS canvas because the algorithm is executed from the Python console Running an external algorithm or command Often there are a bunch of legacy programs or scripts for which there are no resources to port them into another language or framework Thanks to Python and PyQGIS it s simple to integrate your existing programs into QGIS 365 PyQGIS Scripting Running a simple command We can run an external command in different ways but we will explore how to do it with the processing toolbox that supports the progress bar which is often useful to log algorithm steps To execute an external command we will follow these steps 1 Create a processing toolbox script called runping 2 Code the script 3 Test the script Step one is similar to that described in the Creating a test processing toolbox script section The code of the script is in the following code snippet import subprocess import time proc subprocess Popen ping T a 10 Localhose stdout subprocess PIPE stdin subprocess PIPE stderr subprocess PIPE counter 0 for line in iter proc stdout readline print line progress setPercentage counter counter 10 The preceding code runs the ping command to localhost and stops after 10 pings 1 Q In case of Windows OS replace ping c 10 Localhost with ping
123. S does not currently support the creation of color palettes for single band rendering However existing QGIS layer style files qm1 that contain palettes can be applied by clicking on the Load Style button in the layer properties As an example of a raster with a color palette stored within it add NA LC 1km tif from the sample data to the QGIS canvas and open the Style tab under Layer Properties The following figure shows the Paletted band renderer being applied to Band 1 of the raster wY Band rendering Render type Paletted Band Band 1 Palette ei Fok ae pat g eae 7 H8 pe Won Don AUNE To change a color double click on a color in the Color column to open the color picker 66 Chapter 3 Singleband gray raster band rendering The singleband gray band renderer stretches a gradient between black and white to a single raster band Additionally contrast enhancements are available to adjust the way the gradient is stretched across the raster band s values Let s apply a singleband gray renderer and a contrast enhancement to the sample GRAY_50M_SR_W tif raster file that represents shaded relief hypsography and flat water for Earth Add GRAY_50M_SR_W tif to the QGIS canvas and open its Style tab from Layer Properties As this is a singleband raster QGIS defaults the Render type value to Singleband gray with the following parameters as shown in the following screenshot e Gray band Band 1 Gray
124. TestPluginDialogBase QDialog Property Value objectName TestPluginDialogBase windowModality NonModal This is the graphical representation of the test_plugin_dialog_base ui XML file With Qt designer we can reorganize the layout adding new graphical elements and also connect events and triggers related to the interface In the preceding screenshot the four red boxes mark the Designer sections e The Layout Area is the area where the plugin GUI is rendered e The Widget Box section contains the list of predefined GUI components Here we ll look for the combobox to add to the GUI layout e The Object Inspector section gives the hierarchy of graphical components composing the GUI layout e The Property Editor section gives a list of all the properties of the graphical components that can be customized 320 Chapter 9 Adding two pull down menus The next steps will be to create the layout as shown in the following screenshot File Edit Form View Settings Window Help iy B b aa Fee es ay Widget Box aw f Ajea Filte R Object Class asters F Dock Widget a label_2 QLabel B rast ombo QComboBox Input Widgets vect ombo rm QComboBox f a Combo Box E 7 Property Editor A Font Combo Box Hittar ay Line Edit Filte oy All S Text Edit vectorsCombo QComboBox AJ Plain Text Edit Property Value i 13 Spin Box emai Time Edit f Date Edit A
125. This contains the outline pen color for the Circle radius modification circle e Circle radius modification The number of millimeters that the points are displaced from the center symbol This circle can be displayed on the map if the Circle pen width value is greater than 0 e Point distance tolerance For each point if another point s is within this distance tolerance defined in millimeters then all points will be displaced The Labels parameters applies to all points displaced or not in the vector data It is important to use these label parameters rather than the label parameters on the Labels tab of the Layer Properties window because the labels set in the Labels tab will label the center symbol and not the displaced points Inverted polygons vector styling The inverted polygons vector style inverts the area that a polygon covers This vector style only works with the polygon vector geometry type The following figure shows the Inverted polygons style for a polygon of the country of Nigeria on the left and all countries underneath the transparent inverted polygon of Nigeria on the right Notice that the entire canvas is covered by the inverted polygon which has the effect of cutting out Nigeria from the map 90 Chapter 3 The Inverted polygons style parameters rely on a sub renderer to determine the symbol used for the inverted polygons By choosing the sub renderer the polygon rendering is inverted to cover the ent
126. To do this you need to have both QGIS and SAGA installed The processing framework must also be configured properly so that QGIS can access the SAGA command line executable The following are guides for installing and troubleshooting SAGA on each operating system 152 Chapter 5 Windows If you are running Windows and you installed QGIS with either the OSGeo4W or the standalone installer SAGA will be included unless SAGA was unchecked during the OSGeo4W installation More importantly the path to the SAGA executable is automatically configured There is nothing you need to do Just use the SAGA tools Mac OS X When you install QGIS from the Kyngchaos repository at kyngchaos com software qgis SAGA will be included The main error that you may encounter will state Missing dependency This algorithm cannot be run If you encounter this error there are three ways to troubleshoot the problem 1 There may be a conflict with the GDAL plugins When they are enabled the GDAL formats are added to the SAGA file format list and this changes the expected ordering This is mentioned in the Kyngchaos QGIS 2 6 README file The workaround is to disable the GDAL plugins GDAL tools and Georeferencer GDAL when SAGA algorithms are needed This leaves GDAL unable to use these formats so remember to re enable the GDAL plugins when the work with the SAGA tools is complete 2 There may be multiple versions of the Processing Toolbox plugin installe
127. To import color ramps from an XML file navigate to Share Import to open the Import style s window shown in the following figure 4 Import style s Import from file specified below Location 3 Data Selected Blue Color Ramp Styles xml save to group Selected Blue Color Ramp Styl Select symbols to import PuBu PuBuGn RdviBu Spectral iGn YiGnBu Select all Clear selection Close Exported color ramp styles can be imported from an XML file or a URL that is pointing to an XML file To import from a file select file specified below for the Import from parameter then click on Browse to select the XML file Once the Location value is specified the color ramps will be displayed Select the color ramps that you wish to import select the group into which you wish to import the color ramps and then click on Import The imported color ramps and other symbol types if selected will be added to the color ramp list in the Style Manager Adding a color ramp Using the Add item button three types of color ramps Gradient Random and ColorBrewer can be added to the color ramp list in the Style Manager These color ramp types can be created from scratch or they can be selected from a large collection of existing color ramps from the cpt city archive of color gradients Let s add one color ramp of each type and then add a cpt city color ramp 60 Chapter 3 Adding a Gradient color ramp To add a Gradient col
128. Translate command line utility To enable it navigate to Plugins Manage and Install Plugins and then click on the Installed tab and check the box to the left of Georeferencer GDAL plugin Once enabled you can launch the plugin by clicking on Georeferencer under Raster The Georeferencer window has two main windows the image window and the ground control point GCP table window These windows are shown in the following screenshot Z Georeferencer Le o o s File Edit View Settings Help gt 153s FG P i D Ground Control Point GCP Table Transform Not set 0000000 0 000000 EPSG 185 Advanced Data Creation and Editing The general procedure for georeferencing an image is as follows 1 Load the image to be georeferenced into the Georeferencer image window by clicking on Open Raster under File or by using the Open raster button a If you are georeferencing the raster against another dataset load the second dataset into the main QGIS Desktop map canvas Begin to enter ground control points with the Add point tool This tool is also available via Edit Add point Regardless of which of the two ground control point scenarios you are working with you need to click on the GCP point within the Georeferencer image window Use the zoom and pan tools so that you can precisely click on the intended GCP location Once you click on the input raster with the Add point tool the Enter map coor
129. _merge command line tool Overlap among the input rasters is allowed If this happens the data for the last image in the list will be used for the area of overlap In the Merge dialog you have the option to choose whether you wish to merge all the rasters in a folder or you can pick individual rasters to merge This provides a nice built in batch processing option The following are the options for running the Merge tool 1 Depending on how your data is stored you can choose one of the following options O Select Choose input directory instead of files O Click on the Select button and select the individual rasters to merge In the following example the sample data 35106 G4 tif 35106 G5 tif 35106 H4 tif and 35106 H5 tif are being merged Note that this data has a CRS of EPSG 26913 so the project CRS should be set to this Name the output file by clicking on Select If the rasters include a no data value you can select No data value and specify this value 4 Ifthe input rasters cover the same geography but contain different bands of information this tool can be used to create a multiband image By choosing the Layer stack option each input file becomes a separate band in a stacked image 5 The Use intersected extent option specifies the spatial envelope for the output It defaults to the extent of the inputs 6 If the images include a color table that can be passed on to the output by choosing the Grab pseud
130. a Chapter 5 35106 G5 tif D PACKT Data Chapter 5 35106 H4 tif D PACKT Data Chapter 5 35106 H5 tif Help 157 Preparing Raster Data for Processing There are two additional tools in QGIS that can be used to create raster mosaics Both of these require the rasters to be merged and loaded into the QGIS map canvas The first is the GRASS tool r patch It can be found by navigating to Processing Toolbox Grass Commands Raster r patch The second tool is the SAGA tool Merge raster layers This tool can be found by navigating to Processing Toolbox SAGA Grid Tools Merge raster layers This SAGA tool lets you determine how overlapping cells in the set of input rasters will be handled You can choose to use the first value in the order of the grid list or the mean value It also allows you to choose the interpolation method See the following figure Merge raster layers Parameters Log Help Grids to Merge 4 elements selected Preferred data storage type 0 1 bit Interpolation 0 Nearest Neighbor Overlapping Cells 0 mean value Merged Grid Open output file after running algorithm Generating raster overviews pyramids A raster pyramid is a reduced resolution version of a raster The purpose of a pyramid is to reduce the time it takes to display a raster Pyramids can be built at multiple levels to help strike a balance between the pyramid s file size and the display speed 158
131. ables Editing data Styling data Composing a map Finding and installing plugins 1 www allitebooks com A Refreshing Look at QGIS QGIS download and installation QGIS can be installed on Windows Mac OS X Unix Linux and Android operating systems making it a very flexible software package Both the binary installers and source code can be downloaded from download qgis org In this section we will briefly cover how to install QGIS on Windows Mac OS X and Ubuntu Linux For the most up to date installation instructions refer to the QGIS website Installing QGIS on Windows For Windows there are two installation options which are as follows e QGIS Standalone Installer The standalone installer installs the binary version of QGIS and the Geographic Resource Analysis Support System GRASS using a standard Windows installation tool You should choose this option if you want an easy installation experience of QGIS e OSGeo4W Network Installer This provides you with the opportunity to download either the binary or source code version of QGIS as well as experimental releases of QGIS Additionally the OSGeo4W installer allows you to install other open source tools and their dependencies Installing QGIS on Mac OS X To install QGIS on Mac OS X the Geospatial Data Abstraction Library GDAL framework and matplotlib Python module must be installed first followed by the QGIS installation The installation files for GDAL
132. ace mouse movement Register the event above the event handler to the xyCoordinate event ye oe ISD Create an event handler for the Map Tool to trace clicked points by setting the following conditions O If the left button clicked then print coordinates O If the right button clicked then restore the previous Map Tool Register the event handler to the click event generated by the Map Tool Activate the new QgsMapToolEmitPoint Map Tool These steps are coded in the next paragraphs Getting the current Map Tool To return to the current Map Tool after setting the new one we need to save it in a variable that can be used later We can get the current Map Tool by using the following code snippet previousMapTool iface mapCanvas mapTool Creating a new Map Tool We can create the Map Tool with this simple code snippet from qgis gui import QgsMapToolEmitPoint myMapTool QgsMapToolEmitPoint iface mapCanvas You will notice that each Map Tool constructor needs a parameter that is the canvas on which it will operate 370 Chapter 10 Creating a map canvas event handler An event handler is useful to execute actions that are based on user interaction with the canvas First we will create the handler that prints coordinates based on mouse movements on the canvas The event handler will receive the parameters passed by the event to which it is attached The event handler will be attached to the QgsMapCa
133. ackground fill color for the circle 98 Chapter 3 e Line color This contains the color of the lines surrounding the circle and in between slices e Line width This contains the width of the lines surrounding the circle and in between slices e Font This can be used to set the font Open the Select Font dialog to set the font parameters for the attribute labels that appear inside the circle slices e Scale dependent visibility If this is checked the Minimum and Maximum visibility scales can be set Appearance Size Position Options Background color v Line color D Line width 0 20000 Font Scale dependent visibility Minimum 1 000000 Maximum 1 000000 The Options tab shown in the following screenshot provides the Label placement parameter This parameter sets the baseline position for the vertical placement of the text The available options are x height which sets the text at the x height position of the text and Height which sets the text at the bottom position of the text height Appearance Size Position Options Label placement x height 99 Styling Raster and Vector Data Creating a histogram chart diagram The histogram chart diagram displays attributes in a histogram bar chart where each attribute can be visualized as a bar that varies in length in proportion to the attributes values As an example the following figure shows a portion of a state s counties with histogram charts s
134. aded in the canvas or by clicking on the browse button Specify The input data range to be rescaled Specify The output data range Select the output raster layer by clicking on the browse button and choosing Save to a temporary file or Save to file nrescale Rescales the range of category values in a raster layer Parameters Log Help Input raster layer D PACKT Data Chapter 5 RiparianSurface img The input data range to be rescaled Min 10 Max 95 5 The output data range Min Max 100 GRASS region extent xmin xmax ymin ymax GRASS region cellsize leave 0 for default 0 000000 Output raster layer pail Siebel las save to a temporary file Open output file after running algorithm Save to file 155 Preparing Raster Data for Processing Creating a raster mosaic Frequently raster data is made available in tiles In fact some consider the Murphy s Law of GIS to be that your study area lies at the intersection of four topographic quadrangles In this situation the input rasters will need to be combined into a seamless raster that covers the study area When doing this the individual input rasters must all be in the same coordinate reference system and have the same number of bands Assuming that these two conditions have been met the Merge tool that can be found by navigating to Raster Miscellaneous can be used to merge the individual rasters together This tool is a GUI version of the GDAL
135. age of geospatial data to the display of geospatial data The styling capabilities of QGIS will be covered for both vector and raster files Additionally the rendering options that were first introduced in QGIS 2 6 will be covered 49 Styling Raster and Vector Data In this chapter we will cover advanced styling and labeling of raster and vector data in QGIS It is assumed that you are familiar with basic styling in QGIS and are looking to improve your styling techniques The topics that we will cover in this chapter are as follows Choosing and managing colors Managing color ramps Styling single band rasters Styling multiband rasters Creating a raster composite Raster color rendering Raster resampling Styling vectors Vector layer rendering Using diagrams to display thematic data Saving loading and setting default styles 51 Styling Raster and Vector Data Choosing and managing colors As colors are used throughout the styling process we will first review the ways in which you can select and manage color collections in QGIS The color picker is accessible in any window that allows a color selection to take place For example when choosing a color in the Style window under Layer Properties click on the down arrow next to the color display and then select Choose color as shown in the following screenshot HHH E Copy colar Paste color Pick color Choose color JY Select color v Opacity 4
136. ain Looking at the custom script Now run the following processing command processing alglist empty This will generate the following output emptyloop gt script emptyloop This shows that the processing toolbox knows our custom script with the name script emptyloop 364 Chapter 10 Running the script We can run the custom script as usual in the Python console with the following command processing runalg script emptyloop This will increment the progress bar and will print progress in the console as shown in the following screenshot Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help gr Ba otf w S EnA me SPS O Ja sa B gt 7Nn uu Layers Executing algorithm ad 28 Processing Toolbox de Convert format emptyloop gt dp GDAL OGR 34 geoalgorithms gt w GRASS commands 168 geoalgo v y GRASS GIS 7 commands 158 ge p Imagery i gt Miscellaneous m gt Raster r gt Vector v gt Visualization NVIZ Python Console a gt amp Models 5 geoalgorithms 4 gt Orfeo Toolbox Image analysis p QGIS geoalgorithms 89 geoalg gt z QuickOSM 11 geoalgorithms b amp SAGA 228 geoalgorithms E v E Scripts 30 geoalgorithms gt Database gt gt gt processing runalg script emptyLloop k Advanced interface Y Coordinate 0 79 3 15 Scale 1 41 v M
137. al result is generated Using the graphical modeler you can combine these individual steps into a single process The interface to the GM allows you to visually draw inputs GIS algorithms and outputs The entire analysis is then ready to run as a custom tool within the Processing Toolbox The custom tool will look like other tools in the Processing Toolbox After assigning the inputs and naming the outputs the entire analysis will run in a single step A major benefit of this approach is that the completed analytical workflow can be modified and rerun This allows stakeholders to understand how changing thresholds or input values affect the results of an analysis Let s assume that you were assigned the task of developing a site selection model for a new coffee shop To match one of the site selection criteria you buffered railroads by one kilometer However a stakeholder later asks you how the result would change if the one kilometer distance was changed to half a kilometer If you had completed the original analysis with a traditional step by step approach without using a model you would have to start from scratch to answer this question However if you developed this problem as a model you can simply change the distance parameter in the tool and rerun the entire site selection model Similarly the site selection model can also be run in a different city or neighborhood simply by pointing to different but equivalent input layers The mode
138. aling the input raster from source coordinates to destination coordinates Once a data set has been georeferenced it can be brought into a GIS and aligned with other layers Ground control points Georeferencing is done by identifying ground control points GCP These are locations on the input raster where the destination coordinate system is known Ground control points can be identified in one of the following two ways e Using another dataset covering the same spatial extent that is in the destination coordinate system This can be either a vector or a raster dataset In this case GCPs will be locations that can be identified on both the datasets e Using datums or other locations with either printed coordinates or coordinates that can be looked up In this case the locations are identified and the target coordinates are entered Once a set of ground control points has been created a transformation equation is developed and used to transform the raster from the source CRS to the destination CRS Ideally GCPs are well distributed across the input raster You should strive to create GCPs near the four corners of the image plus several located in the middle of the image This isn t always possible but it will result in a better transformation 184 Chapter 6 Using the Georeferencer GDAL plugin The Georeferencer GDAL plugin is a core QGIS plugin meaning it will be installed by default It is an implementation of the GDAL_
139. ality 24 25 point click values current Map Tool obtaining 370 map canvas event handler creating 371 Map Tool event handler creating 371 372 new Map Tool creating 370 new Map Tool setting up 372 obtaining 370 point displacement vector style about 88 90 parameters 89 90 points creating from coordinate data 169 173 polygon centroids creating 110 polygons converting to lines 110 nodes extracting from 112 processing commander 222 383 processing toolbox about 216 222 algorithms accessing 216 configuring 216 218 sources 223 using 223 processing toolbox algorithms custom processing script running 363 information obtaining 361 running 359 360 running at console 362 363 searching 360 Proj 4 about 11 URL 11 Proj 4 definition format about 127 parameters for CRS 128 URL 127 PyDev server QGIS connecting to 330 starting 328 PyQGIS cookbook references 302 URL 302 Python basics 337 model converting to 299 spaces selecting 338 tabs selecting 338 URL 308 Python Console about 306 PyQGIS5 code snippet 307 308 QGIS API exploring 310 sample data obtaining 307 URL 307 PYTHONPATH setting up 327 328 Q QGIS about 1 4 downloading 2 exploring 308 309 installing 2 3 installing and FOSSGIS Packages 3 installing on Mac OS X 2 installing on Ubuntu Linux 2 installing on Windows 2 OGIS Browser interface 5 QGIS Desktop interface 4 reference links 303 URL 2 217 303 309 URL for Ubuntu v
140. all media At Packt we take the protection of our copyright and licenses very seriously If you come across any illegal copies of our works in any form on the Internet please provide us with the location address or website name immediately so that we can pursue a remedy Please contact us at copyright packtpub com with a link to the suspected pirated material We appreciate your help in protecting our authors and our ability to bring you valuable content Questions If you have a problem with any aspect of this book you can contact us at questions packtpub com and we will do our best to address the problem xiv A Refreshing Look at QGIS QGIS is a volunteer led development project licensed under the GNU General Public License It was started by Gary Sherman in 2002 The project was incubated with the Open Source Geospatial Foundation OSGeo in 2007 Version 1 0 was released in 2009 At the time of writing this book QGIS 2 6 was the stable version and new versions are released every four months In this chapter we will review the basic functionality of QGIS which will be assumed knowledge for the remaining chapters in this book If you need a refresher on QGIS or a quick start guide to QGIS you should read this chapter The topics we will cover in this chapter are as follows Downloading QGIS and its installation The QGIS graphical user interface Loading data Working with coordinate reference systems Working with t
141. alysis Random sampling grid Regular sampling grid S SAGA 228 geoalgorithms Geostatistics SS Directional statistics for sin 222 Chapter 7 Notice that by typing grid in either the commander box or the search box we are presented with a list of available algorithms The primary difference between the two is that when a tool is selected in the commander drop down list it automatically opens Therefore once we develop a familiarity with the names of specific tools the processing commander may increase workflow productivity Now that the QGIS processing toolbox is properly configured and we have a basic understanding of its overall functionality and organization we can begin using it to utilize tools that weren t historically available within the QGIS environment The intent with these exercises isn t to provide a comprehensive overview of all the providers or algorithms but to illustrate the power and flexibility that the toolbox brings to QGIS Using the processing toolbox We will begin by using some of the GRASS algorithms and focusing primarily on tools that aren t available through default plugins or drop down menus For example even though GRASS has the ability to calculate aspect this functionality is already available in QGIS and it can be found by navigating to Raster Terrain Analysis The original data used in this chapter can be obtained from these sources e http oe oregonexplorer
142. and healthcare GIS and has completed numerous GIS projects in these disciplines Dr John Van Hoesen GISP serves as an associate professor of geology and is the Environmental Studies Community Mapping Lab Director at Green Mountain College in Vermont USA His passions include open source software environmental science and the great outdoors where he leads students in the discovery of our natural world Luigi Pirelli from Spain is a core contributor to QGIS and a contributing author of this book www allitebooks com He provided us with the chapters on programming for QGIS A huge thanks to our most capable editor Samantha Gonsalves for her management during the creation of this book A former systems engineer at Infosys in Mumbai India and now an editor for Packt Publishing her leadership kept the team on a tight deadline to complete Mastering QGIS while maintaining the highest editorial standards For all of us it has been a fascinating and rewarding experience and now you hold the results of our effort in your hands Best wishes for success on Mastering QGIS now and in the future Phillip Davis Director National Information Security amp Geospatial Technology Consortium Del Mar College Texas USA www allitebooks com About the Authors Kurt Menke a certified GIS Professional GISP has been working in the GIS field since 1997 Prior to this he worked as a professional archaeologist for 10 years in the American Southwe
143. and observer points to evaluate viewshed for any assumptions However setting the position height above ground to 20 meters is an average value for typical fire towers The maximum distance of 32 000 meters is the greatest distance between any of the towers and the edge of the park elevation layer and including Earth s curvature even for small areas at worst increases processing time but provides a more accurate representation of visibility If you have a lot of observers completely fill out the information for the i first observer and after you set the Output raster layer parameter you gt will be prompted to autofill the input boxes If you select yes the interface Q will automatically populate the parameters and you will only need to adjust the parameters that are different For example the coordinates will need to be updated and perhaps not all observers have the same height The output from this algorithm will need to be renamed since they will all be added with the same name fortunately if you hover the mouse over each entry QGIS will report the full path name as illustrated in the following screenshot 71 362 87 6586 El 3 Turbine 3 73 7528 239 The Processing Toolbox Combining viewsheds using r mapcalculator In order to evaluate the cumulative visual impact of all the three towers we need to add them together However the algorithm outputs a grid that contains either a degree angle representing t
144. ander This means that in our PyQGIS script we can refer to the algorithm with the name gdalogr ogr2ogr In this case the ogr2ogr algorithm belongs to the gdalogr backend The alglist method looks for only the left part Notice that the alglist method will also show custom scripts This means that a custom script or model will be accessible in the PyQGIS interface of the processing toolbox Getting algorithm information Every processing algorithm has its own GUI with input and output parameters You may wonder about the parameters their names and the values that may be used in the same algorithm using PyQGIS scripts To discover these elements we will use the alghelp processing command This command accepts the command name that is used internally by processing as a parameter for example the gdalogr ogr2ogr string that we saw in the previous paragraph We can get help for this command by using the following code import processing processing alghelp gdalogr ogr2ogr The preceding code snippet will produce the following output that would depend on the installed version of GDAL OGR processing alghelp gdalogr ogr2ogr ALGORITHM Convert format INPUT LAYER lt ParameterVector gt DEST FORMAT lt ParameterSelection gt DEST DSCO lt ParameterString gt OUTPUT LAYER lt OutputVector gt s DEST FORMAT Destination Format 0 ESRI Shapefile 1 GeoJSON 2 GeoRSS 361 PyQGIS Scripting 3 SQLite ot 5 DOWEL Gesc 1
145. ank If there is a one band raster with a color table use the Expand drop down menu to choose a setting for converting it to a three band image To change the CRS of the raster during the resampling operation specify the new output CRS by clicking on the browse button for the Output projection for output file leave blank to use input projection option box To subset or clip the raster during the resampling operation enter the coordinates for the desired spatial envelope in the Subset based on georeferenced coordinates xmin xmax ymin ymax option box Additional creation parameters can be specified For a full list of options for the gdal_translate utility visit the help page at http www gdal org gdal translate html Use the Output raster type drop down menu to choose the radiometric resolution of the output raster The options are Byte Int16 UInt16 UInt32 Int32 Float32 Float64 CInt16 CInt32 CFloat32 and CFloat64 The Output raster type setting in the Translate tool of A the Processing Toolbox can also be used to convert from Q floating point rasters to integer rasters and vice versa With a floating point raster as the input choose one of the integer settings to convert the raster to an integer 150 Chapter 5 12 Select the output raster by clicking on the browse button You can choose to either Save to a temporary file or Save to file The following screenshot shows the completed Translate tool v Trans
146. ap canvas Explaining Map Tools Setting the current Map Tool Getting point click values Getting the current Map Tool Creating a new Map Tool Creating a map canvas event handler Creating a Map Tool event handler Setting up the new Map Tool Using point click values Exploring the QgsRubberBand class Summary Index viii 352 352 353 355 356 357 358 359 359 359 360 361 362 363 364 365 365 366 367 368 368 369 370 370 371 371 372 373 374 376 379 Preface Welcome to Mastering QGIS The goal of this book is to help intermediate and advanced users of GIS develop a deep understanding of the capabilities of QGIS while building the technical skills that would facilitate in making the shift from a proprietary GIS software package to QGIS QGIS embodies the open source community s spirit It seamlessly works with other free and open source geospatial software such as SAGA GDAL GRASS and fTools and supports standards and formats that are published by myriad organizations QGIS is about freedom in the geospatial world freedom to choose your operating system freedom from licensing fees freedom to customize freedom to look under the hood and freedom to contribute to the development of QGIS As you work through this book we believe that you will be amazed at how much capability and freedom QGIS provides QGIS has rapidly risen from the initial version written by Gary Sherman in 2002 to become a
147. ar and tools for managing saving and exporting compositions the Composer toolbar Maps can be exported as images PDFs and SVG graphic files To export the map click on the Composer menu and select one from among Export as image Export as SVG or Export as PDF depending on your needs The following is a screenshot showing parts of the composer window Z My Map Composer Edit View Layout Atlas Settings BABaLGRePheSeRRo BS PPS OB aE amp Main Menus and Toolbars fe foo 120 140 2160 fio pertartateceetaaeetaceateeaeteseetecnacbeaeebarreteeantenceteveats lpr PEE Naan Print Composer Tabs f 1a OEA A a A in ee Composition Item properties Atlas generation items li v Main properties Cache v Update preview Scale x 4 G Map rotation 0 00 Draw map canvas items Lock layers for map item v Extents Xmin 13678420 565 Ymin 5688125 931 X max 13627676 470 Y max 5726660 920 lt gt Di Set to map canvas extent View extent in map canvas gt Controlled by atlas gt Grids gt Overviews gt Rotation gt Frame gt x Background gt item D P Rendering i t s 3 gt Position and size gt 5 ete i a ig Adding functionality with plugins There are so many potential workflows analysis settings and datasets within the broad field of GIS that no out of the box software could contain the tools for every scenario Fortunately QGIS has been developed with a p
148. articipate in the review of this book I thank my family especially my parents for their physical and moral support Finally I want to thank Cheima Ayachi who helped me a lot when I was reviewing this book Gergely Padanyi Gulyas is a GIS and web developer and remote sensing analyst with over 7 years of experience He specializes in designing and developing web mapping applications and Geographic Information Systems GIS He is a dedicated user developer of open source software and he is also an active member of the OSGeo local chapter He is familiar both with client and server side programming For more than 4 years he worked for archaeologists as a GIS engineer and remote sensing analyst where he contributed to laying the foundation of the Hungarian Archaeological predictive modelling After that he became a Java web developer for a private company Since then he has been working at a state nonprofit corporation as a GIS and web developer where he uses the skills he learned from his previous jobs combining GIS with development During the past few years he has been involved with plugin development in different programming languages such as Java for GeoServer and Python for QGIS He has a website www gpadanyig com www PacktPub com Support files eBooks discount offers and more For support files and downloads related to your book please visit www Packt Pub com Did you know that Packt offers eBook versions of every book p
149. at QGIS Desktop and the Georeferencer window are visible simultaneously 5 Familiarize yourself with both datasets and look for potential GCPs Look for precise locations such as piers corners of roof tops and street intersections 188 Chapter 6 Entering the ground control points 1 Zoom in to the area using the zoom in button ge where you will enter the first GCP in both the QGIS Desktop and Georeferencer windows Zooming in will allow you to be more precise Enter the first GCP into the Georeferencer image window using the Add Point tool y After clicking on the image in the Georeferencer image window the Enter map coordinates window will open Click on the From map canvas button The entire Georeferencer window will momentarily disappear Click on the same location in QGIS Desktop i If you have not first zoomed in to the GCP area in QGIS Desktop you can still do so After zooming in you will need Q to click on the g From map canvas button again to regain the Add Point cursor The Enter map coordinates window will reappear with the destination coordinates populating the X East and Y North boxes Click on OK to complete the Ground Control Point The GCP table in the Georeferencer window will now be populated with the source and destination coordinates for the first ground control point The control point will also be indicated in both the Georeferencer image window and QGIS Desktop by a red dot
150. at better demonstrate the analytical power residing in the processing toolbox We are going to calculate a least cost path for a hypothetical situation where a search and rescue team has been deployed to Crater Lake National Park to extract an injured hiker The team may be able to use roads for part of their approach but would like to identify the least cost or the least rigorous approach to the hiker Essentially we are going to make some simplistic assumptions about how much effort will be required to move across the landscape by incorporating slope and land use into a raster layer representing the cost of movement This cost layer will then be used to identify the least cost path from the search and rescue office to the injured hiker In order to accomplish this analysis we need to complete the following steps Calculate slope using r slope Reclassify new slope raster using rules in slope_recode txt Reclassify the land use raster using rules in lulc_reclass txt Combine reclassified slope and land use layers Calculate cumulative cost raster using r cost oe OW ie pe he Calculate cost path using least cost paths 228 Chapter 7 Calculating the slope using r slope The necessary settings for calculating slope are illustrated in the following screenshot Q rslope Generates raster maps of slope iru elevation raster map Parameters Log Help Name of elevation raster map Elevation EPSG 26710 Format for reportin
151. ation but the technique has other applications such as evaluating which proposed viewing platform offers the greatest viewable area or determining how best to position observers during an aerial threat assessment Although this tool has a specific niche application working through this section will allow us to make use of additional algorithms that have broader applications We will begin by creating a new QGIS project and adding the following files e Elevation file dems _10m dem e Boundary file crlabndyp shp e Surface water file hydp shp e Fire towers file towers shp In this application we are going to assume that the National Park Service has asked us to evaluate the visual impact of building three proposed fire towers We need to perform a viewshed analysis and provide an estimate of the total area impacted within the park 236 Chapter 7 In order to accomplish this analysis we need to complete the following steps Clip elevation to the boundary of the park using GDAL Calculate viewsheds for towers using r viewshed Combine viewsheds using r mapcalculator LS Calculate raster statistics using r stats Clipping elevation to the boundary of a park using GDAL To reinforce the concept that we can make use of a variety of algorithms within the processing toolbox to accomplish our analyses we will use the clip raster by mask layer tool that is available through the GDAL ORG algorithms We will clip the
152. ation tif in combination with a hillshade image Jemez_hillshade img Both data sets are loaded and the vegetation data is dragged to the top of the layer list Vegetation is then styled with a Singleband pseudocolor renderer you can do this by performing the following steps 1 Choose Random colors 2 Set Mode to Equal interval 3 Set the number of Classes to 13 4 Click on Classify 5 Click on Apply The following screenshot shows what the Style properties should look like after following the preceding steps 4 Layer Properties Jemez_vegetation Style DA E General Y Band rendering Render type Singleband pseudocolor a Transparency Band Band 1 Gray Generate new color map A Pyramids Color interpolation Linear Random colors z Invert COMI node Equlintenal Cases 13 1 Min 1 Max 14 Value Color Label 1 000000 Agricultural Classify 2 083330 Conifer 3 166670 Conifer hardwood 4 250000 Developed 5 333330 Exotic herbaceous Estimated min max of full extent 6 416670 Exotic Tree shrub 7 500000 8 583330 Load min max values 9 666670 l 10 750000 Riparian Cumulative 55 11 833300 count cut x 12 916700 14 000000 Sparsely vegetated Min max fi Metadata Min max origin Mean standard deviation x 2 00 Extent Accuracy Full Estimate faster Current Actual slower Restore Default Style 21 www alliteboo
153. atureRequest 599 myFeature features next The feature 599 is the biggest polygon available in the Alaska shapefile Remember that get Features returns an iterator and not the feature directly so it s necessary to use next to get it The next method could generate a StopIteration exception when the iterator arrives at the end of the features list for example when the get Features result is empty In our case we don t care about this exception because we are sure that the feature with ID 599 exists 353 PyQGIS Scripting Changing a feature s geometry After getting feature 599 we can change its geometry We ll substitute its current geometry with its bounding box The code snippet to do this is as follows oldGeom myFeature geometry bbox oldGeom boundingBox newGeom QgsGeometry fromRect bbox newGeomMap myFeature id newGeom myDataProvider changeGeometryValues newGeomMap After we refresh the canvas the shapefile will appear as in the following screenshot The changeGeometryValues method accepts a QgsGeometryMap object This is the Python dictionary that contains the ID of the feature to change as a key and the new geometry as a value Changing a feature s attributes After getting feature 599 we can change its attributes in a way similar to how we changed the geometry of the feature in the previous subsection In this case the map is a QgsChangedAt tributesMap class that wi
154. be noted that the fourth parameter is None instead of an empty string but for the processing toolbox both are equivalent The fifth parameter is the output file name but it could be set to None In this case the output will be generated in a temporary file 362 Chapter 10 The return value of runalg is a dictionary with a key and the output name as specified in the alghelp and the value of the key is the reference of the filename generated by the algorithm In the preceding example the output dictionary is something like the OUTPUT_LAYER F temp alaska sqlite dictionary The last step is displaying the layer This can be done in two ways by loading a vector layer as usual or by running the algorithm with the runandload method as specified earlier In this case the following code will generate and load a new layer processing runandload gdalogr ogr2o0gr alaska 3 None F temp alaska This is shown in the following screenshot Python Console 1 Python Console 2 Use iface to access QGIS API interface or Type help iface for more info 3 gt gt gt processing runandload gdalogr ogr2o0gr alaska 3 None F temp alaska 4 lt processing algs gdal ogr20gr Ogr20gr instance at 0x000000000F678688 gt v a 3 Ei gt gt gt Coordinate 6040523 6802833 Scale 1 63 822 180 v Render EPSG 2964 A The new layer will be called Output layer which is the beautified name of the parameter
155. ce a layer with a binary level of measurement meaning the query is either true or false To execute the raster calculator select the layer representing elevation only in the park enter the formula gt a 1800 name the output file and click on OK 245 The Processing Toolbox The syntax we entered in the formula box tells the SAGA algorithm to look at the first grid in this case a and if it has a value greater than gt 1 800 meters the new grid value should be one otherwise it should be zero The following screenshot illustrates how this appears in the SAGA Raster calculator window We could have also used the native QGIS Raster calculator tool So the intent here is to demonstrate that there are numerous tools at our disposal in QGIS that often perform similar functions However the syntax is slightly different between the QGIS GRASS and SAGA raster calculators so it is important to check the Help tab before executing each of the tools kg Raster calculator Parameters Log Help Main input layer Elevation EP5G 26710 Additional layers optional 0 elements selected Formula gtta 1800 Result CiiMastering_OGIS S5464 eley_1800 tif X Open output File after running algorithm Clipping land use to the park boundary using Clip grid with polygon After executing this tool we will be presented with a new raster layer that identifies the elevations above 1 800 meters with a value of 1 and all other values with a
156. ch type of color ramp was selected Gradient Random ColorBrewer or cpt city Using the opened window the properties of the color ramp can be modified Now that color ramps have been discussed we will put the color ramps to work by styling raster data with color ramps and later we will use color ramps to style vector data Styling single band rasters In this section the three different band render types that are appropriate for single band rasters will be covered Single band rasters can be styled using three different band render types paletted singleband gray and singleband pseudocolor Note that even though raster color rendering and resampling are part of raster style properties they will be discussed separately in later sections as they are common to all single band and multiband raster renderers 65 Styling Raster and Vector Data The raster band render type should be chosen to best match e the type of data For instance a palette renderer is best used ar on rasters that represent discrete data such as land use Q classes The singleband gray would be a good choice for a hillshade while a singleband pseudocolor would work well on a raster containing global temperature data Paletted raster band rendering The paletted raster band renderer applies a single color to a single raster value QGIS supports the loading of rasters with paletted colors stored within and the changing the color assigned to the raster value QGI
157. checked the output composite raster will be added to the map canvas The output raster composite will contain three bands and it will be automatically styled as a multiband raster in QGIS Raster color rendering Raster color rendering modifies the properties of the raster to change the way it displays and interacts with the layer below it in the Layers panel Color rendering is a part of the raster style properties for all band renderer types and works in the same way regardless of the selected band renderer In this section we will discuss the parameters available for change in the Color rendering section of the raster style properties When a raster is first loaded the Color rendering parameters are set to their default values as shown in the following screenshot At any time the default values can be reloaded by clicking on the Reset button Color rendering Blending mode Normal Brightness Contrast Grayscale Off Hue Colorize Strength Saturation There are six parameters that can be set in the Color rendering section and these are as follows e Blending mode This applies a blending method to the raster that mixes with layers below it in the Layers panel A number of blending modes are available to choose from and these are commonly found in graphics editing programs There are 13 blending modes and these are as follows Normal This is the default blending node If the raster has any transparent cells the col
158. creenshot The ellipsis drop down button to the right of the palette select box provides seven handy functions to manage palettes Let s review each one Copy Colors This copies the selected color s in the current palette to the clipboard Paste Colors This pastes color s stored in the clipboard to the current palette Import Colors This imports colors from a GPL palette file and places them into the current palette Export Colors This exports the current palette to a GPL palette file New Palette This create a new empty palette that you can name The palette will persist in the color picker until it is removed Import Palette This imports a GPL palette file into the list of palettes The palette will persist in the color picker until it is removed Remove Palette This removes the current palette from the list Note that the three default palettes cannot be removed 56 Chapter 3 Color sampler The color sampler sets the current color to a color sample collected from the screen using the mouse pointer The color sample is based on the average of all colors under the mouse pointer within the specified Sample average radius value To collect a sample color click on the Sample color button then move the mouse cursor to a location where you want to sample a color and then either press the spacebar or click to collect the sample As you move the mouse cursor around a preview of the sample color will appear under the Sa
159. ct all these events to print commands This is a good way to learn the event sequence that is generated during vector editing For example the following code snippet in the editing console will print a message every time someone starts to edit the vector layer myVector def printMessage print Editing is Started myVector editingStarted connect printMessage In the last line we instruct Python to call the printMessage function every time the editingStarted event is emitted by myVector In this case the printMessage function is usually known as a callback or a listener Discovering the QgsVectorLayerEditBuffer class It s possible to have a fine grained control of the editing session managing event of the QgsVectorLayerEditBuffer class that stores all modifications of the layer It s also possible to access the buffer using the vector layer with the following code myEditBuffer myVector editBuffer A detailed description of this class is outside the scope of this chapter but it s strongly suggested to explore it to discover all the PyQGIS editing opportunities such as setting the attribute of a new feature based on some parameters of the nearest geometry of another layer Changing a feature s geometry After we have reloaded the original Alaska shapefile and got feature 599 we can change its geometry We ll substitute the current geometry with its bounding box and the code snippet to do this is as follows oldGeom
160. ct this option as illustrated in the following screenshot The resulting batch processing interface allows us to enter the coordinates for all three towers and export a separate viewshed for each tower The toolbox options can be set to the following values e Elevation This is the elevation for the park e Coordinate identify viewing position This contains the coordinates for each tower e Viewing position height above ground This is the viewing position height for example 20 e Maximum distance from the viewing point This is the maximum distance from the viewing point for example 32000 e Consider Earth Curvature This contains a Yes value if Earth s curvature should be considered e GRASS GIS Region 7 extent This option is set to default e GRASS GIS 7 region cellsize This option is set to 0 0 e Output raster layer Set output names as towerl tower2 tower3 e Load in QGIS This option is set to Yes if viewshed loads in QGIS 238 Chapter 7 1 Q If we had more than three towers we could click on the Add row button at the bottom of the batch processing interface We can begin entering the necessary parameters using the coordinates provided in the following table and the guidelines 574599 082827 4749967 314004 580207 025953 4752197 343687 571656 050455 4750321 28697 It is worth exploring the rationale behind some of the input parameters The first two are hopefully obvious we need an elevation layer
161. cterize potential habitat If the intent is to merely show a visual and not perform any spatial analyses remember from Chapter 3 Styling Raster and Vector Data that we can symbolize the elevation layer using colors rather than greyscale to better visualize changes in elevation Adjusting default algorithm settings for cartographic reasons The default altitude and azimuth settings specify the position of the sun relative to the landscape which isn t an unrealistic value for Crater Lake However it is possible to move the sun to an unrealistic position Q to achieve better contrast between topographic features For an extensive exploration of shaded relief techniques visit http www Shadedrelief com To calculate accurate azimuth and elevation values for varying latitudes visit http www esrl noaa gov gmd grad solcalc azel html 227 The Processing Toolbox Calculating the least cost path Least cost path LCP analyses have been used to model historical trade routes and wildlife migration corridors plan recreation and transportation networks and maximize safe backcountry travel in avalanche prone areas to name just a few applications To perform a LCP analysis in QGIS we are going to use a variety of tools from the processing toolbox and combine the resulting output from the tools Although there are numerous useful geoprocessing algorithms in the GRASs library we are going to focus on more advanced spatial analyses th
162. culation uses two functions to insert the x coordinate for a point and the x coordinate of a specific vertex for a line or polygon First we will calculate the x coordinate of a point To do this perform the following steps 1 Open the Field calculator window 2 Select Create a new field and set the following options O Output field name XCoord O Output field type Decimal number real Output field width 10 Output field precision 7 3 In the Function list field expand Geometry and then double click on x to add it to the Expression area This function returns the x coordinate of a point geometry The following figure shows the completed calculation XCoord 7010162 7910105 7630980 8 125640 Now let s calculate the first and last x coordinate for a line or polygon 1 Open the Field calculator window 2 Select Create a new field and set the following options O Output field name XCoord1 O Output field type Decimal number real Output field width 10 Output field Precision 7 3 Inthe Function list field expand Geometry and then double click on xat to add it to the Expression area This function returns the x coordinate of a vertex specified by a 0 based index number Inside the parentheses you will need to specify the index of the vertex whose coordinates you wish to retrieve For example to retrieve the x coordinate of the first vertex the command will be xat 0 You can also s
163. d Shut down QGIS Move the qgis2 python plugins processing folder to your desktop and relaunch QGIS The folder will be rebuilt and the most recent version of SAGA will be used 3 Identify the installation path for SAGA In QGIS navigate to Settings Options System and make sure that the PATH variable includes the path to the SAGA binaries This can be found under Current environment variables Linux The SAGA binaries must be installed They can be found at http sourceforge net p saga gis wiki Binary 20Packages There are packages available for Debian Ubuntu and FreeBSD For example SAGA can be installed on Ubuntu by running the following commands sudo add apt repository ppa johanvdw saga gis sudo apt get update sudo apt get install saga 153 Preparing Raster Data for Processing If you encounter the missing dependencies error perform the following steps 1 Open SAGA by navigating to Processing Options Providers and uncheck Use SAGA 2 0 8 syntax Processing options Setting v amp Providers gt GDAL OGR gt y GRASS GIS 7 commands gt 4 GRASS commands e 4 Modeler only tools gt Orfeo Toolbox Image analysis gt QGIS geoalgorithms gt R scripts v SAGA Activate Zj amp Enable SAGA Import Export optimizations Log console output Sj amp Log execution commands i amp Use SAGA 2 0 8 syntax a gt i TauDEM hydrologic analysis 2 If you
164. d all the available methods for this reason it s best to read the API documentation Here we will give you only a brief introduction to some of the useful and commonly used methods It s possible to have the length and area methods of the geometry when these values have sense depending on geometry type For the geom get in the previous paragraph the following code print Is this a Polygon geom type QGis Polygon print it s length is geom length print it s area measure geom area print Is it multipart geom isMultipart This will generate the following output Is this a Polygon True it s length is 19143 8757902 it s area measure 8991047 15902 Is it multipart False The area and length unit depend on the myVector CRS and can be obtained with the following code myVector crs mapUnits This returns 0 for QOGis Meters 1 for QGis Feet or 2 for QGis Degrees Other interesting methods of the QgsGeomet ry class are related with spatial operators such as intersects contains disjoint touches overlaps simplify and so on Useful methods can be found to export geometry as a Well Know Text WKT string or to GeoJSON with export ToWkt and export ToGeoJSON 349 PyQGIS Scripting There are a bunch of static methods that can be used to create geometry from a WKT or QGIS primitives such as QgsPoint QgsPolygon and so on A static method is a method that can be called without an instance va
165. d and output to a new layer In the following example we are interested in identifying street intersections By extracting the nodes from the Selected_streets shp layer a point for each intersection is generated as shown in the following screenshot 112 Chapter 4 Street lines and extracted nodes representing intersections Simplifying and densifying features The Simplify Geometries and Densify Geometries tools which can found by navigating to Vector Geometry Tools remove and add vertices respectively They are only suitable for line and polygon data Simplifying data may be desirable to make it more suitable for use at a smaller scale It may also be helpful if the data is to be used in an online interactive mapping scenario The simplify tool uses a modified Douglas Peucker algorithm that reduces the number of vertices while attempting to maintain the shape of the features In the following example the polygons of the Neighborhoods_pdx shp data are simplified with a simplify tolerance of 20 The tolerance is specified in map units 113 Preparing Vector Data for Processing Here the data is in a State Plane coordinate reference system with units in feet With the Simplify tolerance field set to 20 the algorithm will try to eliminate vertices within 20 feet of one another With this setting the number of vertices is reduced from 31 637 to 6 189 with almost no change in the shapes of the polygons This reduced the size o
166. d maximum value button Increase size of small diagrams If checked this parameter sets the Minimum size value to which the charts will be scaled Appearance Size Position Options Fixed size Size units mm Scale linearly between 0 and the following attribute value diagram size Attribute POP10 Find maximum value 0000 Size 40 lt Scale Area M X Increase size of small diagrams Minimum size 17 00 Diagram position parameters The Position tab shown in the following screenshot provides the following parameters e Placement This sets the placement of the chart The available options are Around point Over point Line Horizontal and Free 94 Chapter 3 Data defined position If this is checked the x and y positions of the chart can be set by attributes Automated placement settings This provides more parameters to fine tune the placement of charts such as showing all charts and showing partial labels Appearance Size Position Placement Over Point Distance 0 00 Data defined position Automated placement settings Adding attributes to diagrams Each of these diagram types supports the display of multiple attributes To add or remove attributes you must move or build an expression from the Available attributes list to the Assigned attributes list Attributes in the Assigned attributes list will be used in the diagram There are two ways to add an attribute to the Assigned attributes list
167. d naming a model 274 Adding inputs 276 Adding algorithms 279 Running a model 284 Editing a model 286 Documenting a model 288 Saving loading and exporting models 290 Executing model algorithms iteratively 292 Nesting models 294 Using batch processing with models 298 Converting a model into a Python script 299 Summary 300 Chapter 9 Creating QGIS Plugins with PyQGIS and Problem Solving 301 Webography where to get API information and PyQGIS help 301 PyQGIS cookbook 302 API documentation 302 The QGIS community mailing lists and IRC channel 303 Mailing lists 303 IRC channel 304 The StackExchange community 304 Sharing your knowledge and reporting issues 304 The Python Console 306 Getting sample data 307 My first PYQGIS code snippet 307 My second PyQGIS code snippet looping the layer features 308 Exploring iface and QGis 308 vi Table of Contents Exploring a QGIS API in the Python Console 310 Creating a plugin structure with Plugin Builder 310 Installing Plugin Builder 311 Locating plugins 311 Creating my first Python plugin TestPlugin 312 Setting mandatory plugin parameters 313 Setting optional plugin parameters 314 Generating the plugin code 315 Compiling the icon resource 315 The plugin file structure where and what to customize 317 A simple plugin example 319 Adding basic logic to TestPlugin 319 Modifying the layout with Qt Designer 319 Modifying the GUI logic 321 Modifying the plugin logic 322 Setting up a
168. d numbers to make it easier for humans to delineate classes 3 Create the classes to list by either clicking on the Classify button to add a class for each unique attribute that is found otherwise click on the Add Class button to add an empty class and then double click in the Value column to set the attribute value range to be used to create the class Classes can be removed with the Delete or Delete All buttons They can be reordered by clicking and dragging them up and down the list Classes can also be modified by double clicking in the Value and Label columns 4 Set the symbol for all classes by clicking on the Symbol button to open the Symbol selector window Individual class symbols can be changed by double clicking on the Symbol column of the class list 5 Choose the color ramp to apply to the classes Individual class colors can be changed by double clicking on the Symbol column of the class list The Legend Format field sets the format for all labels Anything can be typed in the textbox The lower boundary of the class will be inserted where 1 is typed in the textbox and the upper boundary of the class will be inserted where 2 is typed If Link class boundaries is checked then the adjacent class boundary values will be automatically changed to be adjacent if any of the class boundaries are manually changed Other symbol options such as transparency color and output unit are available by right clicking on a category row
169. d to the boundary of the left side parcel and the overlap has been repaired Click on Validate Extent in the Topology Checker panel to ensure that the overlap has been solved If so no errors will be listed in the Topology Checker error table 5 Any remaining overlaps can be fixed repeating steps 1 4 The repaired parcel is shown in the following figure Topology Checker Error Layer Feature ID Overlap repaired 212 Chapter 6 Example 3 repairing a gap between polygons This example we will continue to work with the parcel shp polygon layer Here we will focus on the first gap error listed in the Topology Checker error list It has a feature ID of 0 1 Ensure that editing is still toggled on for the parcels layer 2 Double click on the error in the table so that QGIS will zoom to the area You will see a small horizontal gap between two parcel polygons as shown in the following figure A small gap between two parcels The same editing parameters that were set in Example 2 will be used here Zoom in a bit closer to the problem area Select the parcel to the north with the Select Feature by Rectangle tool ON OM eS 2 Using the node tool select the lower left vertex and drag it to snap with the other two parcels and close the gap Moving a vertex to repair or close the gap 213 Advanced Data Creation and Editing 7 Verify that the issue has been resolved by clicking on Validate Extent in t
170. d with a refresher on composing maps and how to find install and manage plugins The next chapter will cover creating spatial databases Data is the foundation of any GIS Now that you have had a refresher on the basics of QGIS it is time to learn how to expand your work to include spatial databases In Chapter 2 Creating Spatial Databases you will learn how to create and manage spatial databases within QGIS 26 Creating Spatial Databases This chapter covers the creation and editing of spatial databases using QGIS The core concepts of databases will be briefly reviewed however we have assumed that you are generally familiar with database concepts and SQL for most of the content covered in this chapter The topics that we will cover in this chapter are as follows Core concepts of database construction Creating spatial databases Importing and exporting data Editing databases Creating queries Creating views Fundamental database concepts A database is a structured collection of data Databases provide multiple benefits over data stored in a flat file format such as shapefile or KML The benefits include complex queries complex relationships scalability security data integrity and transactions to name a few Using databases to store geospatial data is relatively easy considering the aforementioned benefits There are multiple types of databases however the most common type of database and the type of database
171. debugging environment 326 What is a debugger 326 Installing Aptana 32 Setting up PY THONPATH 327 Starting the Pydevd server 328 Connecting QGIS to the Pydevd server 329 Debugging session example 330 Creating a PyDev project for TestPlugin 331 Adding breakpoints 333 Debugging in action 334 Summary 335 Chapter 10 PyQGIS Scripting 337 Where to learn Python basics 337 Tabs or spaces make your choice 338 Loading layers 338 Managing rasters 339 Exploring QgsRasterLayer 340 Visualizing the layer 341 Managing vector files 342 Managing database vectors 343 Vector structure 345 The basic vector methods 345 Describing the vector structure 346 Describing the header 347 Describing the rows 348 Iterating over features 350 Describing the iterators 352 vii Table of Contents Editing features Updating canvas and symbology Editing through QgsVectorDataProvider Changing a feature s geometry Deleting a feature Adding a feature Editing using QgsVectorLayer Discovering the QgsVectorLayerEditBuffer class Changing a feature s attributes Adding and removing a feature Running processing toolbox algorithms Looking for an algorithm Getting algorithm information Running algorithms from the console Running your own processing script Creating a test processing toolbox script Looking at the custom script Running the script Running an external algorithm or command Running a simple command Interacting with the map canvas Getting the m
172. dels We also covered how to execute models iteratively nest models within models and run them in batch mode With what has been covered to this point you should understand how to work with a variety of vector raster and tabular data You should also be well versed with the geoprocessing and analytic capabilities of QGIS In the next chapter we will switch from conducting analyses with the graphical modeler and the Processing Toolbox to expanding the functionality of QGIS with Python In Chapter 9 Creating QGIS Plugins with PyQGIS and Problem Solving you will learn how to create a QGIS plugin from scratch The chapter will begin with a primer on PyQGIS You ll learn where you can get API information and other PyQGIS help We will then explore plugin file structure and the available functions The chapter will conclude with a simple step by step example of writing a QGIS plugin This will also include information on debugging your code 39 J Creating QGIS Plugins with PyQGIS and Problem Solving This chapter focuses on the basic information necessary to start developing a QGIS plugin Topics that will be approached will include the following e Where to get help to solve your PyQGIS problems e How to setup a development environment that can resolve PyQGIS5 and PyQt API names during code editing e How to interactively test your snippet using the QGIS Python console and some useful classes that can be used everywhere e Creating a basic
173. dinates dialog box will open Enter map coordinates Enter X and Y coordinates DMS dd mm ss ss DD dd dd or projected coordinates mmmm mm which correspond with the selected point on the image Alternatively cick the button with icon of a pendil and then dick a corresponding point on map canvas of QGIS to fill in coordinates of that point Te Snap to background layers OK From map canvas Cancel You are now halfway done entering the GCP Follow the appropriate directions below These will be different workflow depending on whether you are georeferencing against a second loaded dataset or against benchmarks datums or other printed coordinates on the input raster If you are georeferencing against a second loaded dataset you need to follow these steps 1 i Click on the From map canvas button Locate the same GCP spatial location on the data loaded in the main QGIS Desktop map canvas Click on that GCP location Use the zoom and pan tools so that you can precisely click on the intended GCP location If you need to zoom in to the dataset within the QGIS Desktop map canvas you will have to first zoom in and then click on the From map canvas button again to regain the Add point cursor and enter the GCP 186 Chapter 6 5 The Enter map coordinates dialog box will reappear with the X East and Y North coordinates entered for the point where you clicked on the QGIS Desktop map canvas 4 Enter map coo
174. ding address based data 176 How address geocoding works 177 The first example geocoding using web services 178 The second example geocoding using local street network data 180 Georeferencing imagery 184 Ground control points 184 Using the Georeferencer GDAL plugin 185 The first example georeferencing using a second dataset 188 Getting started 188 iv Table of Contents Entering the ground control points 189 Transformation settings 191 Completing the operation 197 The second example georeferencing using a point file 197 Checking the topology of vector data 200 Installing the Topology Checker 200 Topological rules 200 Rules for point features 201 Rules for line features 201 Rules for polygon features 202 Using the Topology Checker 203 Repairing topological errors via topological editing 207 Example 1 resolving duplicate geometries 208 Example 2 repairing overlaps 208 Setting the editing parameters 208 Repairing an overlap between polygons 211 Example 3 repairing a gap between polygons 213 Summary 214 Chapter 7 The Processing Toolbox 215 About the processing toolbox 216 Configuring the processing toolbox 216 Understanding the processing toolbox 219 Using the processing toolbox 223 Performing raster analyses with GRASS 223 Calculating the shaded relief 225 Calculating least cost path 228 Calculating the slope using r slope 229 Reclassifying a new slope raster and the land use raster 230 Combining reclassi
175. e For this example we will format the current date as dd mm yyyy 1 Open the Field calculator window 2 Select Create a new field and set the following options O Output field name Updated O Output field type Text string Output field width 10 3 Inthe Function list field expand the String node and then double click on format_date to add it to the Expression area This function takes two arguments a time string and a string representing the format to convert the time string to We will use the current date function for the time and write a format string 4 Inthe Function list field expand the Date and Time node and then double click on now to add it to the Expression area after the open parenthesis 5 Type a comma after Snow and enter dd MM yyyy followed by a closed parenthesis The snow function returns a string representation of the current time and date The following figure shows the completed calculation 6 Click on OK to execute the calculation This will enable editing on the layer and calculate the field values 7 Save the edits to the layer and disable the editing mode The calculated values are now stored in the layer The following figure shows a sample of the calculated and formatted date Updated 29 11 2014 29 11 2014 9 35 Preparing Vector Data for Processing The second example inserting geometric values The second example of an advanced field cal
176. e you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then click on Refresh under Database or press the F5 key on your keyboard Note that the Peaks table has the point icon indicating that it is a geometry table Renaming a table To rename a table perform the following steps 1 Open DB Manager by clicking on DB Manager under Database In the Tree panel expand the tree and select the database that contains the table that you wish to rename In the Tree panel select the table you wish to edit Right click on the table and choose Rename from the contextual menu to rename it Editing table properties To edit table properties perform the following steps 1 Open DB Manager by clicking on DB Manager under Database In the Tree panel expand the tree and select the database that contains the table that you wish to edit In the Tree panel select the table that you wish to edit Navigate to Table Edit table to open the Table properties window The Table properties window shown in the following screenshot has three tabs Columns Constraints and Indexes that allow the editing of their respective table properties Table properties Columns Constraints Indexes Table columns Type Null Default INTEGER True REAL True TEXT True TEXT True True True ELEVATION REAL True QUADNAME TEXT True Northing REAL True Easting REAL True geom POINT True Add column
177. e Fire towers file towers shp Calculating shaded relief The basic requirement for many of the tools within the GRASS library is a digital elevation model DEM or digital terrain model DTM However since a DEM is a layer that contains continuous data representing elevation when we load a DEM into QGIS or any GIS for that matter it has a flat appearance Therefore it is sometimes difficult to visually evaluate how topography might influence the results of our analyses So our first foray into the GRASS library will make use of the r shaded relief tool to create a shaded relief map or hillshade which can provide some topographic context for spatial analyses Remember that you can access this algorithm by using the processing commander search bar or by navigating through the GRASS GIS commands list Once the dialog box is open we need to select the elevation layer of interest in this case the elevation layer Leave all the default parameters the way they are as illustrated in the next screenshot and click on Run 225 The Processing Toolbox Changing the default save option M By default QGIS saves any new layer as a temporary file in memory To save all your output files to a directory you need to click on the small button containing three dots and specify the location where file needs to be saved x Eel Q rshaded relief Creates shaded relief from an eivai layer DEM Parameters Log Help Input elevat
178. e ID of the layer inside QGIS and the value is an instance of the QgsMapLayer class that can be a vector or raster layer The variable layersDict has the suffix Dict only for a didactic J reason but it could simply be named layers 322 Chapter 9 Classifying layers The next step is creating a list of vector and raster layer names This can be achieved by looping layersDict in the following way from gqgis core import QgsMapLayer vectors rasters for id map in layersDict items if map type QgsMapLayer VectorLayer vectors append map name elif map type QgsMapLayer RasterLayer rasters append map name else print Not Raster nor Vector for layer with id id The first line which is the import line is necessary to allow the use of the QgsMapLayer class The next two lines initialize two empty Python lists that will be filled with the layer names Adding an element to an array is done with the Python command append The layers are looped separating each key value couple directly into a couple of variables named id and map The id variable is used to display a warning message in line 10 The map variable is used with the methods type and name of the QgsMapLayer class The map type call returns an enumerator value that can be 0 1 or 2 but it s better to compare it with the symbolic name of this constant to allow more readability Populating the combobox After
179. e PyQGIS classes However we will not explain all the possibilities of the Python Console These are well documented and you can find them at http docs qgis org testing en docs pyqgis developer cookbook intro html python console Getting sample data To continue experimenting with PyQGIS we need a test dataset The QGIS project has training material and sample data that we ll use in our snippet The QGIS sample data can be downloaded from http qgis org downloads data My first PyQGIS code snippet To break the ice we will create our first PyQGIS code to show the unique ID of a selected layer loaded in QGIS 1 To start with we will first load the layer airports shp which is available in the shapefiles directory of qgis_ sample data 2 After it has been loaded successfully select it in the list of layers doing this will make it the active layer for QGIS We can also do this by writing the following line in the the Python Console command line layer iface activeLayer 3 After editing the code and hitting Return the edited command code is shown in the console 4 The reference of the current active layer is archived in the variable named layer Now we can show the layer ID by typing the following code command in the command line after pressing Return print layer id The output will display something similar to airports20141001174143539 307 Creating QGIS Plugins with PyQGIS and Problem Solving My seco
180. e Symbol component tree in the bottom left corner lists the layers of symbol components Clicking on each layer changes the symbol parameters so that the symbol can be changed As an example of how to use the Single Symbol style properties to create a circle around a gas pump a second layer with the SVG marker symbol layer type was added by clicking on the Add symbol layer button and was then moved on top of the circle by clicking on the Move up button The following figure shows the parameters used to create the symbol 0 200000 Aschor point HCenter Dala defieed properties achor pont Verto Data defined properties KR iH To save your custom symbol to the Style Manager click on the Save button to name and save the style The saved style will appear in the Style Manager and the list of library symbols Categorized vector styling The categorized vector style applies one symbol per category of the attribute value s This vector style is the best when you want a different symbol that is based on attribute values such as when styling country polygons or classes of roads lines The categorized vector style works best with nominal or ordinal attribute data 81 Styling Raster and Vector Data The following screenshot shows the Categorized style type with parameters for point vector data of schools Its properties will be very similar to those for line and polygon vector data a Categorized Co
181. e The plugin_upload py file is a command line utility to upload the plugin in the QGIS plugin repository e The README files contain the messages displayed at the end of plugin generation A simple plugin example The goal of this section is to customize TestPlugin to classify the loaded layers in the raster and vectors and respectively populate two comboboxes with the layer names Adding basic logic to TestPlugin As said previously to customize TestPlugin we have to modify some code portions in the files test_plugin_dialog_base ui for the GUI layout test_plugin_ dialog py for the GUI logic and test_plugin py for the plugin logic Modifying the layout with Qt Designer The default plugin GUI layout has only two buttons Ok and Cancel Here we will add two comboboxes that will be populated by the logic of the plugin in test _ plugin py 319 Creating QGIS Plugins with PyQGIS and Problem Solving To edit the test_plugin_dialog_base ui GUI layout open it with Qt Designer which will show the interface of the following screenshot File Edit Form View Settings Window Help AaREaMWSeHteese ow OU My First jest Plugin test plugin dialop bas e Layouts Ik rt button_box Vertical Layout I Horizontal Layout Layout Area 323 Grid Layout Property Editor Fit 33 Form Layout Spacers pad Horizontal Spacer 8 Vertical Spacer Buttons ex Push Button Cancel WN Tool Button Radio Button
182. e center pixel would have the same value In QGIS there are several tools that can be used to resample a raster In this example the GDAL Translate tool will be used The resolution of the elevation tif sample data will be changed from 27 3526 meters to 100 meters While the Translate tool can be found by navigating to Raster Conversion Translate you will use the Processing Toolbox implementation of it here because it has better options for specifying the output pixel size As you ll see this tool can be used for a variety of raster conversions during the resampling process As you ll see in the following steps this tool can be used for a variety of raster conversions during the resampling process 1 Open the Processing Toolbox 2 Switch to the advanced view 3 Locate the tool by navigating to GDAL OGR Conversion Translate convert format 4 Select the Input layer raster by clicking on the down arrow to choose a raster loaded in the canvas or by clicking on the browse button 149 Preparing Raster Data for Processing 5 10 11 For a specific output resolution enter the number in the Set the size of the output file In pixels or box To change the resolution by a percentage click on the Output size is a percentage of input size box If there are cells to designate as Nodata cells enter the value in the Nodata value leave as none to take the nodata value from input field Otherwise leave this option bl
183. e doing 32 Chapter 2 Importing data into a SpatiaLite database Importing data into a SpatiaLite database is easy using the DB Manager SpatiaLite supports the following formats for importing files e Shapefile shp e Dbase dbf e Text txt Commas Separate Values csv and Excel spreadsheets x1s e Well known Text wkt and Well known Binary wkb e PostGIS ewkt ewkb e Geography Markup Language gm1 e Keyhole Markup Language km1 e Geometry JavaScript Object Notation geojson e Scalable Vector Graphics svg Let s use DB Manager to import data in a few different formats into our GiffordPinochet sqlite database Importing KML into SpatiaLite To import a KML file into a SpatiaLite database complete the following steps 1 Open DB Manager by clicking on DB Manager under Database Expand SpatiaLite and select GiffordPinochet sqlite on the Tree panel Navigate to Table Import layer file to open the Import vector layer dialog Click on the ellipsis button at the right hand side of the Input drop down box and select and open streams km1 from the sample dataset that is available for download on the Packt Publishing website 4 Click on the Update options button to load the remainder of the dialog box The output table name will populate as streams and it will match the base name of the input file 5 Set the following options as shown in the next screenshot Select Source SRID and enter 432
184. e how multiband rasters are rendered in QGIS Add TL_ASTER jpg from the sample data to the QGIS canvas This sample image is a TerraLook image derived from an ASTER image Open the Style tab from Layer Properties As this is a multiband raster QGIS defaults the Render type field to Multiband color with the parameters shown in the following screenshot Band rendering Render type Multiband color Red band Band 1 Red Load min max values Minimax 0 a Cumulative 59 e count out Green band Band 2 Green Min max Minimax Mean 300 Blue bane Band 3 Blue standard deviation x z Min max Extent Accuracy Contrast SS E Full Estimate faster enhancement Current Actual slower Load 73 Styling Raster and Vector Data The multiband color renderer allows you to designate which raster band will be applied to each of the three color bands which you can think of as color ramps In the preceding screenshot the raster has three bands with Band 1 Red applied to Red band Band 2 Green applied to Green band and Band 3 Blue applied to Blue band To change which raster band is applied to the color bands select the band from the drop down boxes The drop down boxes will list all the bands stored in the raster as well as Not set that is shown in the following screenshot Choosing Not set does not apply a raster band to a color band Band 1 Red Band 2 Green Band 3 Blue After exploring
185. e method The connection is inside try catch to allow catching an exception raised in case settrace cannot connect The connection will take some seconds to connect If the connection fails the Python Console will show a message similar to the following one Could not connect to 127 0 0 1 5678 Traceback most recent call last File mnt data PROGRAMMING IDE Aptana Studio 3 plugins org python pydev 3 8 0 201409251235 pysrc pydevd_ comm py line 484 in StartClient s connect host port File usr lib python2 7 socket py line 224 in meth return getattr self sock name args error Errno 111 Connection refused If the connection is successful the Aptana Debug Perspective will change showing the connected clients Debugging session example Here we will show how to debug the TestPlugin remotely We ll also learn how to insert a code breakpoint to stop executions and to show variable values during executions The steps to follow are as follows 1 Create a PyDev project that points to the source code of the TestPlugin 2 Adda breakpoint to the TestPlugin run function in the Aptana Debug Perspective Start the PyDev Debug server Connect to the PyDev server from QGIS Run the plugin Explore the variable values ei SON ey 2 Continue the execution of the plugin 330 Chapter 9 Creating a PyDev project for TestPlugin To be able to add code breakpoints it s necessary to load test_plugin py
186. e minimum and maximum values change Click on Apply Default singleband contrast enhancement left and more accurate contrast enhancement right You can specify the default settings for rendering rasters RS by navigating to Settings Options Rendering Here Q the defaults for the Contrast enhancement Load min max values Cumulative count cut thresholds and the standard deviation multiplier can be set Blending modes The blending modes allow for more sophisticated rendering between GIS layers Historically these tools have only been available in graphics programs and they are a fairly new addition to QGIS Previously only layer transparency could be controlled There are now 13 different blending modes that are available Normal Lighten Screen Dodge Addition Darken Multiply Burn Overlay Soft light Hard light Difference and Subtract These are much more powerful than simple layer transparency which can be effective but typically results in the underneath layer being washed out or dulled With blending modes you can create effects where the full intensity of the underlying layer is still visible Blending mode settings can be found at the bottom of the Style menu under Layer Properties in the Layer Rendering section along with the Layer transparency slider They are available for both vector and raster datasets 20 Chapter 1 In this example of using blending modes we want to show vegetation data Jemez _ veget
187. e the field calculator will report a syntax error instead of an output preview The following are common syntax rules for expressions Operators should be placed without any special formatting For example Fields should be surrounded by double quotes For example State Name Text string values should be surrounded by single quotes For example Washington 133 Preparing Vector Data for Processing e Whole numbers integer and decimal numbers float should be entered without any surrounding characters For example 153 27 e Functions come in two types as follows O Functions requiring parameters These begin with a function name followed by a set of parentheses Inside the parentheses are function parameters separated by commas For example log base value Functions not requiring parameters These begin with a dollar sign followed by the function name For example area If this is a little confusing don t worry you can rely on the field calculator to enter a portion of the syntax for you correctly To add an operator field or function to the expression double click on the desired item in the function list and it will be added to the cursor location in the expression In addition to adding functions through the function list the field calculator can also add to the expression any value that currently exists in any field To do this expand the Fields and Values branch of the function list tree A lis
188. e to it Clicking on the close button X will delete the input from the model For our example we will add a second vector layer Trees is added as a required polygon layer Finally we will add a number input This will allow us to expose the buffer distance value as an input that can be changed when the model is executed It will be named Buffer distance and it will be given a default value of 100 since 100 meters is the distance that we initially want to use 277 Automating Workflows with the Graphical Modeler al Q In the graphical modeler distances are expressed in coordinate reference system units Parameter definition Parameter name Buffer distance The following screenshot shows the model with the two vector layer inputs and a number input 4 Processing modeler amp gt BH s amp Parameters SP Boolean oP Extent dP File cP Buffer distance de Number qP Raster layer TP String oP Table SP Table field qP Vector layer A model with three inputs 278 Chapter 8 Adding algorithms Algorithms are added to the graphical modeler in the same way as inputs Find the algorithm from the Algorithms tab and either double click on it or drag it onto the modeler canvas You can search for tools as you would in the Processing Toolbox Type the name into the search box at the top of the Algorithm tab The Processing Toolbox has two interface settings Advanced and Simplified The interface setting t
189. e written to this file This allows you to easily see and troubleshoot any unmapped records 10 Click on OK The status of the geocoding operation can be seen in the lower left corner of QGIS The word Geocoding will be displayed followed by the number of records that have been processed 11 The output will be a point shapefile and a CSV file listing the addresses that were not matched The following screenshot shows the completed Web Service Geocode tool Web Service Geacode Input CSV File UTF 8 D PACKT Data Chapter 6 Addresses csv Address Field City Field STREE CITY State Field Country Field STATE COUNTRY Web Service Google Maps Output Shapefile D PACKT Data Chapter 6 Addresses shp Not Found Output List D PACKT Data Chapter 6 notfound csv OK 179 Advanced Data Creation and Editing The second example geocoding using local Two additional attribute columns will be added to the output address point shapefile addrtype and addrlocat These fields provide information on how the web geocoding service obtained the location These may be useful for accuracy assessment addrtype is the Google lt t ype gt element or the OpenStreetMap class attribute This will indicate what kind of address type was used by the web service highway locality museum neighborhood park place premise route train station university and so on addrlocat is the Google lt location_type gt element or Op
190. eaningful categories For example elevation may be an important input in a habitat model for species X However you may only be interested in identifying several broad elevation thresholds that help to define the habitat In the following example you will use the elevation tif sample data You will reclassify the elevation data into several categories less than 2000 meters 2000 to 2500 meters and greater than 2500 meters This will result in a raster with three values one for each group of elevation values The following steps outline how to use the r recode GRASS tool found in the processing toolbox to accomplish this 1 Load elevation tif and set the project s CRS to EPSG 26912 145 Preparing Raster Data for Processing 2 Turn on the Processing plugin by navigating to Plugins Manage and Install Plugins if it is not enabled Open the Processing panel by clicking on Toolbox under Processing l The Processing Toolbox is covered in more detail in Chapter 7 The Processing Toolbox To help locate the tool type recode into the Processing Toolbox search bar and hit the Enter key Double click on the tool to open it Select the input layer by clicking on the down arrow to choose a raster loaded in the canvas or by clicking on the browse button Next the tool will ask for a value to be filled in the File containing recode rules optional field This file has to be created in a text editor The syntax for the recode rul
191. eas that are less than 1 000 meters of surface water but greater than 0 If we just query for values less than 1 000 meters we will produce an output that will suggest that the roosting habitat exists within bodies of water Querying the proximity for 1 000 meters of water using the GDAL Raster calculator The easiest way to perform this query is by using the native QGIS Raster calculator tool by clicking on Raster Calculator under Raster The following screenshot illustrates how to enter the Proximity to Water 1 gt 0 AND Proximity to Water 1 lt 1000 syntax to identify a range between 0 and 1 000 meters Raster calculator Raster bands Result layer Land Use Land Class 1 Output layer lastering OGIS SAGA buf water tif m Park Elevation 1 tput lay ing _QGIS SAGA buf pn Proximity to Water 1 Elevation gt 1800 1 Current layer extent Land Use water 1 Water 0 500 a 1 min 556959 77007 XMax 584576 59726 Elevation 1 hillshade 1 Y min 4735343 81175 Ymax 4770938 57566 Columns 1350 Rows 1740 Output format x Add result to project Y Operators Raster calculator expression Proximity to Water 1 gt 0 AND Proximity to Water 1 lt 1000 Expression valid 249 The Processing Toolbox The resulting output will contain the values of 0 and 1 where 1 represents the cells that are within 1 000 meters of surface water and 0 represents those that are beyond 1 000 m
192. eate a single watershed However if we wanted to identify sub watersheds for each individual reach we could select No for the Delineate Single Watershed option The resulting watershed and stream reach shapefile are shown in the next screenshot with the original river s layer illustrating the improved visualization of potential surface flow upstream of the town Hopefully this brief exercise demonstrates the potential applications of TauDEM for exploring hydrologic conditions using high resolution elevation data Although the final output consists of only models of how water might flow across the surface the clear alignment with the provided river s shapefile and the resulting stream network shapefile suggests that the default assumptions are useful for delineating watersheds from user specified pour points and for estimating potential stream networks within this watershed 261 The Processing Toolbox R R is a standalone open source language and environment that is useful for performing statistical analyses and graphically visualizing data Users typically make use of this language within a command line interface or a GUI based software such as RStudio Within either environment users can work with geospatial data by installing additional packages such as SP RGDAL SpatioTemporal and so on However we can also access a number of these powerful statistical tools through the processing toolbox Similar to all the previous algorit
193. eatures Copy Features and Paste Features tools Snapping Snapping is an important editing consideration It is a specified distance tolerance within which vertices of one feature will automatically align with vertices of another feature The specific snapping tolerance can be set for the whole project or per layer The method for setting the snapping tolerance for a project varies according to the operating system which is as follows For Windows navigate to Settings Options Digitizing For Mac navigate to QGIS Preferences Digitizing For Linux navigate to Edit Options Digitizing 15 A Refreshing Look at QGIS In addition to setting the snapping tolerance here the snapping mode can also be set to vertex segment or vertex and segment Snapping can be set for individual layers by navigating to Settings Snapping Options Individual layer snapping settings will override those of the project The following screenshot shows examples of multiple snapping option choices Snapping options pping opt Layer Mode Tolerance Units Avoid intersections x Parks pax tovertex 10 00000 map units Y schools to segment 5 00000 map units a kaai k r ae Y Streets_pdx to vertex and segment 2 00000 pixels X Enable topological editing Enable snapping on intersection OK Cancel Apply There are many digitizing options that can be set by navigating to Settings Options Digitizing These include
194. eclassification resampling rescaling mosaicking interpolation and so on Many of the tools shown in this chapter rely on GDAL to perform the calculations while others are included as part of plugins or QGIS Where applicable alternative tools were noted in information boxes as different tools may provide different functionality or parameters In the next chapter we will switch from modifying and preparing existing data as discussed in Chapter 4 Preparing Vector Data for Processing and this chapter to creating data The next chapter will cover creating points from raw coordinate data geocoding georeferencing imagery and topology operations 167 Advanced Data Creation and Editing This chapter will provide you with more advanced ways to create vector and raster data There is a great deal of spatial data held in tabular format Readers will learn how to map coordinate and address based data Other common sources of geospatial data are historic aerial photographs and maps in hard copy Readers will learn how to georeference scanned imagery and transform it into a target coordinate reference system The final portion of the chapter will cover testing topological relationships in vector data and correcting any errors via topological editing The topics that you will cover in this chapter are as follows Creating points from coordinate data Geocoding address based data Georeferencing imagery Checking the topology of vector data
195. ect the endpoints of the second layer are flagged as errors must be inside This evaluates how a point layer interacts with a second polygon layer Points not covered by the polygons are flagged as errors must not have duplicates This evaluates if point features are stacked on top of one another Additional points are with the same x and y position stacked as the first point queried are flagged as errors must not have invalid geometries This checks whether the geometries are valid and if they are not then it flags those features as errors must not have multi part geometries This flags all multi part points as errors Rules for line features The rules for line features are as follows end points must be covered by This relationship test evaluates how a line layer interacts with a second point layer The features that do not intersect the point layer are flagged as errors must not have dangles This test will flag features that are dangling arcs must not have duplicates This flags additional duplicate line segments stacked as errors must not have invalid geometries This checks whether the geometries are valid and if they are not then it flags those features as errors must not have multi part geometries This flags features that have a geometry type of multi line as errors must not have pseudos This tests lines for the presence of pseudo nodes This is when there is a pair of nodes where there should only be o
196. ed until all the records are matched Use the Identify tool a to inspect the mapped points and the roads to ensure that the operation was successful Never take a GIS operation for granted Check your results with a critical eye The following figure shows the results of geocoding addresses via street address ranges The addresses are shown with the street network used in the geocoding operation Fos fan ie AA ge a Ee sea ffi a 183 Advanced Data Creation and Editing Georeferencing imagery Maps and aerial photographs in hard copy have a lot of valuable data on them When this data needs to be brought into a GIS they are digitally scanned to produce raster imagery The output of a digital scanner has a coordinate system but it is a local coordinate system created by the scanning process The scanned imagery needs to be georeferenced to a real world coordinate system before it can be used in a GIS Georeferencing is the process of transforming the coordinate reference system CRS of a raster dataset into a new coordinate reference system Often the process transforms the CRS of a spatial dataset from a local coordinate system to a real world coordinate system Regardless of the coordinate systems involved we ll call the coordinate system of the raster to be georeferenced as the source CRS and the coordinate system of the output as the destination CRS The transformation may involve shifting rotating skewing or sc
197. efile type Input directory Browse Output shapefile ring OGIS Chapter 4 data Merged Neighborhoods shp Browse Add result to map canvas In the Merge shapefiles dialog you have the option to choose whether you wish to merge all shapefiles in a folder or pick individual shapefiles to merge 1 Depending on how your shapefiles are stored you can do either of the following O Keep Select by layers in the folder unchecked to merge all shapefiles in a directory O Check Select by layers in the folder to select individual files to merge 906 J Chapter 4 2 If the previous option is enabled choose the shapefile type Point Line or Polygon Set the input directory files by clicking on Browse Name the output shapefile by clicking on Browse Choose whether you wish to select Add result to map canvas a oot Te a Click on OK to merge the shapefiles Creating spatial indices Large data layers with hundreds or thousands of features will render much more quickly with a spatial index To create a spatial index choose the Create Spatial Index tool by navigating to Vector Data Management Tools Select the loaded canvas layers or check the Select files from disk option and navigate to a folder and select layers on disk Click on OK to create the spatial indexes 4 Build spatial index Se Select files from disk Select files Major Streets_pdx Neighborhoods pdx PDX cty limits Parks_pdx Portland FireStations
198. els folder You can set the location of the Models folder by navigating to Processing Options in QGIS Desktop Under the Models section of the Processing options window you can specify the location of the Models folder Click on the default folder path and the browse ellipses button will appear allowing you to select a different location Processing options Desr T Setting H 3 General la Models at Activate a Models folder E Prowiders E Scripts To demonstrate the basics of using the graphical modeler we will use a simple example that identifies riparian tree stands in Alaska It will have three inputs and two algorithms First we will give our model a name and a group name For this example as shown in the following screenshot we have opened the graphical modeler and named the model as Riparian trees and the model group as Landcover This is the group and the name by which the model will be displayed within the Processing Toolbox 274 Chapter 8 4 Processing modeler 68 RA amp amp Parameters Riparian trees Boolean Extent File Number Raster layer String Table cf Table field qP Vector layer Inputs Algorithms Graphical modeler with the model named as Riparian trees and the group named as Landcover Then we will click on the save button F The Save Model dialog will open defaulting to the Models folder Here you need to choose a name for the model file We are naming it as R
199. enStreetMap type attribute This indicates the relationship of the coordinates to the addressed feature approximate geometric center node relation rooftop way interpolation and so on If the web service returns more than one location for an address the first of the locations will be used as the output feature Use of this plugin requires an active Internet connection Google places both rate and volume restrictions on the number of addresses that can be geocoded within various time limits You should visit the Google Geocoding API website http code google com apis maps documentation geocoding for more details current information and Google s terms of service Geocoding via these web services can be slow If you don t get the desired results with one service try the other street network data Here are the steps for geocoding the Addresses csv sample data using local street network data 1 Load the Addresses csv and Streets shp sample data 2 Open the Addresses csv sample data and examine the table This contains the addresses of municipal facilities Notice that there is an address column for example 150 Easy Street along with separate columns for number 150 and street Easy This tool requires that the number and street address components be held in separate fields 3 Install and enable the MMQGIS plugin Navigate to the MMQGIS Geocode Geocode from Street Layer menu and open the Geocode from Street
200. ent map tool and print it iface mapCanvas setMapTool newMapTool print Current map tool is a iface mapCanvas mapTool H here is your code H set the previous map tool and print it iface mapCanvas setMapTool oldMapTool print Current map tool is a iface mapCanvas mapTool The preceding code will generate an output that is similar to the following console lines Previous map tool is a lt qgis gui QgsMapToolPan object at 0x7 fe4a0e2e4d0 gt Current map tool is a lt qgis gui QgsMapToolZoom object at 0x7 fe4a13a04d0 gt Current map tool is a lt qgis gui QgsMapToolPan object at 0x7 fe4a0e2e4d0 gt You will notice that changing the current Map Tool with setMapTool will also change the cursor icon 369 PyQGIS Scripting Getting point click values In this paragraph we will create a code that will be useful to get point click coordinates and print them in the console We will use the QgsMapToolEmit Point Map Tool but the structure of the following code can be applied to other available Map Tools as well We will write all the code in the Python console but it can be used in a custom plugin for example to create a GUI interface to trace mouse movement or to plot a polygon that is based on clicked points We will use the following steps for getting the clicked points 1 Save the previous Map Tool Create a QgsMapToolEmitPoint Map Tool Create an event handler for the map canvas to tr
201. entering a name in the textbox at the top and then click on the Store button To load and run the stored query select the query name in the drop down box at the top To delete a stored query select the query in the drop down box and then click on the Delete button Creating a spatial view Creating a spatial view on a SpatiaLite database using the SQL window in DB Manager is a two step process The first step is to create a view that includes a field with unique identifiers and the geometry column The second step is to insert a new record in the views_geometry_columns table to register the view as a spatial view In this section we will create a spatial view on the Waterfalls table to show all the waterfalls in the Mowich Lake quad to do this perform the following steps 1 Open DB Manager by clicking on DB Manager under Database 2 Inthe Tree panel navigate to and select the GiffordPinochet sqlite database 46 Chapter 2 Navigate to Database SQL window or press F2 on your keyboard to open the SOL window Enter the following query CREATE VIEW mowich lake waterfalls AS SELECT w pk as ROWID w NAME w TYPE w geom from Waterfalls as w WHERE w quadname Mowich Lake In the CREATE VIEW query two fields are required to be included in the SELECT statement the unique identifier field should be renamed to ROWID and the geometry field You must rename the unique identifier to ROWID or the view cannot be registered as a spatia
202. ents 0 and 172 of line 8 intersect at 509892 890 Geometry has 1 errors Total encountered errors 267 Press Ctrl C to copy results to the clipboard Save errors location Output point shapefile b PACKT Data Portland Output Chapter4 Errors shp Browse 3 Add result to canvas BERET E EE 108 Chapter 4 If you have a layer with hundreds or thousands of errors the most elegant way to repair them is to use the GRASS GIS plugin to import them into a GRASS database GRASS uses a topological vector data model When importing you can set a snapping tolerance below which vertices will be snapped together This will likely clean up the majority of the errors For the remainder you can use the v clean GRASS tool Once the errors have been cleaned up the data can be exported out of GRASS into the vector format that you require Converting vector geometries Sometimes it is necessary to make conversions among point line and polygon vector geometries For example you may need to generate point centroids from zip code polygons or a town boundary polygon from a line layer Such conversions may be necessary to put the data into the most appropriate geometry for analysis For example if you need to determine the acreage of parcels but they are provided in a line format you will need to convert them to polygons to calculate their areas Sometimes you may want to convert geometries for cartographic reasons such as co
203. er This parameter sets the orientation of the bars The options available are Up Down Right and Left Appearance Size Position Options Bar Orientation Up Down Right Left Saving loading and setting default styles Now that you have set the styles that you want for raster and vector layers you will likely want to save the styles so that they can be used again later or applied to other layers There are four buttons that are used to manage styles These four buttons shown in the following screenshot are always displayed near the bottom of the Layer Properties window Load Style Save As Default Restore Default Style Save Style In this section we will use these four buttons to save a style to a style file load a saved style file and set and restore a default style 101 Styling Raster and Vector Data Saving a style QGIS can save styles in two file formats qm1 and sid The qm1 style file is specific to QGIS while the s14 style file is useable by other programs to style files In general you should plan on saving styles using the qm1 file type as it does the best job of saving your styles however if portability is a priority then the s14 file is the better choice To save a style open the Layer Properties window set the style that you wish to save then click on the Save Style button and save the style as either a qm1 ora sld file The saved style file can later be loaded and applied to other da
204. er transparency This contains the percentage of transparency for the layer The higher the transparency value the more the layers below will be visible through this layer Layer blending mode This applies a blending method to the vector that mixes with layers below in the Layers panel A number of blending modes are available to choose from and are commonly found in graphics editing programs In fact there are 13 blending modes Each of these blending modes is discussed in more detail in the Raster color rendering section of this chapter Feature blending mode This applies a blending method to the vector that mixes with other features in the same vector layer A number of blending modes are available to choose from and are commonly found in graphics editing programs There are 13 blending modes Each of these blending modes is discussed in more detail in the Raster color rendering section of this chapter The new blending modes should be explored before you use transparency for overlays Let s consider an example where we want to add a hillshade to our map to give it some depth In the following figure the top left map shows Africa s countries by using polygons and Normal Layer blending A common way to place a hillshade behind polygons is to make the polygons semi transparent so that the hillshade can give depth to the polygons However this tends to wash out the colors in the polygons and the hillshade is muted which is illustrated at the t
205. errors 202 Chapter 6 Using the Topology Checker The parcels shp sample data will be used to demonstrate how to set up and test topological relationships Here the parcels polygon shapefile is loaded and the Topology Checker panel has been enabled by clicking on Topology Checker under Vector The following figure shows the parcels shp sample data loaded into QGIS desktop and the Topology checker plugin enabled 4 QGIS 2 6 0 Brighton Project Edit View Layer Settings Plugins Vector Raster Database Web MMQGIS Processing Help DEBRA 408MS 2 RPP Ane 4o H R40R A rA g R 7 z 2 abs a oe abe ate abe csw g o we M a F A ws Ab AS b a r Ry fe a 3 Q Ve 2a a 4 ed LIe E Configure E validate All IE validate Extent Show errors Topology not checked yet Coordinate 1537940 1486931 v Render EPsc 2903 A 203 Advanced Data Creation and Editing Here are the steps for configuring the Topology Checker plugin and evaluating the topology of the parcels shp sample data 1 Click on the Configure button in the Topology Checker panel to open the Topology Rule Settings dialog To set a rule choose the target layer parcels Next choose the must not have gaps rule from the central drop down menu Topology Rule Settings Current Rules parcels must contain must contain must not have duplicates must not have gaps must not have invalid geo
206. ers 65 Paletted raster band rendering 66 Singleband gray raster band rendering 67 Singleband pseudocolor raster band rendering 70 ii Table of Contents Styling multiband rasters 73 Creating a raster composite 74 Raster color rendering 76 Raster resampling 79 Styling vectors 80 Single symbol vector styling 80 Categorized vector styling 81 Graduated vector styling 83 Rule based vector styling 85 Point displacement vector styling 88 Inverted polygons vector styling 90 Vector layer rendering 91 Using diagrams to display thematic data 93 Parameters common to all diagram types 94 Diagram size parameters 94 Diagram position parameters 94 Adding attributes to diagrams 95 Creating a pie chart diagram 96 Creating a text chart diagram 98 Creating a histogram chart diagram 100 Saving loading and setting default styles 101 Saving a style 102 Loading a style 102 Setting and restoring a default style 102 Summary 103 Chapter 4 Preparing Vector Data for Processing 105 Merging shapefiles 106 Creating spatial indices 107 Checking for geometry errors 107 Converting vector geometries 109 Creating polygon centroids 110 Converting polygons to lines and lines to polygons 110 Creating polygons surrounding individual points 111 Extracting nodes from lines and polygons 112 Simplifying and densifying features 113 Converting between multipart and singlepart features 115 Adding geometry columns to an attribute table 117 Using basic vector geoprocessin
207. ersions 3 QGIS API exploring in Python Console 310 OGIS Browser about 5 data tree view 6 information panel 6 toolbar 5 6 QGIS color ramp collection managing 58 65 QGIS community 303 QGIS Desktop map display 4 menu bar 4 panels 4 toolbars 4 5 OGIS installation on Windows OSGeo4W Network Installer option 2 QGIS Standalone Installer option 2 QGIS issue reporting 304 reporting URL 305 QGIS Python Plugins Repository URL 25 QGIS Redmine bug tracker URL 305 QGIS sample data URL 307 QgsRasterLayer class 340 QgsVectorDataProvider editing through 353 feature adding 356 feature deleting 355 feature geometry changing 354 355 QgsVectorLayer feature adding 359 feature attribute changing 359 feature removing 359 384 OgsVectorLayerEditBuffer class discovering 358 used for editing 357 358 Qt Designer URL for downloading 318 used for modifying TestPlugin layout 319 320 queries creating 45 spatial view creating 46 48 spatial view dropping 48 49 SQL query creating 45 46 R about 262 histograms exploring 262 268 summary Statistics exploring 262 268 URL 262 Random color ramp adding 62 raster analyses performing with GRASS about 223 224 LCP calculating 228 shaded relief calculating 225 227 slope using r slope calculating 229 viewshed evaluating 236 raster composite r composite tool creating 74 76 parameters 75 raster mosaic creating 156 158 raster overviews pyramids generating 158 16
208. ert the layer to a standalone shapefile or another vector format click on Save as under Layer or right click on the layer in the Layers panel and click on Save as Here you can choose any OGR supported file format along with an output CRS of your choice The cougar_sightings csv sample data has coordinates in UTM zone 13 NAD83 or EPSG 26913 The following screenshot shows the mapped data in the cougar_sightings csv sample data Z QGIS 2 6 0 Brighton Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help rh SS A os X M D amp PI ID O Q Ne BRRA AUSHPAPLHPLRS s 8 4 444 X Ve Fes o amp Q v 95 Vo bd Q Coordinate 57808 3643023 Scale 1 4 754 239 v X Render EPSG 26913 Sample data in cougar _ sightings csv mapped by x and y coordinate values 173 Advanced Data Creation and Editing Mapping well known text representations of geometry As mentioned earlier the Add Delimited Text Layer tool can also be used to map Well Known Text WKT representations of geometry WKT can be used to represent simple geometries such as Point LineString and Polygon along with MultiPoint MultiLineString and MultiPolygon It can also represent more complex geometry types such as geometry collections 3D geometries curves triangular irregular networks and polyhedral surfaces WKT geometries use geometry primitives such as Point LineString and Polygon followed by the coordi
209. es file is as follows input _value_low input_value_high output_value_low output_value__ high input value low input value high output_ value input _value output value inpub value output value The following are the recode rules for this example The first line tells the tool to recode the values less than 2000 meters with a value of 1 in the output raster The first asterisk is a wildcard for every value less than 2000 The second line recodes the values greater than and equal to 2000 and less than 2500 as 2 in the output raster The third line recodes all values greater than 2500 as 3 in the output raster e22000 2 1 2000 2500 2 25003 3 Save the preceding code to a text file named Elevation_rRecode_ Rules txt 146 Chapter 5 9 Select the output raster by clicking on the browse button You can choose either to Save to a temporary file or Save to file The following screenshot shows the completed r recode tool 4 nrecode Recodes categorical raster maps Parameters Log Help Show advanced parameters Input layer elevation EP5G 26912 File containing recode rules optional DB PACKT Data Chapter 5 Elevation_rRecode Rules txt GRASS region extent xmin xmax ymin ymax GRASS region cellsize leave 0 for default 0 000000 Output raster layer ao p Temporary mie Open output file after running algorithm Run There is a similar GRASS tool in the Processing Toolbox named r reclass The r reclas
210. ese can serve as part of the documentation of the georeferencing operation and can be reloaded with the Load GCP Points tool a to redo the operation at a later date Transformation settings The following steps describe how to set the Transformation settings 1 Once all the eight GCPs have been created click on the Transformation settings button gh Here you can choose appropriate values for the Transformation type Resampling method fields and other output settings There are seven choices for Transformation type This setting will determine how the ground control points are used to transform the image from source to destination coordinate space Each will produce different results and these are described as follows for this example choose Polynomial 2 O Linear This algorithm simply creates a world file for the raster and does not actually transform the raster Therefore this option is not sufficient for dealing with scanned images It can be used on images that are already in a projected coordinate reference system but are lacking a world file It requires a minimum of two GCPs Helmert This performs a simple scaling and rotational transformation This option is only suitable if the transformation simply involves a change from one projected CRS to another It requires a minimum of two GCPs 191 Advanced Data Creation and Editing O al Polynomial 1 Polynomial 2 Polynomial 3 These are perhaps the most
211. esign with the Qt framework is beyond the scope of this book but the framework has good tutorials explaining how to customize graphic interfaces The test_plugin_dialog py file contains the logic of the preceding plugin GUI layout This is the place where you add the logic of the plugin related to the GUI For example buttons that are disabled when a specific value is inserted and so on The test_plugin py file is the container of the business logic of the plugin It is complex but usually only some parts have to be modified to insert the plugin logic Modifying how this code will be managed will be dealt with shortly 318 Chapter 9 Plugin Builder generated files The Plugin Builder generates more than basic files because it creates a template to manage complex Python plugin projects A project can involve unit testing detailed documentation translation code analysis and so on Here is a summary of these files e The help directory contains all the files necessary to automatically generate documentation in different formats from HTML to PDF e The i18n directory contains files where we can add translations in other languages e The script directory contains some tools to facilitate the plugin development and deployment e The test directory contains unit tests for the plugins It also contains utility classes to support unit testing e The pylintrc file is a configuration file for Pylint a framework of code analysis
212. eters The next screenshot illustrates what this layer looks like after you set the value of zero to transparent and the buffer itself to red The last habitat variable that we need to evaluate is the preference towards roosting in dense forest cover We are going to reclassify or recode the land use layer assuming that the owls will make use of three primary classes that are deciduous evergreen and mixed forest types in decreasing preference This means that we are going to use a simple ordinal ranking scheme to assign a new value of 3 to grid cells that represent evergreen forest a value of 2 for shrub cover a value of 0 for water and a value of 1 for the remaining land types We could assume zero for all other cover types but owls are unpredictable 250 Chapter 7 Reclassifying land use using the Reclassify grid values tool To accomplish this we could use r recode as we did in a previous exercise but instead we are going to use the SAGA Reclassify grid values tool The advantage of this tool is that we can enter our reclassification rules directly in the tool interface rather than reading them into the algorithm from a separate file The following screenshot provides the necessary values to reclassify the land use layer where 11 represents surface water 42 represents evergreen forest 52 represents shrub cover and all other values are equal to 1 Fixed Table If your version of QGIS doesn t allow you to add o
213. ething if loading failed gt 339 PyQGIS Scripting Exploring QgsRasterLayer The myRaster variable is an instance of the QgsRasterLayer class This means that all methods of the raster are documented at http qgis org api classQgsRasterLayer html This class is a specialization of the generic QgsMapLayer class lt i Remember that API documentation refers to C APIs but not all methods are visible to Python If you want to have all the methods Q available in Python use the Python help command by typing the help QgsRasterLayer command in the Python console For example we can get some raster information by calling the methods as follows print myRaster height myRaster width The preceding code will produce the following output 5046337 5374023 To get the extent of the layer it is necessary to use the extent method of the QgsMapLayer class so execute the following code print myRaster extent This will generate a strange result that is similar to the following output lt qgis core QgsRectangle object at Oxaa55dd0 gt This shows that the result of the extent method is a QgsRectangle instance where it is possible to call all the methods belonging to QgsRectangle For example the bounding box coordinates can be printed with the following code snippet ext myRaster extent print ext xMinimum ext yMinimum ext xMaximum ext yMaximum This will produce the following resu
214. examples in greater detail Here you will learn all the options for using the Georeferencer GDAL plugin The first example georeferencing using a second dataset In this example the scanned1990 tif aerial photograph will be georeferenced by choosing ground control points from a more recent aerial photograph of the bridgeport_nj sidarea The scanned1990 tif image is the result of scanning a hard copy of an aerial photograph The bridgeport_nj sid image file covers the same portion of the planet and is in the EPSG 26918 NAD83 UTM zone 18N CRS Once the georeferencing operation is completed a new copy of the scanned1990 tif image will be created in the EPSG 26918 NAD83 UTM zone 18N CRS Getting started 1 Launch QGIS Desktop and load the bridgeport_nj sid file into the QGIS Desktop map canvas Note that you may need to navigate to Properties Style for this layer and set the Load min max values option to Min max so that the image renders properly 2 Click on Georeferencer under Raster to launch the plugin The Georeferencer window will open 3 Load the scanned1990 tif aerial photograph into the Georeferencer image window by clicking on Open Raster under File or by using the Open raster button E The Coordinate Reference System Selector window will open This is because the scanned1990 tif image does not have a defined CRS Set the CRS to EPSG 26918 NAD83 UTM zone 18N and click on OK 4 Arrange your desktop so th
215. exercises in this chapter to do this perform the following steps 1 Open QGIS Desktop and open the Browser panel If the Browser panel is missing click on Browser by navigating to View Panels In the Browser panel you will find the SpatiaLite entry below your hard drive folders 30 Chapter 2 2 Create a new SpatiaLite database by right clicking on SpatiaLite and then choose Create database as shown in the following screenshot 3 Create a new folder on disk and save the new database as GiffordPinchot sqlite The newly created database will appear under the SpatiaLite database entry uh Home q Favourites C D E F H I i L YJ Z j Ye mssoL 4 E E E F Fostols ra E ows New Connection WCS Create database G2 wes EA wes Now that we have a new SpatiaLite database let s look at its initial structure and contents To do this we will use DB Manager a built in QGIS plugin DB Manager provides a simple graphical interface to manage PostGIS and SpatiaLite databases Using DB Manager we will be able to view and manage our SpatiaLite database Let s start by getting familiar with the DB Manager interface 1 Click on DB Manager under Database to open the DB Manager The DB Manager interface as shown in the following screenshot is composed of four parts menu bar toolbar tree view and information panel 31 www allitebooks com
216. extent xmin xmax ymin ymax Leave blank to use min covering extent GRASS region cellsize leave 0 for default 0 000000 Output RGB image ive Publications Mastering QGIS Chapter 3 Data composite LC_Composite tif anal X Open output file after running algorithm The r composite tool has a number of parameters that can be set and these are as follows e Red This includes the raster that will be assigned to the red band e Green This include the raster that will be assigned to the green band e Blue This includes the raster that will be assigned to the blue band e Number of levels to be used for lt band gt This includes the number of levels that the input raster values will be mapped to e Dither If this is checked will dither the image to reduce banding and loss of detail e Use closest color If this is checked the original pixel colors will be translated into the closest palette color No dithering will occur if this is enabled 75 Styling Raster and Vector Data e GRASS region extent This sets the region extent of the output composite raster Uses the minimum of inputs by default otherwise the extent can be set from the current canvas extent or by a layer in the Layers panel e GRASS region cell size This sets the cell size of the output composite raster e Output RGB image This includes the name and location of the output composite raster e Open output file after running algorithm If this is
217. f Aptana and then starting the server with the relative start stop buttons The buttons and Debug perspective are shown here 328 Chapter 9 File Edit Source Relactorlee thvigate Search Project Pydey Commands Run Window Help amp A P pEi start the pydev server TestPluginGialog test_plugin_dialog os path writable insert 189 1 The Debug perspective can be opened by clicking on the right hand side button circled in red If the Debug perspective button is not available it can be added to the Aptana menu Navigate to Window Open Perspective Other The Start Stop server button is pointed out by the red circle on the left While starting the server some messages will appear in the Debug window highlighted by the red box in the upper left section this means that the server is running In the Debug window all connected clients will be listed In the red box on the bottom the Aptana console window shows that the server answers to the port 5678 this is the information that we ll use to connect from QGIS Connecting QGIS to the Pydevd server After running the PyDev server we ll connect to it from QGIS In the QGIS Python Console type the following code import pydevd try pydevd settrace port 5678 suspend False except pass 329 Creating QGIS Plugins with PyQGIS and Problem Solving The preceding code first imports the pydevd module and then connects to the server with the settrac
218. f the data on disk from 1 05 MB to 208 KB w Simplify geometries Pity g Input line or polygon layer Neighborhoods pdx Use only selected features Simplify tolerance 20 0000 Save to new file Chaptbr4 Neighborhoods_simple shp Add result to canvas The following figure shows the result of the simplification operation When zoomed in to 1 1 000 the difference between the input and output geometries can be seen The black lines are the original neighborhood boundaries and the red lines are the boundaries after being simplified The largest shift in position represents 15 feet in real world units Chapter 4 The Densify Geometries tool adds vertices to a line or polygon layer The operation is run per polygon or line segment The tool asks for vertices to be added The default is one per segment but can be set to the desired number This might be the first step before the Extract Nodes tool is used or there may be other reasons for densifying the geometries Converting between multipart and singlepart features In a typical vector layer one feature corresponds to one record in the attribute table In a multipart layer there are multiple features that are tied to one record in the attribute table This is often the case with data representing islands For example we have some sample data of county boundaries for Hawaii Hawaii has five counties and the GIS data has five records in the attribute table However several of these
219. fied slope and land use layers 232 Calculating the cumulative cost raster using r cost 233 Calculating the cost path using least cost paths 234 Evaluating a viewshed 236 Clipping elevation to the boundary of a park using GDAL 23 Calculating viewsheds for towers using r viewshed 238 Combining viewsheds using r mapcalculator 240 Calculating raster statistics using r stats 242 SAGA 244 Evaluating a habitat 245 Calculating elevation ranges using the SAGA Raster calculator 245 Clipping land use to the park boundary using Clip grid with polygon 246 Querying land use for only surface water using the SAGA Raster calculator 247 Finding proximity to surface water using GDAL Proximity 248 Querying the proximity for 1 000 meters of water using the GDAL Raster calculator 249 Reclassifying land use using the Reclassify grid values tool 251 v www allitebooks com Table of Contents Combining raster layers using the SAGA Raster calculator 252 Exploring hydrologic analyses with TauDEM 254 Removing pits from the DEM 255 Calculating flow directions across the landscape 256 Calculating the upstream area above Fort Klamath 257 Calculating a stream network raster grid 259 Creating a watershed specific vector stream network 260 R 262 Exploring summary statistics and histograms 262 Summary 269 Chapter 8 Automating Workflows with the Graphical Modeler 271 An introduction to the graphical modeler 272 Opening the graphical modeler 272 Configuring the modeler an
220. fire sta Select all Select none Clear list Checking for geometry errors Data even from reputable sources can contain geometry errors These can often be tiny geometry errors that are not obvious but that prevent geoprocessing tools from running or producing valid results 107 Preparing Vector Data for Processing The Check Geometry Validity tool which can be found by navigating to Vector Geometry Tools Check Geometry Validity takes an input vector layer that is loaded in the canvas and scans the data for errors such as geometric intersections The errors can be displayed in a window on the tool or can be output to a point layer The resulting point layer will have an attribute describing the error In the following screenshot you can see that the tool has been run using the DRECP_Alternativel_ Integrated shp sample dataset This data is portion one of five alternatives for the Desert Renewable Energy Conservation Plan for Southern California The tool found 267 errors The errors still need to be repaired but at least you now know where they are 4 Check geometry validi g ry ity Input Vector Layer BRECP_ Alternative 1_Integrated Use only selected features Geometry errors segments 5 and 100 of line 0 intersect at 477152 3544 Geometry has 1 errors segments 440 and 458 of line 0 intersect at 477402 3 Geometry has 1 errors segments 469 and 482 of line 0 intersect at 518082 3 Geometry has 1 errors segm
221. following figure shows a portion of a state with pie charts showing the proportion of different racial population in each county Chapter 3 By reviewing the pie charts in the preceding figure let s note down a few things The pie slices differ for each county s race attributes and are colored based on each attribute s selected color The size of the pie charts vary based on the total population in each county The pie slice outlines are black and thin The pie charts are displayed above the centroid of each polygon These four noted items among others that are not noted are all customizable using parameters available in the Diagrams tab of the Layer Properties window The Diagrams tab for pie charts has three tabs with parameters Appearance Size and Position The Appearance tab shown in the following screenshot provides the following parameters Transparency This is used to specify the transparency percentage for the pie chart Line color This contains the color of the lines surrounding the pie and in between pie slices Line width This contain the width of the lines surrounding the pie and in between pie slices Start angle This is used to specify the angle from which the pie slices will begin to rotate in a clockwise manner The available options are Top Right Bottom and Left Scale dependent visibility If this is checked the Minimum and Maximum visibility scales can be set Appearance Size Position Opt
222. formation settings 191 196 Geospatial Data Abstraction Library GDAL about 2 8 URL 8 GitHub URL 314 Google Geocoding API URL 180 Gradient color ramp adding 61 graduated vector style 83 84 graphical modeler about 272 configuring 274 275 differences 281 opening 272 273 GRASS URL 222 233 251 used for performing raster analyses 223 225 ground control points GCP about 184 entering 189 191 H habit evaluation SAGA about 245 elevation ranges calculating with SAGA Raster calculator 245 GDAL Raster calculator using 249 250 land use clipping with clip grid with polygon 246 land use querying with SAGA Raster calculator 247 proximity to surface water finding with GDAL Proximity 248 249 Reclassify grid values tool using 251 SAGA Raster calculator using 252 histogram chart diagram creating 100 101 hydrologic analyses exploring with TauDEM 254 261 flow directions calculating across landscape 256 381 pits removing from TauDEM 255 stream network raster grid calculating 259 upstream area above Fort Klamath calculating 257 258 watershed specific vector stream network creating 260 261 Identify tool 183 iface class exploring 308 309 imagery georeferencing about 184 Georeferencer GDAL plugin using 185 188 eround control points GCP identifying 184 point file using 197 200 second dataset using 188 inputs adding 276 278 installations QGIS 2 OGIS on Mac OS X 2 OGIS on Ubun
223. g the native QGIS Raster calculator tool to use with the r cost algorithm Since neither layer contains any zero values which would need to be preserved through multiplication we can combine them using addition as shown in the next screenshot We could also use the r mapcalculator tool to combine these layers but this demonstrates how easy it is to move between native QGIS tools and those housed in the processing toolbox Q Raster calculator Raster bands Cumulative costa 1 Slope a1 TULC Recodea 1 Slope Recodea 1 Alevationa 1 hillshade 1 wlc_dnp i slope _lulcg 1 Raster calculator expression LULC Recode m1 Slope Recodea 1 Result layer Output layer astering_QGIS GRASS slope_lulc tif Current layer extent x min 50845 00000 XMax 592075 00000 min 4733025 00000 Ymax 4775095 00000 Columns 2015 Rows 2057 Output format X Add result to project 232 Chapter 7 In this example we used r cost to create a new layer representing the cost of traveling across the landscape If we know that the path will be traveled exclusively on foot it may make more sense to use the r walk algorithm available through the GRASS library For more information about this visit http grass osgeo org grass63 manuals r cost html Calculating the cumulative cost raster using r cost To summarize our progress so far we have reclassified the slope and land use layers and combined the
224. g the slope percent Type of output aspect and slope maps float Multiplicative factor to convert elevation units to meters 1 000000 Minimum slope val in percent for which aspect is computed 0 000000 Do not align the current region to the elevation layer GRASS region extent min xmax ymin ymax oVverine wtemnt egy hlari tn GRASS region cellsize leave 0 for default 0 000000 Name for output slope raster map C Mastering_OQGIS GRASS slope tif X Open output file after running algorithm This dialog box indicates that slope is being calculated in percent which will need to be reflected in the rule set used to reclassify this layer Essentially we are making the assumption that increasing slope values equate to increased physical exertion and thus inflicts a higher cost on members of the search and rescue team 229 The Processing Toolbox Reclassifying a new slope raster and the land use raster To accomplish this we are going to input the slope raster into r recode and use the slope _recode txt file to inform the tool how to reclassify the slope values It is worth opening up the slope_recode txt file to understand the GRASS formatting requirements and evaluate the assumptions within this reclassification scheme which are also summarized in the following table Land use Land class Land use Land class Travel cost Recode value type code assumption Developed open land 21 Developed low 22 Lowest cost in
225. g this area 4 Processing modeler a aR L Z Parameters P Boolean ap Extent oP File lt P Number qP Raster layer TP String oP Table ee dP Table field it eee qP Vector layer aaa Clip grid with polygon Out 292 Chapter 8 If the model is run normally it will clip the DEM to the extent of all 21 watersheds and produce one output elevation raster However if the Iterate over this layer button is clicked see the following screenshot the model will cycle through each feature in the watershed layer and output a DEM that covers each individual watershed This will result in 21 individual elevation rasters This sort of automation is very easy to generate and can save you a lot of time DEMs Clipped to Watersheds DEM elevation FPS6 26915 Watersheds watersheds EPS5G 26913 Watershed DEM Save to temporary file Open output file after running algorithm 293 Automating Workflows with the Graphical Modeler The following figure shows the resulting 21 DEMs that are clipped to individual watersheds The resulting 21 clipped DEMs Nesting models As we previously covered when a model is saved to the Models folder it will appear in the Models category of the Processing Toolbox panel What we didn t mention earlier was that it will also appear in the Algorithms tab of the Processing modeler window This means that a previously written model can be used as an algor
226. g to Vector Geometry Tools Delaunay Triangulation Delaunay triangulation creates a series of triangular polygons The method creates a triangle in such a way that a circle drawn through the three nodes of the triangle will contain no other nodes This is the same technique that is used to generate triangulated irregular networks TINs 111 Preparing Vector Data for Processing Here we ll use the High_schools_pdx shp data to compare these two methods The two tools are intuitive You need to provide the input point vector layer and specify the output polygon shapefile name The Voronoi polygons tool has an option to set a buffer region This is the amount by which the resulting polygons will extend beyond the perimeter points In the following example the Buffer region field has been set to 10 oronai on mv polyg Input point vector layer HighSchools_pdx Buffer region 10 0000 o Output polygon shapefile Portland Output Chapter4 High_school_voronoi_10 shp Add result to canvas Cos The following figure shows the output differences between Voronoi polygons and Delaunay polygons Input Points Voronoi Polygons Delaunay Triangulation a Extracting nodes from lines and polygons The Extract Nodes tool which can found by navigating to Vector Geometry Tools Extract Nodes can be used to convert either line or polygon layers into a point layer Each individual vertex from the input layer will be extracte
227. g tools 118 Spatial overlay tools 119 Using the Clip and Difference tools 120 iii Table of Contents Using the Intersect and Symmetrical Difference tools 121 Overlaying polygon layers with Union 123 Creating buffers 123 Generating convex hulls 125 Dissolving features 126 Defining coordinate reference systems 127 Understanding the Proj 4 definition format 127 Defining a new custom coordinate reference system 129 Advanced field calculations 131 Exploring the field calculator interface 132 Writing advanced field calculations 134 The first example calculating and formatting the current date 135 The second example inserting geometric values 136 The third example calculating a population dependent label string 137 Complex spatial and aspatial queries 140 Summary 144 Chapter 5 Preparing Raster Data for Processing 145 Reclassifying rasters 145 Converting datasets from floating point to integer rasters 148 Resampling rasters 149 Installing and troubleshooting SAGA on different platforms 152 Rescaling rasters 155 Creating a raster mosaic 156 Generating raster overviews pyramids 158 Converting between raster and vector data models 161 Converting from raster to vector 161 Converting from vector to raster rasterize 162 Creating raster surfaces via interpolation 164 Summary 167 Chapter 6 Advanced Data Creation and Editing 169 Creating points from coordinate data 169 Mapping well known text representations of geometry 174 Geoco
228. ge window The To and From coordinates are displayed in the GCP table window along with the RMSE values File Edit View Settings Help Eo Gla Pt O PPO RA nq teh onoff onjoff u sa A 0 was 86 RISS 34 478363 61 4407951 56 478368 78 4411005 76 474774 80 4407777 55 i 476776 99 4410568 52 1476344 17 ee Maa 478103 67 475602 69 4408 130 27 4408838 90 4408496 53 Transform Polynomial 2 Mean error 1 03886 2 Mean error 1 03686 608 240 aca A 196 Chapter 6 Completing the operation 1 Click on the Start Georeferencing button button to complete the operation The georeferenced raster will be written out and added to the QGIS Desktop map canvas if the option was checked in the Transformation Settings window The Generate GDAL Script tool L g will write out the GDAL command line syntax for the current georeferencing operation This code can be copied to the clipboard and used to write out to a file GDAL script Fe Copy in dipboard The second example georeferencing using a point file This example will use the zone_map bmp image It displays zoning for a small group of parcels in Albuquerque New Mexico This image has five geodetic control points that are indicated by small points with labels for example 125 28 These control points are maintained by the U S National Geodetic Survey
229. globally used and developed volunteer led project In 2009 QGIS version 1 0 was released as an Open Source Geospatial Foundation OSGeo project and continues to be rapidly adopted worldwide The enduring support of the open source community has really delivered QGIS to a point where it is now a top shelf product that should be in all GIS users toolboxes and we want this book to be your tour guide and reference as you learn use and contribute to QGIS ix Preface What this book covers Chapter 1 A Refreshing Look at QGIS reviews the installation and basic functionality of QGIS that will be the assumed knowledge for the remainder of the book Chapter 2 Creating Spatial Databases covers how to create and edit spatial databases using QGIS While QGIS supports many spatial databases SpatiaLite will be used in this chapter First core database concepts will be covered followed by the creation of a spatial database Next importing exporting and editing data will be covered The chapter will conclude with queries and view creation Chapter 3 Styling Raster and Vector Data covers styling raster and vector data for display First color selection and color ramp management are covered Next singleband and multiband raster data are styled using custom color ramps and blending modes Next complex vector styles and vector layer rendering are covered Rounding out the chapter is the use of diagrams to display thematic map data Chapter
230. gt dh h 4h 4h 4h 4D HTML notation fff ane sas oa om a ene Let s take a tour of the color picker tool by starting with the components that are always available and conclude our tour by looking at the four changeable panels 52 Chapter 3 Always available color picker components The current and previous colors are displayed in the bottom left corner of the color picker tool In the preceding screenshot the Old field depicts the color that is currently chosen and the color mentioned in the Current field will replace the one in the Old field if the OK button is clicked The current color can be saved into a quick access color collection the 16 colored squares in the bottom right corner using either of the following two ways e By clicking on the button with the blue arrow This will save the current color in the first column of the top row of the color collection overwriting any existing color Subsequent clicks on the blue arrow button will store the current color in the next column until all 16 boxes are full and then will loop back to the beginning e By Dragging and dropping the current color on top of a quick access color box The old color can also be saved using the drag and drop method The color picker displays and allows manipulation of the value in the Current field in two color models HSV and RGB On the right half of the color picker the hue H saturation S value V red R
231. gured so far 6 Set the Color interpolation field to Linear to assign all unique values a unique color Optionally you could check Clip below classification list this would not assign colors to values outside the maximum and minimum values listed in the classification list 7 Click on Apply or OK to render the raster 72 Chapter 3 These three single band render types paletted singleband gray and singleband pseudocolor provide a large amount of flexibility and customization to fit your styling needs The next section covers the remaining band render type that is best applied to multiband rasters multiband color Styling multiband rasters The multiband color band renderer stretches three gradients red green and blue to three separate raster bands The basic idea is that the computer will display natively used combinations of red green and blue lights to create the desired image By matching individual raster bands to the red green and blue lights used by the display the three bands colors will mix so that they are perceived as other colors thereby creating a red green and blue image composite that is suitable for display Contrast enhancements are available to adjust the way the gradients are stretched across the raster bands values Contrast enhancements have already been covered in the Singleband gray raster band rendering section so refer to this section for an in depth coverage of the topic Let s se
232. hange to set the target CRS and save the file 10 Chapter 1 To create anew CRS or modify an existing CRS perform the following steps 1 Click on Custom CRS under Settings to open the Custom Coordinate Reference System Definition window Click on the Add new CRS button to add a new entry to the CRS list 3 With the new CRS selected we can set the name and parameters of the CRS The CRS properties are set using the Proj 4 format To modify an existing CRS click on Copy existing CRS and select the CRS from which you wish to copy parameters otherwise enter the parameters manually Proj 4 is another Open Source Geospatial Foundation http i osgeo org project used by QGIS and it is similar to OGR and GDAL This project is for managing coordinate systems and Q projections For a detailed user manual for the Proj 4 format used to specify the CRS parameters in QGIS download it from ftp ftp remotesensing org proj OF90 284 pdf Working with tables There are two types of tables you can work with in QGIS attribute tables and standalone tables Whether they are from a database or associated with a shapefile or a flat file they are all treated the same Standalone tables can be added by clicking on the Add Vector Layer menu by navigating to Layer Add Layer QGIS supports the table formats supported by OGR along with database tables Tables are treated like any other GIS layer they simply have no geometry Both types of tables
233. hat is currently being used in the Processing Toolbox determines how the algorithms in the graphical modeler will be organized This setting cannot be changed from within the graphical modeler Instead to change the setting the GM has to be shut down the interface setting has to be changed in the Processing Toolbox and then the graphical modeler has to be reopened Other than this finding algorithms is the same as it is within the Processing Toolbox buffer 5 Geoalgorithms c w GRASS commands Raster Raster r 8 General tools y buffer Creates a raster map layer s amp amp Grid buffer Vector v Grid proximity buffer y v buffer column Creates a buffer aro Threshold buffer y v buffer distance Creates a buffer ar Vector 3 QGIS geoalgorithms B Geometry operations Vector geometry tools Fixed distance buffer Fixed distance buffer Shapes buffer Variable distance buffer Variable distance buffer S amp SAGA E Grid Tools Grid buffer Grid proximity buffer Threshold buffer Shapes Tools S Shapes buffer The Algorithms tab with buffer being used as the search term The simplified interface is shown on the left and the advanced interface is on the right 279 Automating Workflows with the Graphical Modeler In the Algorithms tab there is a special category named Modeler only tools The three tools Calculator Raster layer bounds and Vector layer bounds
234. he Topology Checker panel The remaining gaps can be repaired using these steps 1 7 Toggle off editing for the parcels layer and save the edits If you have too many topological errors to repair manually gt you can import your data into a GRASS database GRASS has a topological vector data model The GRASS command v clean will repair a lot of errors The cleaned GRASS vector can then be exported into the file format of your choosing Summary This chapter covered more advanced ways to create GIS data from different sources We provided explanations and step by step examples of mapping raw coordinate data geocoding address based data georeferencing imagery validating vector data with topological rules and topological editing With the topics covered to this point you will be able to work with a variety of vector raster and tabular input data In the next chapter we will switch from preparing and editing data to performing spatial analyses We will cover the QGIS processing toolbox We will begin with a comprehensive overview and a description of layout of the toolbox We will then explore the various algorithms and tools that are available in the toolbox with real world examples 214 The Processing Toolbox In this chapter we will explore the structure of the QGIS processing toolbox identify which algorithm providers are available and how to use these specialized algorithms To accomplish these goals we will ensure tha
235. he filename and add more icon resources Q but this is out the scope of the current chapter so please refer to the Qt documentation for this 316 Chapter 9 The plugin file structure where and what to customize Our TestPlugin code has been created in this folder lt your home path gt qgis2 python plugins Here we can find a complex file structure where only a subset of files are strictly necessary for plugins and are in the scope of this book The basic files are the following ones Ae py metadata txt e Makefile e icon png e resources qgrc e resources rc py e test plugin dialog base ui e test plugin dialog py e test plugin py Each file has its own role inside the plugin but only a few of them have to be modified to develop a custom plugin Other than basic files the Plugin Builder generates other files and directories useful to manage more complex plugin projects The files and directories are as follows help e il8n sCripcs e test py liner e plugin _upload py README html README txt 317 Creating QGIS Plugins with PyQGIS and Problem Solving Exploring main plugin files Here we will describe the role of each of the main files that compose a plugin The init__ py file is the common Python module starting file and it s also the entry point for QGIS to load the plugin Usually it doesn t have to be modified to create a plugin The metadata txt file is a te
236. he following query in the SQL query text area UPDATE Waterfalls SET geom MakePoint Easting Northing 26910 38 Chapter 2 Let s discuss the MakePoint function MakePoint Easting Northing 26910 is a SpatiaLite function that creates a new point geometry object Easting and A i Northing are the columns in the same row that hold the values for the x and y coordinates respectively 26910 is the SRID of the x and y coordinates Click on the Execute F5 button to execute the query The query will return no result but will indicate that 100 rows were affected This indicates that the geometry field of 100 rows have been populated with point geometry The following screenshot shows the query and the indication that 100 rows were affected SQL window GiffordPinochet sglite SpatiaLite SOL query Y Store 1 UPDATE Waterfalls 2 SET geom MAKEPOINT Easting Northing 26910 Execute F5 100 rows 0 0 seconds Result Load as new layer On the SQL window click on the Close button to close the window To view the changes made to the Waterfalls table you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then clicking on Refresh under Database or press the F5 key on your keyboard 39 Creating Spatial Databases 6 Note that the Waterfalls table now has the point icon next to it Click on the Info tab on the information panel Under the SpatiaLite section of the i
237. he geometry of this new feature will be set as the bounding box of feature 599 this is done using the following code get data provider myDataProvider myVector dataProvider get feature with id 599 features myVector getFeatures QgsFeatureRequest 599 myFeature features next create geometry from its bounding box bbox myFeature geometry boundingBox newGeom QgsGeometry fromRect bbox create a new feature newFeature QgsFeature set the fields of the feature as from myVector this step only sets the column characteristic of the feature not its values newFeature setFields myVector pendingFields set attributes values newAttributes 1000 Alaska 2 newFeature setAttributes newAttributes set the geometry of the feature newFeature setGeometry newGeom add new feature in myVector using provider myDataProvider addFeatures newFeature The preceding code is explained in the inline comments and it adds a new feature The new feature can be checked by opening the attribute table and selecting the last record This will produce an interface that is similar to the following screenshot 356 Chapter 10 Attribute table alaska Features total _ 4 amp Base Pals cat r NAME AREA_MI bla 646 647 648 649 650 651 652 b5U Alaska 651 Alaska 652 Alaska 653 Alaska 655 Alaska 657 Alaska 658 Alaska 665 Alaska U J05499U 0 44360500 1 71062900
238. he plugin The make command will use the instructions included in the file called Makefile that instructs us about all the aspects of plugin compilation The result of the make command is shown in this next screenshot gt QSGeo4W Shell H4 29 PH 934 README txt H4 29 PH 181 resources qrc 65 65 PH 5 443 resources_re py H5 09 PH 1 932 resources_re pyc 61 26 PH scripts H4 29 PM test Ai 56 PM 8 309 test3 py ii 5i AN 167 test pyc 12 52 PA 7 076 test3_csv_nonfunziona py Hi 713 PH 2 295 test3_dialog py H1i 13 PH 2 767 test3_dialog pyc 14 26 PH 2 496 test3_dialog_hbase ui 18 13 AN 3 584 ui_used_in_developing py 5 46 PM 1 455 _ init__ py 45 42 PM 1 662 _ init__ pyc 19 Filets gt 68 933 bytes 6 Dirt s gt 23 884 546 048 bytes free Co Users qino qgis2s pythonsplugins Test3 gt MAKE pyrec4 o Pesources_FPC py FPesources qre C2 Users gino qgis2spython plugins Test3 gt make Pyrcc o Pesources_re py Pesources qre C gt Users qino qgis2spythonsplugins lest3 gt In the preceding screenshot we can see that the make command instructs pyrcc4 to compile the resources qrc file generating the Python version resources_rc py After compiling the icon resource and restarting QGIS the plugin button will have an icon which is the default icon set by the Plugin Builder To change the icon just change icon png with a new image leaving the filename unchanged and then recompile the icon resource A It s possible to change t
239. he plugin logic Our core plugin logic has to be added This can be done by modifying the test_ plugin py code In many cases such as in simple plugins or batch processing instructions only the run self function has to be modified inside the plugin code The run function is the function that is called every time the plugin button is clicked on in the QGIS toolbox or the plugin is run from the plugin menu In the run function we have to e Collect all loaded layers e Classify in raster and vector layers e Populate the comboboxes in the plugin GUI e Show the GUI with the new values loaded To get all the listed layers we ll have to ask the container of all the displayed layers that is the OgsMapLayerRegistry class To do this we will use the following code from qgis core import QgsMapLayerRegistry layersDict QgsMapLayerRegistry instance mapLayers The first line imports the class QgsMapLayerRegistry from the Python module qgis core You can import classes everywhere in the code but it s good j practice to import them at the beginning of the file Forgetting to import the class will cause an error during runtime The error will be as follows Traceback most recent call last File lt input gt line 1 in lt module gt NameError name QgsMapLayerRegistry is not defined In the second line the result variable layersDict is a Python dictionary it contains a set of key value pairs where the key is the uniqu
240. he radius of the rings are determined by a distance value stored in an attribute column Generating convex hulls The Convex Hull tool will take a vector layer point line or polygon and generate the smallest possible convex bounding polygon around the features It will generate a single minimum convex hull around the features or allow you to specify an attribute column as input In the latter case it will generate convex hulls around features with the same attribute value in the specified field 125 Preparing Vector Data for Processing The result will be the bounding area for a set of points and it will work well if there are no outlying data points Here a convex hull has been generated around Portland FireStations shp Dissolving features The Dissolve tool merges the features of a GIS layer It will merge all the features in a layer into one feature This can be done via the values of an attribute field or with the Dissolve all option In the following example the neighborhoods of Portland Neighborhoods _pdx shp are dissolved which creates the city boundary Tool g Dissolve 2 e Input vector layer Neighborhoods _pdx v Use only selected features Dissolve field Dissolve all X Output shapefile nd Output Chapter4 PDX_neighborhoods_Dissolve shp 3 Add result to canvas ll 0 OK Close 126 Chapter 4 Most tools have a checkbox for using only selected features Def
241. he resource management office of Crater Lake National Park has requested an analysis of potential habitat for the endangered Northern Spotted Owl We are informed that the owls prefer to roost at higher elevations approximately 1 800 meters and higher in dense forest cover and in close proximity to surface water approximately 1 000 meters In order to accomplish this analysis we need to complete the following steps Calculate elevation ranges using the SAGA Raster calculator tool Clip land use to the park boundary using Clip grid with polygon Query land use for only surface water using SAGA Raster calculator Find proximity to surface water using GDAL Proximity Query the proximity for 1 000 meters of water using GDAL Raster calculator Reclassify land use using the Reclassify grid values tool ls ON er ee he Combine raster layers using SAGA Raster calculator Calculating elevation ranges using the SAGA Raster calculator There are multiple ways to create a layer that represents elevation ranges or in this case elevation zones that relate to potential habitat One method would be to use r recode as we did in the GRASS exercise another would be to use the Reclassify grid values tool provided by SAGA which we will use later in this exercise but another very quick way is to only identify the areas above a certain elevation in this case greater than 1 800 meters using a raster calculator This type of query will produ
242. he vertical angle with respect to the observer or null values If we attempt to add three layers that contain null values the resulting output will not accurately reflect the total visible area within the park To address this issue we need to make use of the isnul11 function within r mapcalculator We will use this function within a conditional statement to identify where there are null values and replace them with a zero so that we can accurately combine all the three layers We need to open r mapcalculator and use this conditional statement if isnull A 0 1 2i asnuli B 50 10 if isnull C 0 100 The query that we are asking the calculator to execute is if layer A is null then replace it with a value of zero otherwise give the resulting grid a value of 1 and then add it to the results from the other three layers which are also evaluated for null values By replacing the original values with either 0 1 10 or 100 we are able to evaluate the total cumulative viewshed and also differentiate between the impacts of individual towers The following screenshot illustrates how to ask these questions within the raster calculator 240 Chapter 7 Q r mapcalculator Calculate new raster map from a r mapcalc expression Turbine 1 EPSG 26710 Raster layer B optional Turbine 2 EPSG 26710 Raster layer C optional Raster layer D optional Raster layer E optional Raster layer F optional Formula iffisnull A 0 1 iff
243. hm 259 The Processing Toolbox Creating a watershed specific vector stream network Essentially this algorithm identifies grid cells that have values greater than the default threshold of 100 Cells with values greater than the threshold represent areas of high flow accumulation in other words areas of the landscape that accumulate surface flow and represent potential river networks Our last step is to extract a vector layer from this raster stream network To accomplish this we are going to use the Stream Reach And Watershed tool and populate the interface as shown in the next screenshot Q Stream Reach and Watershed Parameters Log Help l Pit Filled Elevation Grid Elevation pits removed EPSG 26710 D8 Flow Direction Grid D8 Flow Direction Grid EPSG 26710 DS Drainage Area D8 Contributing Area Grid Sun Creek EPSG 26710 Stream Raster Grid Stream Raster Grid Sun creek EPS6 26710 Outlets Shapefile as Network Nodes optional Gauge EP5G 26710 Delineate Single Watershed Stream Order Grid EL LENIN al y Open output file after running algorithm Watershed Grid C G1S_Projects Scratch QGIS TauDEM dnp_wat tif Open output file after running algorithm Stream Reach Shapefile C G1S_Projects Scratch QGIS TauDEM clnp_sr shp Open output file after running algorithm Network Connectivity Tree Cove to temnorary oo Network Coordinates 260 Chapter 7 In this example we chose to cr
244. hm providers we need to make sure that R is properly configured Although QGIS gets installed with the required R scripts in order to run them we also need to download and install R from http www r project org The integration of R within QGIS offers access to some highly specialized spatial analysis tools that are focused on home range analysis and point pattern analysis To explore some of the R algorithms available through the toolbox we will use our existing data to perform some common statistical tasks that are useful for evaluating spatial data We will use the following data layers in this section e Elevation file dems_10m dem available in the GRASS data folder e Hillshade file hillshade tif created in the GRASS section e Rivers hyd1 shp e Volcanic Vents file vents shp in the R data folder e Slope file slope tif created in GRASS section Exploring summary statistics and histograms A common starting point for exploring spatial data is to evaluate the range of values within a given data layer Although QGIS offers similar information natively for vector data when you navigate to Vector Analysis Tools Basic Statistics and for raster layers through the Histogram window under Properties R offers a few more options and the ability to better visualize the data For example we might be interested in the range of values for stream length within a given watershed To evaluate this using R we can use the Summa
245. howing the Hispanic population as one bar and non Hispanic population as the other bar By reviewing the histogram charts in the preceding figure let s note down a few things e The bars report each race s attribute and are colored according to the selected attribute colors e The bar outlines are black and thin e The histogram charts are displayed above the centroid of each polygon These three noted items among others that are not noted are all customizable using parameters available in the Diagrams tab of the Layer Properties window The Diagrams tab for text charts has four tabs with parameters Appearance Size Position and Options The Appearance tab shown in the following screenshot provides the following parameters e Bar width This contain the width in millimeters of each bar in the histogram Transparency This is used to specify the transparency percentage for the histogram chart 100 Chapter 3 Line color This contains the color for the lines surrounding the bars e Line width This contains the width of the lines surrounding the bars e Scale dependent visibility If this is checked the Minimum and Maximum visibility scales can be set Appearance Size Position Options Bar width 3 00 Transparency 0 Line color as v a Line width 0 20000 Scale dependent visibility Minimum 1 000000 Maximum The Options tab shown in the following screen provides the Bar Orientation paramet
246. ich is applied to the Stacked Points shp sample data Each point within the Point distance tolerance value of at least one other point is displaced at a distance of the Circle radius modification value around a newly created center symbol In this example three groups of circles have been displaced around a center symbol Point displacement Center symbol Renderer Single Symbol Renderer settings Displacement circles Circle pen width 0 40 Grae color O Circle radius modification 3 00 Point distance tolerance 0 0000100 l Labels Label attribute Label font Label color COC M __ Use scale dependent labelling Max scale denominator The Point displacement style parameters shown in the preceding figure provide multiple parameters to displace the points set the sub renderer style the center symbol and label the displaced points Let s review parameters that are unique to the point displacement style e Center symbol This contains the style for the center symbol that is created at the location from where the point symbols are being displaced 89 Styling Raster and Vector Data e Renderer This contains the renderer that styles the displaced points Click on the Renderer settings button to access renderer settings e Circle pen width This sets the outline pen width in millimeters that visualizes the Circle radius modification value e Circle color
247. ie chart diagram creating 96 97 text chart diagram creating 98 99 used for displaying thematic data 93 Difference tool 120 digital elevation model DEM 225 digital terrain model DTM 225 dir function Python 310 Dissolve tool using 126 E Edit rule button 86 external algorithm running 365 simple command running 366 external command running 366 367 Extract Nodes tool 112 380 F feature attributes method 308 feature geometry method 308 features dissolving with Dissolve tool 126 editing 352 iterating over 350 351 iterators describing 352 features editing canvas updating 352 OgsVectorDataProvider using 353 QesVectorLayer using 357 symbology updating 352 field calculations about 131 advanced calculations writing 134 field calculator interface exploring 132 field calculator interface exploring 132 134 function types 134 floating point and integer rasters converting 148 functionality adding with plugins 24 25 G GDAL used for clipping evaluation layer 237 GDAL Proximity used for finding proximity to surface water 248 249 GDAL Raster calculator using 249 250 GDAL Script tool 197 Geographical Resources Analysis Support System See GRASS geometry errors checking for 107 109 geometry values adding to attribute table 117 Georeferencer GDAL plugin using 185 188 georeferencing with second dataset about 188 eround control points entering 189 191 operation completing 197 trans
248. ields found in the source Addresses csv files The first example geocoding using web services Here are the steps for geocoding the Addresses csv sample data using web services 1 Load the Addresses csv and Streets shp sample data into QGIS Desktop 2 Open the Addresses csv sample data and examine the table These are addresses of municipal facilities Notice that the street address for example 150 Easy Street is contained in a single field There are also fields for the city state and country Since both Google and OpenStreetMap are global services it is wise to include such fields so that the services can narrow down the geography 3 Install and enable the MMQGIS plugin Navigate to MMQGIS Geocode Geocode CSV with Google OpenStreetMap The Web Service Geocode dialog window will open 178 Chapter 6 5 Select an appropriate value for the Input CSV File UTF 8 field by clicking on Browse and locating the delimited text file on your system 6 Select the address fields by clicking on the drop down menu and fill suitable values in the Address Field City Field State Field and Country Field fields MMQGIS may identify some or all of these fields by default if they are named with logical names such as Address or State Choose the web service Name the output shapefile by clicking on Browse Select a value for the Not Found Output List field by clicking on Browse Any records that are not matched will b
249. in the dictionary returned by runalg Running your own processing script Running a custom processing script is not much different from running a generic processing command We only need to find out how the custom script is addressed by the toolbox To discover all this information we will create and run a simple processing script 363 PyQGIS Scripting Creating a test processing toolbox script We will start by opening the processing toolbox by navigating to Processing Toolbox Next navigate to Scripts Tools Create new script in the toolbox These actions will open a new interface where we can paste the following code snippet import time for index in range 100 progress setPercentage index print index time sleep 0 1 We will save it with the name emptyloop The code is shown in the following screenshot Script editor A ZA go lt 1 import time 24 for index in range 100 progress setPercentagelindex print index time sleep 0 1 3 fil 3 The default directory where processing will look for user scripts is lt your home dir gt qgis2 processing scripts So our file script will be available with the name lt your home dir gt qgis2 processing scripts emptyloop py The preceding script will count from 0 to 99 In addition the algorithm will set the progress bar and write the step number in the Python console The last instruction will wait for 0 1 seconds before running a loop ag
250. indow The model can also be deleted here by clicking on Delete model 286 Chapter 8 Processing Toolbox Recently used algorithms a GDAL OGR 34 geoalgorithms w GRASS commands 168 geoalgorithms 8 Models 1 geoalgorithms Landcover i Tools Execute Execute as batch process Z Qas pRa Edit rendering styles for outputs Edit model Delete model If a model is opened in the Processing modeler window individual model input and algorithms can be modified As we mentioned in the Adding inputs section of this chapter clicking on the pencil icon of a model s input will open the Parameter definition dialog so that changes can be made Clicking on the close button X will delete the input from the model From the modeler canvas information about algorithm parameters can be exposed by clicking on the signs above and below an algorithm This is a convenient way to see algorithm parameters without opening each algorithm Right clicking on an algorithm opens a context menu as you can see in the following screenshot Clicking on Remove deletes the algorithm from the model as long as there are no other algorithms depending on its output Dissolve result Segments Distance Input layer In g g Fixed distances Remove Out Buffer e Deactivate 287 Automating Workflows with the Graphical Modeler If you attempt to delete an algorithm in the middle of a workflow you wi
251. ing feature QGIS will cut the new polygon to create the shared boundary Enable snapping on intersection This allows you to snap to an intersection of another background layer For this example leave the above two options unchecked 5 Click on OK to close the Snapping options window The completed Snapping options are displayed in the following screenshot Snapping options x parcels to vertex and segment 10 00000 Enable topological editing Enable snapping on intersection Cancel 6 There are additional editing parameters that need to be set from Options under Settings on the Digitizing tab Set the value for Search radius for vertex edits field to 10 pixels as shown in the figure below Setting this value to something larger than zero helps to ensure that QGIS finds the correct vertex during an editing operation Ww Snapping LE Map Tools Open snapping options in a dock window QGIS restart required E Composer Default snap mode Off ka F Default snapping tolerance 0 00000 map units Digitizing Search radius for vertex edits 10 00000 pixels r 209 Advanced Data Creation and Editing RS Adjusting the layer transparency can help when you work with overlaps A 50 percent transparency setting will allow the overlaps to be visible 7 Uncheck the Show errors option on the Topology Checker panel This declutters the map canvas 8 Here we will work on the first overlap in the list that has a fea
252. ining coordinate reference systems QGIS supports hundreds of coordinate reference systems for data display and analysis In some cases however the supported CRS may not suit your exact needs QGIS provides the functionality to create custom CRS using the Custom Coordinate Reference System Definition tool that can be found by navigating to Settings Custom CRS In QGIS a CRS is defined using the Proj 4 definition format We must understand the Proj 4 definition format before we can define a new or modify an existing CRS therefore in the first part of this section we will discuss the basics of Proj 4 and in the second part we will walk you through an example to create a custom CRS Proj 4 is another Open Source Geospatial Foundation http osgeo gt org project used by QGIS similar to OGR and GDAL This project is for managing coordinate systems and projections For a detailed user manual for the Proj 4 format used to specify the CRS Parameters in QGIS visit the project website at https trac osgeo org proj Understanding the Proj 4 definition format The Proj 4 definition format is a line composed of a series of parameters separated by spaces Each parameter has the general form of parameter value The parameter starts with the character followed by a unique parameter name If the parameter requires a value to be set then an equal sign character will follow the parameter name and the value will follow the equal sign If a
253. ins In Chapter 1 A Refreshing Look at QGIS we covered basic vector data editing that included layer based snapping In this final section we will cover how to repair topological geometry errors via topological editing We will continue to use the parcels shp data as an example Topological editing only works with polygon geometries The editing approach taken depends on the topological error you are addressing In the last section three types of errors were found gaps overlaps and duplicate geometries These are three of the most common errors associated with polygon data and we will look at how to repair these three types of errors 207 Advanced Data Creation and Editing Example 1 resolving duplicate geometries Duplicate geometries are the most straightforward errors to address Here are the step by step directions for resolving this type of topological conflict 1 Toggle the editing option on for the parcels layer 2 Double click on the first instance of a duplicate geometry in the Topology Checker error table to zoom to it 3 Use the Select Features by Rectangle tool iy to select the duplicate parcel Open the attribute table Change the display filter in the lower right corner to Show Selected Features VA Attribute table parcels Features total 6970 filtered 2 selected 2 SUBDIVISIO STREETNUMB STREETNAME STREETDESI 0000 CACTUS SHALIT PARCEL 1700 YALE BLVD 0000 CACTUS SHALIT PARCE
254. ion layer Elevation EPSG 26710 Altitude of the sun in degrees above the horizon 30 000000 Azimuth of the sun in degrees to the east of north 270 000000 Factor for exaggerating relief 1 000000 Scale factor for converting horizontal units to elevation units 1 000000 et scaling factor applies to lat Jong locations only none scale 1 none GRASS GIS 7 region extent xmin xmax ymin ymax wtemnt ee mn cowering GRASS GIS 7 region cellsize feave 0 for default 0 000000 Output shaded relief layer C Mastering_OGIS GRASS hillshade tif Open output file after running algorithm We could have easily used the built in Terrain Analyses tools and executed the Hillshade tool by navigating to Raster Terrain Analyses but the decision to use GRASS was deliberate to illustrate that more often than not algorithms in the processing toolbox offer more optional parameters for better control over the resulting output In this case the output can be moved to the bottom of the data layer panel and the blending mode of the elevation layer can be set to Darken The results of this blending operation are shown in the following screenshot 226 Chapter 7 Although this tool would typically be considered more of a geoprocessing action than an analytical tool this type of algorithm has been used to evaluate topographic shading at various times throughout a given year to estimate the persistence of snow and chara
255. ions Transparency 0 Line color D v Line width 0 20000 Start Angle Top z Scale dependent visibility Minimum 1 000000 97 AA Styling Raster and Vector Data Creating a text chart diagram The text chart diagram displays attributes in a round circle where each attribute occupies a horizontal slice of the circle and the attribute value is labeled inside the slice As an example the following figure shows a portion of a state s counties with text charts showing the Hispanic population in the top half and non Hispanic population in the bottom half of the circle By reviewing the text charts in the preceding figure let s note down a few things e The labels report each race s attribute and are colored based on the selected attribute colors e The size of the chart varies according to the total population of each county e The horizontal slice outlines are black and thin e The text charts are displayed above the centroid of each polygon These four noted items among others that are not noted are all customizable using parameters available in the Diagrams tab of the Layer Properties window The Diagrams tab for text charts has four tabs with parameters Appearance Size Position and Options The Appearance tab shown in the following screenshot provides the following parameters Transparency This is used to specify the transparency percentage for the text chart e Background color This contains the b
256. iparianTreeClipper model model can be saved Q The model name and group name must be set before the Save Model Hes J qgis processing models v File name SEWED S T efa Save as type Processing models model If models are saved to the Models folder they will appear as model tools in the Processing Toolbox panel Once a model has been named and saved to the Models folder it will appear under its group in the Processing Toolbox Again the model will appear with the name that was entered into the graphical modeler versus the name of the model file Models can be saved outside the Models folder but they won t appear in the Processing Toolbox panel 275 Automating Workflows with the Graphical Modeler Q You will need to close the model before it appears in the Processing Toolbox panel Processing Toolbox 4 Recently used algorithms E Recently used algorithms uenee en Geoalgorithms E GDAL OGR 34 geoalgorithms 8 Models 1 geoalgorithms E y GRASS commands 168 geoalgorithms E Landcover a Models 1 geoalgorithms Riparian trees Landcover Tools df Riparian trees Scripts 0 geoalgorithms Tools Orfeo Toolbox Image analysis 82 geoalgorithms QGIS geoalgorithms 85 geoalgorithms r scripts 19 geoalgorithms SAGA 227 geoalgorithms Scripts 0 geoalgorithms TauDEM hydrologic analysis 26 geoalgorithms A Tools for LIDAR data 58 geoalgorithms 1
257. ire map canvas The following screenshot shows the Inverted polygons style parameters that created the inverted polygon of Nigeria Inverted polygons Sub renderer Single Symbol z Merge polygons before rendering slow Single Symbol Categorized Unit eg Graduated Transparen Rule based a Color a v Symbols in group Open Library 5 W Fill E Simple fill Pe a ie corne diago dotte green land water wine Advanced v If multiple polygons are going to be inverted and the polygons overlap Merge polygons before rendering slow can be checked so that the inverted polygons do not cover the area of overlap Vector layer rendering Layer rendering modifies the properties of the vector to change the way it displays and interacts with the layer below and the features within the vector Layer rendering is a part of vector style properties for all style types and works in the same way regardless of the selected style type In this section we will discuss the parameters that are available for change in the Layer rendering section of vector style properties When a vector is first loaded the Layer rendering parameters are set to their default values as shown in the following screenshot Layer rendering Layer transparency Layer blending mode Normal Feature blending mode Normal 91 Styling Raster and Vector Data The Layer rendering section has three parameters which are as follows Lay
258. irections Pit Filled Elevation Grid DEM Pits Removed EPSG 26710 D3 Flow Direction Grid C Mastering_QGIS TauDEM dnp_d fd tif X Open output file after running algorithm D8 Slope Grid C Mastering_QGIS TauDEM dnp_d sip tif Open output file after running algorithm Calculating the upstream area above Fort Klamath These two grids cover the entire area of the DEM but we are only interested in evaluating what the watershed looks like along Sun Creek upstream of Fort Klamath Many rivers are monitored by USGS gauging stations which can be used as points of interest to delineate the upstream contributing area However more often than not smaller streams typically aren t monitored even though they are important for local communities So we can create arbitrary outlet points that are defined along the stream network To focus our analyses on Sun Creek we will make use of the Gauge shapefile and use as our outlet what is often called a pour point We will use the D8 Contributing Area tool to identify the cells that drain through this pour point In other words we are going to calculate the watershed above this particular point on Sun Creek The following screenshot indicates that we need the D8 flow direction and the Gauge shapefile as input For simplicity we are going to use all the algorithm defaults but it is important to clarify that these parameters may need to be changed depending on local conditions 257 The Processi
259. irelli is a freelance software analyst and developer with an honors degree in computer science from the University of Bari He worked for 15 years in Satellite Ground Segment and Direct Ingestion for the European Space Agency Since 2006 he has been involved with the GFOSS world contributing to QGIS GRASS and MapServer core and developing and maintaining many QGIS plugins He actively participates in QGIS Hackmeetings He is the founder of the OSGEO Italian Local Chapter http gfoss it drupal and now lives in Spain and contributes to this GFOSS community During the last few years he started teaching PyQGIS by organizing trainings from basic to advanced levels and supporting companies to develop their specific QGIS plugins He is the founder of the local hackerspace group Bricolabs cc that is focused on Open Hardware He likes cycling repairing everything and trainings groups on conflict resolution Besides this book he has also contributed to Lonely Planet Cycling Italy A special thanks to the QGIS developer community and core developers as the project is managed in a really open way by allowing contributions from everyone I want to thank everyone with whom I have worked I learned from each of them and without them I wouldn t be here giving my contribution to free software and to this book A special thanks to my friends and neighbors who helped me with my son during the writing of this book I would like to dedicate this boo
260. is the constructor which means that it is called every time you find a call such as Test Plugin iface as you can find in the init py code In our case the constructor needs the iface variable passed as a parameter during construction The constructor has the role of initializing the current translation creating the plugin dialog GUI and also creating the toolbar where the plugin button is to be added The tr self message method is just a shortcut to access the Qt translation engine of string messages The initGui self method gets the icon resource and instructs how to interact with QGIS menu calling the add_action method This method is always called when the plugin is loaded in QGIS It is important to remember the difference between loading a plugin and running a plugin Loading is done using the Plugin Manager or automatically at the start of QGIS if the plugin was already loaded in the previous session Running the plugin is when the user starts the plugin by clicking on the plugin icon or activating it in the plugin menu 325 Creating QGIS Plugins with PyQGIS and Problem Solving e The add_action method has a lot of parameters that allow for fine configuration but most of them are used with their default values True or None The main goal of this method is to create the menu in the plugin menu and to create the button to call the run method In Ot these kind of buttons are objects of the
261. ish to export Navigate to Table Export to file to open the Export vector file dialog Click on the ellipsis button at the right hand side of the Output file text box and name the output file Note that you can only export to the shapefile format using this tool 40 Chapter 2 5 Set the Source SRID Target SRID and Encoding options or leave them unselected to use the default values Select Drop existing one if you wish to overwrite an existing shapefile The following screenshot shows the Export to vector file dialog ready to export to waterfalls shp Export to vector file Options Source SRID Encoding UTF 8 Drop existing one Managing tables DB Manager provides functions to create rename edit delete and empty tables using tools found under the Table menu In this section we will discuss each tool Creating a new table Creating new tables using DB Manager is fairly straightforward When creating a new table you can specify whether it will be a spatial table or a nonspatial table In this section we will create a new spatial table in SpatiaLite to hold data about mountain peaks in a park to do this perform the following steps A To quickly create a new SpatiaLite layer and optionally a Q database in one dialog box in QGIS Desktop navigate to Layer Create Layer New SpatiaLite Layer 1 Open DB Manager by clicking on DB Manager under Database Expand SpatiaLite and select GiffordPinochet
262. isible Inverted polygons This is a new renderer that allows a feature polygon to be converted into a mask For example a city boundary polygon that is used with this renderer would become a mask around the city It also allows the use of Categorized Graduated and Rule based renderers and SQL expressions The following screenshot shows the Style properties available for a vector data layer lt Single Symbol Single Symbol Categorized n Millimeter Size Graduated Transparency 0 J Rotation 0 00 Rule based i s Color Point displacement Inverted polygons Symbols in group oe D isplay O Actions O Marker airport Simple marker 4 Joins arrow capital Diagrams S Oo o i WS Tel e ellipse pentagon square A gt triangle triangle2 eE sxe v Layer rendering y Open Library fa diamond K circle star2 Advanced 7 Layer transparency O Layer blending mode Normal Load Style Save As Default vy Feature blending mode Restore Default Style Cancel Normal Save Style Apply Help In the preceding screenshot the renderer is the layer symbol For a given symbol you can work with the first level which gives you the ability to change the transparency and color You can also click on the second level which gives you control over parameters such as fill border fill style border style joi
263. isnull B 0 10 iffisnull C 0 100 GRASS region extent xmin xmax ymin ymax Leave blank to use min covering extent GRASS region cellsize leave 0 for default 0 000000 Output raster layer C Mastering_OGIS GRASS View cum_view tif Open output file after running algorithm 241 The Processing Toolbox The resulting output will contain values that can be used to interpret which towers contribute to the cumulative viewshed These values are summarized in the following screenshot To better visualize the cumulative viewshed within the park you can load the view_style qml layer and adjust the colors to your preference as follows 0 000000 Nat visible 10 000000 5 11 000000 Tower 1 and 2 100 000000 Tower 3 101 000000 Towers 1 and 3 110 000000 Towers 2 and 3 111 000000 Towers 1 2 and 3 Calculating raster statistics using r stats To evaluate the total cumulative impact we can use r stats to summarize the number of pixels with these eight corresponding values In the r stats dialog we need to select the cumulative viewshed as the input raster make sure that the Print area totals and Print Category labels options are checked and set an output filename By default the One cell range per line option is checked and we need to uncheck this option The results of this algorithm will summarize the area in square meters by category In this case the categories are equal to the eight values in the previous screenshot
264. isplayed in the QGIS canvas 377 A Add item button 60 address based data address geocoding working 177 178 geocoding 176 177 address geocoding local street network data using 181 183 web services using 178 180 working 177 178 Add rule button 86 advanced field calculations writing about 134 current date calculating 135 current date formatting 135 geometric values inserting 136 population dependent label string calculating 137 139 algorithms adding 279 284 Application Programming Interfaces API 301 Aptana installing 327 URL for downloading 327 B basic vector geoprocessing tools Buffer tool 123 Convex Hull tool 125 Dissolve tool 126 127 spatial overlay tools 119 using 118 Index batch processing using with model 298 299 buffers creating with Buffer tool 123 124 C Calculator tool 280 CamelCase format URL 313 categorized vector style 81 82 changeable panels color picker about 54 color ramp panel 54 color sampler 57 color swatches panel 55 56 color wheel panel 55 Clip grid with polygon tool 292 Clip tool 120 code refactoring URL 302 ColorBrewer color ramp adding 63 color picker about 52 available components 53 54 changeable panels 54 color ramp adding 60 ColorBrewer color ramp adding 63 cpt city color ramp adding 64 65 editing 65 exporting 59 Gradient color ramp adding 61 379 importing 60 managing 57 QGIS color ramp collection managing 58 Random color
265. ithm in another model Models won t appear as algorithms if some of their component algorithms are not available This can happen if an algorithm _ provider is deactivated in Providers and you can find this by amp navigating to Processing Options For example if you have f used a SAGA tool in a model but have subsequently deactivated SAGA tools that model will not be available As long as all the algorithms in a model are visible in the Processing Toolbox panel a model will be available as an algorithm 294 Chapter 8 To demonstrate this feature we will build on the model that we used in the previous section The model clipped elevation data by watershed boundaries With a DEM you can generate a metric called Topographic Wetness Index TWI The QGIS sample model TWI_from_DEM mode1 shown in the following screenshot takes one input a DEM From this input it generates slope and catchment areas These then feed into the Topographic Wetness Index algorithm 4 Processing modeler e A B Parameters TWI From DEM Hydrology P Boolean ap Extent oP File P Number x dp Raster layer catchment Area P String P Table dP Table field qP Vector layer Out In F S Slope aspect curvature Out In S Topographic wetness inde Out 295 Automating Workflows with the Graphical Modeler First we will create a new copy of the DEMs Clipped to Watersheds model using the
266. ithms a G GDAL OGR 32 geoalgorithms y GRASS commands 167 geoalgorithms ft n Models 3 geoalgorithms A Orfeo Toolbox Image analysis 83 ge OGIS geoalgorithms 79 geoalgorithms R scripts 19 geoalgorithms SAGA 228 geoalgorithms Scripts 13 geoalgorithms TauDEM hydrologic analysis 26 geoa kq Tools for LIDAR data 55 geoalgorithms B B B B B B E Initially you will only see a list of the various providers and a summary of the total algorithms available from each provider When you click on the icon next to any of the entries the list expands to reveal subdirectories that group related tools In addition to manually navigating these directories there is a search box at the top of the toolbox So if you are already familiar with these third party packages or are looking for a specific tool this may be a more efficient way to access the algorithms of interest You can search algorithms by topic Even if you aren t familiar with the algorithm providers you can still A use the search box to explore what tools are available from multiple Q providers For example if you are interested in finding different ways to visualize or explore topographic relationships you could search for it by typing topographic in the text box and discover that there are ten tools from three different providers that relate to topography 220 Chapter 7 To open any algorithm of interest you
267. just need to double click on the name and the algorithm dialog interface will open It looks similar to other tools that we have already used in QGIS For example click on the icon next to the GDAL OGR entry and double click on Aspect You will see the dialog interface as shown in the following screenshot Aspect Save to temporary file sof 221 The Processing Toolbox Any algorithm that you select will present you with a similar algorithm dialog box so it is worth exploring the functionality of the interface Similar to other tools that we have already used clicking on any inverted black triangle will reveal drop down options that will allow us to select an option that is passed to the algorithm The dialog boxes within the processing toolbox also provide two additional tabs that may provide additional information The Log tab will record the history of any operations performed using the tool which is often useful for debugging errors and the Help tab provides a brief summary of the functionality and explanation of the various options presented in the interface However if we click on the Help tab for the Aspect tool we are presented with the message Sorry no help is available for this algorithm This is not an uncommon experience so if the functionality or optional parameters are unclear we need to visit the website of the algorithm provider You can explore the functionality of each of the algorithms that we are going
268. k to my partner and especially to my son for his patience when he used to see me sitting in front of a computer for hours instead of playing with him www allitebooks com Dr John Van Hoesen is an associate professor of geology and environmental studies at Green Mountain College in rural west central Vermont He earned an MS in 2000 and a PhD in geology from the University of Nevada Las Vegas in 2003 He is a certified GIS Professional GISP with a broad background in geosciences and has been using some flavor of GIS to evaluate and explore geologic processes and environmental issues since 1997 He has used and taught some variant of FOSS GIS since 2003 and over the last 3 years he has taught graduate undergraduate and continuing education courses using only FOSS GIS software www allitebooks com About the Reviewers Paolo Corti is an environmental engineer based in Rome Italy He has more than 15 years of experience in the GIS field after working with proprietary solutions for some years he proudly switched to open source technologies and Python almost a decade ago He has been working as a software architect developer and analyst for organizations such as the United Nations World Food Programme the European Commission Joint Research Centre and the Italian Government Currently he is working within the GeoNode project for which he is the core developer in the context of emergency preparedness and response He is an OS
269. ks com A Refreshing Look at QGIS At the bottom of the Style menu under Layer Properties set the Blending mode to Multiply and the Contrast to 45 and click on Apply The blending mode allows all the details of both the datasets to be seen Experiment with different blending modes to see how they change the appearance of the image The following screenshot shows an example of how blending and contrast settings can work together to make a raster pop off the screen 4 QGIS 659cada BlendingModes Project Edit View Layer Settings Plugins Vector Database Web Processing Help DEBERA MOS se sRPN AAs A A j 4 Pi rs PR abs aby gt Ge abo a abe csw 44 S aAa Aa eA RO F Aa Layers Ex A w Yodo E Jemez_vegetation Agricultural Conifer Conifer hardwood Developed Exotic herbaceous Exotic Tree shrub Grassland Hardwood Non vegetated Riparian Shrubland Recently burned Sparsely vegetated 5 8 Jemez_hillshade 132 206 Ve r k fa Q Q va Sy 4 rf Seat Layers Browser licors sonas see issoro vm Rere scan OE 22 Chapter 1 Composing maps With QGIS you can compose maps that can be printed or exported to image and graphic files To get started click on New Print Composer under Project Give the new composition a name click on OK and the composer window will open The composer presents you with a blank sheet of paper upon which you can craft your map Along the left hand side there
270. l be constructed in three parts and then concatenated together using the concatenation operator two vertical bars Replace result with the STATE_NAME field Add a concatenation operator after STATE NAME by either typing two vertical bars or by clicking the concatenation operator button i _ This allows the following text to be concatenated with the contents of the STATE _ NAME field After the concatenation operator type nPopulation and keep a space between the colon and closing single quote The n is interpreted as a new line and starts a new line in the string 138 10 11 12 13 Chapter 4 Add a concatenation operator to the end of the line The last item to add to the string is the population value stored in the POP1996 field However the population is stored as an integer and an integer or any other number cannot be concatenated to a string Therefore we need to convert the integer to a string so that we can concatenate Luckily for us the format_number function converts a number to a string and adds thousands separators and rounds the number although rounding is not needed in this case To convert a number to a string without formatting use Js the tostring function After the concatenation operator add the format_number function by expanding String in Function list and double click on format_number You can also manually type in the function Inside the parenthesi
271. l can also be shared with others Opening the graphical modeler The graphical modeler can be opened from QGIS Desktop using either of the following two ways By clicking on Graphical Modeler under Processing e By enabling the Processing Toolbox panel navigating to Models Tools and then clicking on Create new model 272 Chapter 8 The processing modeler opens as a new window On the left hand side of the window there are two tabs Inputs and Algorithms These are used to add both types of elements to the modeler canvas that takes up the remainder of the window Above the modeler canvas there are the Enter model name here and Enter group name here input boxes to enter the model name and the group name The buttons for managing models can be found above the Input and Algorithm tabs as shown in the following screenshot 4 Processing modeler amp 8 n DE Parameters P Boolean dp Extent qP File dP Number qP Raster layer oP String dP Table dP Table field qP Vector layer Modeler Canvas Inputs and Algorithms graphical modeler The window itself is called processing modeler and not 7 273 Automating Workflows with the Graphical Modeler Configuring the modeler and naming a model Before starting a model it is a good practice to configure the modeler Models are saved as JSON files with a model extension When you save a model QGIS will prompt you to save the model file to the Mod
272. l view Click on the Execute button to create the view The following screenshot displays a successfully written and executed view of the Waterfalls table SOL window GiffordPinochet sglite SpatiaLite SQL query Store Delete 1 CREATE VIEW mowich_lake_waterfalls as 2 SELECT w pk as ROWID w NAME w TYPE w geom from Waterfalls as w 3 WHERE w quadname Mowich Lake Execute F5 1rows 0 1 seconds Clear Now that the view is created we need to register it as a spatial view by inserting a new row in the views_geometry_columns table This table links the view s geometry to the geometry of the table it selects from In the SQL window click on the Clear button to clear the SOL query textbox Enter the following query INSERT INTO views geometry columns view name view geometry view rowid f table name f geometry column read only VALUES mowich lake waterfalls geom rowid waterfalls geom 1 In this INSERT query six fields have values inserted in them view name This contains the name of the view that we wish to register as spatial view _geometry This contains the name of the geometry field in the view 47 Creating Spatial Databases view rowid This contains the name of the rowid field Note that it must be rowid If the rowid field is named something else you will need to recreate the view with a rowid field table _name The name of the table the view is selecting from
273. late convert format Parameters Log Help Input layer elevation EPS6 26912 Set the size of the output file In pixels or 4 100 Output size is a percentage of input size Nodata value leave as none to take the nodata value from input none Expand none Output projection for output file eave blank to use input projection Subset based on georeferenced coordinates xmin xmax ymin ymax Leave blank to use min covering extent Copy all subdatasets of this file to individual output files Additional creation parameters optional Output raster type UInti6 Output layer Save to temporary file Open output file after running algorithm 151 Preparing Raster Data for Processing The following figure shows the result of raster resampling The original elevation raster with 27 3526 meter pixel resolution is on the left and the resampled raster with 100 meter pixel resolution is on the right There are two additional tools that can be used to resample raster data and both are found in the Processing Toolbox Under GRASS commands Raster tools there is the r resample tool Q Under SAGA Grid Tools there is the Resampling tool Both these tools have similar options to the GDAL Translate tool and are included with most installations of QGIS Installing and troubleshooting SAGA on different platforms SAGA like GRASS GIS is a standalone application whose tools can be accessed from within the QGIS Processing Toolbox
274. lator window 2 Select Create a new field and set the following options Output field name StateLabel Output field type Text string Output field width 35 3 Inthe Function list field expand the Conditionals node and then double click on CASE ELSE to add it to the Expression area This will add CASE WHEN expression THEN result ELSE result END to the Expression area We will replace expression with the test for populations greater than five million The result after THEN will be replaced with the label we wish to create when expression is true The result after ELSE will be replaced with the label we wish to create when expression is false 137 Preparing Vector Data for Processing 4 Let s start by setting the label for states with a population of less than five million Replace the result after ELSE with the field name STATE_NAME Now we will set the condition to check for population greater than five million Replace condition with PoP1996 gt 5000000 The following figure shows the expression with optional formatting to make it easier to read Expression WHEN POP1996 gt 5000000 THEN result STATE NAME END The last step is to calculate the string for states with a population greater than five million The format of the string will be lt state name gt Population lt population gt with the state name on the first line and the population on the second line As this is a complex string it wil
275. lbox In this section we will ensure that the processing toolbox is correctly configured to access and execute the algorithms within the GRASS SAGA TauDEM and LAStools libraries Many of the required libraries are automatically installed but configuring these tools will vary depending on your operating system and how you choose to install QGIS 216 Chapter 7 Finding support for your installation To use some of the GRASS algorithms in this chapter you will need to make sure that you properly install GRASS 7 If you used the OSGeo installer you can use the advanced installer option to add GRASS 7 to your installation otherwise you will need to manually install GRASS 7 and set the path directory in the processing toolbox mS In depth instructions for configuring SAGA are provided in Chapter 5 Preparing Raster Data for Processing Instructions for configuring most third party algorithms on different operating systems can be found on the QGIS website athttp docs qgis org 2 6 en docs user_ manual processing 3rdParty html Support for installing LAStools on Windows Mac OS X and Linux can also be found at http rapidlasso com category tutorials To begin configuring the toolbox we need to click on Options and configuration under Processing which is illustrated in the following screenshot Note that if you are using a Linux distribution this configuration can be found by navigating to Processing Options Providers
276. ld can also be created by selecting Create a new field and Create virtual field A virtual field is not stored in the dataset instead it is stored as an expression in the QGIS project file and will be recalculated every time the field is used Function list This contains a tree of field calculation functions available for insertion into the expression e Function help This displays the help documentation for the selected function in the function list e Operators This ensures quick button access to insert commonly used operators into the expression These operators are also in the function list under the Operators branch e Expression This is an editable text area that contains the expression that will calculate field values Underneath the expression is a preview of the output for a sample record If the expression is invalid a notice will appear witha link to more information about the expression error 132 Chapter 4 Field calculator M Create a new feld A Update existing field Create virtual field Field Designation Output field name Output field type Whole number integer Output field width 10 gt Precision 0 AREA Function list F Operators Conditionals Function Help Math Conversions Date and Time String w Operators perators Ph P P ad Expression Expression OEa Output preview The expression must meet strict syntax guidelines otherwis
277. lick currentPos clickedButton if clickedButton Qt LeftButton provider iface activeLayer dataProvider result provider identify currentPos QgsRaster IdentifyFormatValue if result isValid print Value at d d currentPos x currentPos y print result results if clickedButton Qt RightButton reset to the previous mapTool iface mapCanvas setMapTool previousMapTool clean remove myMapTool and relative handlers myMapTool deleteLater 373 PyQGIS Scripting Running the code of the previous paragraph with the manageClick function will generate an output similar to the following screenshot Python Console 3 gt gt gt exectile u mnt data Libri gt GIS PyQGIS Link to Dropbox QG rasterIdentify py IS Mastering Packt Book raster from PyQt4 qt Identify py encode LUTF 8 from ggis cor 4 Value at 540057 4274114 from qgis qui 5 1 7 0 6 Value at 10501 5414559 inf w A e C c gt previousMapTo 7 1 6 0 A myMapTool Ei a Value at 679038 5886467 ii 7 km 9 1 6 0 def managecli 10 Value at 1111620 5257256 if click i3 11 1 6 0 12 Value at 1229597 4942651 13 1 12 0 14 Value at 2134088 3133670 15 1 0 0 16 gt gt gt The result of the identify call is a QgsRasterIdentifyResult object that contains all the information of the result In this case the return of results is a Python dictionary with the band nu
278. limited Text File File Name 0 PACKT Data Chapter 6 cougar_sightings csv Layer name cougar_sightings File format CSV comma separated values Custom delimiters Regular expression delimiter Comma Tab Space Colon Semicolon Other delimiters Quote Escape Record options Number of header lines to discard 0 X First record has field names Field options Trim fields Discard empty fields Decimal separator is comma Geometry definition Point coordinates Well known text WET No geometry attribute only table X field UTM_X N 19 11 field UTM_Y N 19 11 DMS coordinates Layer settings Use spatial index Use subset index Watch file L SAMPID C 20 SEX C SEX C 10 UTM_X N 19 11 UTM_X N 19 11 UTM_ N 19 11 a paos7 PAGE 115556 044021 3486272 88304 2 PADS 116870 545644 37489102 55056 2 PAOS8 116148 894117 3483420 30411 4 PNoo1 i 482000 018751 3700998 34463 5 PNoo2 296192 720405 4053069 38808 E PNOO3 o47990 948525 3990302 276593 172 Chapter 6 Once the tool has been run a new point layer will be added to QGIS with all the attributes present in the original file unless you chose to discard empty fields However this is not a standalone GIS layer yet It is simply a rendering of the tabular data within the QGIS project As such it will behave like any other layer It can be used as an input for other tools records can be selected and it can be styled However it cannot be edited To conv
279. ll be a Python dictionary This is composed of a key the ID of the feature to change and another dictionary as a value This last dictionary will have the index of the column to change as the key and the new value that has to be set as the value For example the following code snippet will change the area value to 0 for column 2 of feature 599 354 Chapter 10 columnIndex myVector pendingFields fieldNameIndex AREA MI newColumnValueMap columnIndex 0 newAttributesValuesMap myFeature id newColumnValueMap myDataProvider changeAttributeValues newAttributesValuesMap You can check whether the value of the parameter has changed by navigating to View Identity Features If you want to change more than one attribute you just have to add more key value pairs in the newColumnValueMap dictionary using the following syntax newColumnValueMap columnindexl newValuel columnIndexN newValueN Deleting a feature A feature can be deleted by pointing at it with its ID The ID of the feature can be obtained with the id method of the QgsFeature class The following snippet will remove feature 599 myDataProvider deleteFeatures 599 After you refresh the canvas myVector will be shown as in the following screenshot 355 PyQGIS Scripting Adding a feature After we reload the original Alaska shapefile we will again get the feature 599 that we will use as the base to create a new feature T
280. ll see the following message The dependent downstream elements will have to be deleted prior to deleting the algorithm re Could not remove element i Other elements depend on the selected one Remove them before trying to remove it Clicking on Edit from the algorithm context menu opens the algorithm dialog so that changes can be made to the model After editing an algorithm the connections to other model elements in the canvas will be updated The algorithm parameters exposed by clicking on the signs above and below the algorithm will also be updated Dissolve result Segments Distance Input layer In E Remove cA Fixed distance bu Out E Buffer z Deactivate Clicking on Deactivate from the algorithm context menu will deactivate the algorithm and all algorithms downstream that depend on that algorithm An algorithm can be reactivated at any point by right clicking on it and choosing Activate When you do this any other downstream algorithms that were deactivated earlier will have to be individually reactivated Documenting a model Model help can be written for any model by clicking on the Edit model help button C3 within the Processing modeler window This will open the Help editor window that has three panels At the top is an HTML page with placeholders for the Algorithm description Input parameters and Outputs sections At the bottom left corner there is an element selection box and there is a box for
281. ll values To access the Rasterize tool click on Rasterize Vector to Raster by navigating to Raster Conversion The Rasterize tool shown in the following figure uses the DFW_Land_Cover shp sample file as input 162 To convert a vector to a raster the following options are available Rasterize Vector to raster Input file shapefile ar DFW Land _Covershp Y Attribute field LC Output file for rasterized vectors raster _landcover ConvertedLc tif Keep existing raster size and resolution Raster size in pixels Lear Tata Width 3000 oa aa i Raster resolution in map units per pixel Horizontal 60 00000000 Vertical 60 00000000 4 Load into canvas when finished odal_rasterize a LC tr 60 0 60 0 4 DFW _Land Cover D Users Rick OneDrive Publications Mastering QGIS Chapter 5 Data nled2006_TX_landcover DOFW_Land_Covershp D Users Rick OneDrive Publications Mastering OGIS Chapter 5 Data nied2006_TX_landcover ConvertedLc tif Chapter 5 mx Select Input file shapefile The input vector file to be converted The tool supports multiple vector formats Attribute field The attribute field holds the value to assign to the raster cells Output file for rasterized vectors raster The output raster file The tool supports multiple raster formats Keep existing raster size and resolution This is only selectable if the output file already exists This sets the
282. ll within specified Hue Saturation and Value ranges The Classes parameter determines how many colors to generate Colors are randomly generated each time any one of the parameters are changed As an example in the following screenshot five random colors are generated with different hues between 100 and 320 but with the same saturation and value Random color ramp Hue from Saturation from Value from When all the parameters have been set click on OK to save the Random color ramp QGIS will prompt you to name the color ramp Once it is named the Random color ramp will appear in the Style Manager s list of color ramps 62 Chapter 3 Add a ColorBrewer color ramp To add a ColorBrewer color ramp click on the Add item button and then choose ColorBrewer This opens the ColorBrewer ramp window shown in the following figure A ColorBrewer color ramp generates three to eleven colors using one of the available schemes The Colors parameter determines how many colors to generate and the Scheme name parameter sets the color scheme that will be used As an example in the following screenshot five colors are generated using the RdGy Red to Grey color scheme ColorBrewer ramp Scheme name Colors Preview When all the parameters have been set click on OK to save the ColorBrewer color ramp QGIS will prompt you to name the color ramp Once it is named the ColorBrewer color ramp will appear in the St
283. lows you to add oN base maps from OpenStreetMap Google Maps Bing Maps Map Quest OSM Stamen and Apple Maps This plugin requires an Internet connection Search sb Installed p Azimuth and Distance Plugin Installed Plugins Color Ramp Manager j i ee eee ere you only see plugins insta on your Not installed amp Coordinate Capture d x p wo os DB Manager Click on the name to see details Settings Pr Digitizing Tools Click the checkbox or doubleclick the name to activate or deactivate the plugin A f2Sh nv eg BEE You can change the sorting via the context menu right click eVis y FlowMapper ad fTools Bi Gdaltools Georeferencer GDAL ci Geotag and import photos Globe SB GPS Tools H GRASS po Group Stats Heatmap A InsSAEE Upgrade all Uninstall plugin Reinstall plugin Close Help 1 gt You can also browse the QGIS Python Plugins Repository athttps plugins gqgis org plugins 25 A Refreshing Look at QGIS Summary This chapter provided a refresher in the basics of Desktop and QGIS Browser We covered how to install the software on several platforms and described the layout of both QGIS Desktop and QGIS Browser We then covered how to load vector raster and database data layers Next you were shown how to work with coordinate reference systems and style data We covered the basics of working with tables including how to perform a table join The chapter conclude
284. lt 7117600 0 1367760 0 4897040 0 7809680 0 340 Chapter 10 Visualizing the layer Finally we can visualize the raster using the centralized QGIS layer manager called QgsMapLayerRegistry This class is like the hub where we can manage layer loading and unloading It s useful to read the list of its methods in the QGIS API documentation QgsMapLayerRegistry is a singleton class This means that it can t be instantiated multiple times like o9gsRasterLayer For example we can have different loaded raster layers and each one is an instance of the QgsRasterLayer class but it s not possible to have different OgsMapLayerRegistry instances This is because it is blocked by code and it s possible to get only the unique instance using the instance method Finally to visualize the layer we have to execute the following code QgsMapLayerRegistry instance addMapLayer myRaster This will produce an output similar to the following lt qgis core QgsRasterLayer object at 0x955f3b0 gt Another way to load it is by using a method similar to the one shown in the following code QgsMapLayerRegistry instance addMapLayers myRaster This gives a similar result as before but with two more brackets because it returns a list lt qgis core QgsRasterLayer object at 0x955fc20 gt The latter addMapLayers method differs from addMapLayer because it accepts a list of layers mixing rasters and vectors Layers will be displayed a
285. lugin architecture Plugins are add ons to QGIS that provide additional functionality Some are written by the core QGIS development team and others are written by QGIS users 24 Chapter 1 You can explore the QGIS plugin ecosystem by navigating to Plugins Manage and Install Plugins This opens the Plugins Manager window shown in figure below that will allow you to browse all plugins those that are installed and those that are not installed and adjust the settings If there are installed plugins with available upgrades there will also be an Upgradable option The search bar can be used to enter search terms and find available plugins related to the topic This is the first place to look if there s a tool or extra type of functionality that you need To install a plugin simply select it and click on the Install Plugin button Installed plugins can be toggled on and off by checking the box next to each You will be notified by a link at the bottom of the QGIS Desktop application if there are updates available for your installed plugins Clicking on the link will open the Plugins Manager window where the Upgrades tab will allow you to install all or some of the available updates Plugins themselves may show up as individual buttons toolbars or as items under the appropriate menu such as Plugins Vector Raster Database Web or Processing To add a base map to QGIS enable the OpenLayer plugin It appears under the Web menu and al
286. lugin run fest plugin py ii No details to display for the current selection lt terminated gt Debug Server The line where the breakpoint is added is marked with the red box on the left and a new breakpoint will be listed in the Breakpoints list as marked by the red box on the upper right corner Debugging in action Now it s time to test the debug session Let s start the PyDev debug server and connect to it as described previously In QGIS run Test Plugin QGIS will now freeze because it s starting to execute the run method of the test_plugin py code and a breakpoint is encountered So the control is passed to the PyDev debugger the Aptana Debug perspective will appear similar to this 334 Chapter 9 Debug Server Python Server nenioan W Mainthread pid 12659 se q1 rm test_plugin py 1947 B Gebug Server r TestPlugin lt TestPlugin test_plugin TestPlugin idg Hsr Here the red box on the left shows where the code is stopped adding an arrow to the previous green bullet The Variables Aptana section marked with the red box on the right displays variables used in the code and their values There are buttons marked in the preceding screenshot by the upper red box to run statements in a step by step way to enter in a function or continue with an execution A detailed description of how to work with the PyDev Debug perspective is out of the scope of this chapter but it s po
287. lumn LEVEL E change Color ramp E BuGn Value Legend High school High school Elementary Elementary Classify Delete Delete all Join Advanced v Styling vector data with the Categorized style type is a four step process which is as follows 1 Select an appropriate value for the Column field to use the attributes for categorization Optionally an expression can be created for categorization by clicking on the Expression button to open the Expression dialog Create the classes to list by either clicking on the Classify button to add a class for each unique attribute that is found otherwise click on the Add button to add an empty class and then double click in the Value column to set the attribute value to be used to create the class Classes can be removed with the Delete or Delete all buttons They can be reordered by clicking and dragging them up and down the list Classes can also be modified by double clicking in the Value and Label columns Set the symbol for all classes by clicking on the Symbol button to open the Symbol selector window Individual class symbols can be changed by double clicking on the Symbol column of the class list Choose the color ramp to apply to the classes Individual class colors can be changed by double clicking on the Symbol column of the class list Other symbol options which will change availability based on vector layer type such as transparency color size and outpu
288. lumns table by using the SQL DELETE command For example to drop the entry related to the mowich_lake_ waterfalls view you will need to execute the following SQL command DELETE FROM views geometry columns WHERE view name mowich lake waterfalls Downloading the example code AI You can download the example code files from your account at http www packtpub com for all the Packt Publishing books Q you have purchased If you purchased this book elsewhere you can visit http www packtpub com support and register to have the files e mailed directly to you Summary This chapter provided you with the steps to handle databases in QGIS While QGIS can handle multiple databases we used SpatiaLite as it provides a good amount of functionality with little overhead or administration Using the DB Manager you can perform a number of operations on databases Operations of note are creating indices spatial and aspatial views importing and exporting and performing queries From the introduction to DB Manager and SpatiaLite in this chapter you are now well equipped to write more complex queries that take full advantage of the SQL commands and SpatiaLite SQL extension commands A full listing of SQLite SQL commands are available at http www sqlite org lang html A full listing of the SpatialLite SQL extension commands are available at http www gaia gis it gaia sins spatialite sql 4 2 0 html The next chapter moves us from the stor
289. m so that we can now create a cost grid that can be used to evaluate the cost associated with moving between individual cells This analysis requires two additional inputs a starting and ending point that are provided as separate shapefiles Start shp and End shp These points serve as a guide for how the algorithm should characterize the cost of moving through an area of interest The following screenshot illustrates how to populate the r cost tool er QG r cost Creates a raster layer of cumulative cost of moving across a raster layer whose cell va Hu g F Unit cost layer Slope LULC Recode EPSG 26710 Start points Start FPSG 26710 Stop points End EPSG 26710 Use the Knight s move slower but more accurate Keep null values in output raster layer GRASS region extent xmin xmax ymin ymax Fave Udik WU Use Till DYE EA LET GRASS region cellsize leave 0 for default 0 000000 Cumulative cost C Mastering_QGIS GRASS cost tif X Open output file after running algorithm 233 The Processing Toolbox Calculating the cost path using least cost paths This land use layer can now be used to calculate the least cost path between each individual cell To accomplish this we are going to make use of a tool from the SAGA library which is explored in more depth later in this chapter This approach again demonstrates the flexibility of the processing toolbox and how easy it is to combine tools from various librarie
290. mber as the key and the raster value in the clicked point as the value Exploring the QgsRubberBand class A rubber band is the graphical canvas item that can be used to draw geometry elements on the canvas for example points lines or polygons It is generally used in combination with a customized Map Tool to get click coordinates that are used to add or move points of the rubber band object 374 Chapter 10 The following code snippet will upgrade the previous example to get canvas click coordinates to draw a polygon in the canvas from PyQt4 QtCore import Qt from PyQt4 QtGui import QColor from gqgis core import QGis from gqgis gui import QgsMapToolEmitPoint QgsRubberBand previousMapTool iface mapCanvas mapTool myMapTool QgsMapToolEmitPoint iface mapCanvas create the polygon rubber band associated to the current canvas myRubberBand QgsRubberBand iface mapCanvas QGis Polygon set rubber band style color QColor red color setAlpha 50 myRubberBand setColor color def showCoordinates currentPos if myRubberBand and myRubberBand numberOfVertices myRubberBand removeLastPoint myRubberBand addPoint currentPos iface mapCanvas xyCoordinates connect showCoordinates def manageClick currentPos clickedButton if clickedButton Qt LeftButton myRubberBand addPoint currentPos terminate rubber band editing session if clickedButton Qt RightButton remove
291. metries Rule must not have multi part geometries must not overlap must not overlap with The list of available rules changes depending upon which Q target layer is chosen 4 Since this rule involves only the target layer the final drop for a second layer disappears 5 Now we will add a second rule Set the target layer to parcels and the rule as must not overlap Click on the Add Rule button SH Add Rule 7 For the third rule again set the target layer to parcels and set the rule as must not have duplicates 204 Chapter 6 8 Click on the Add Rule button 9 For this example we will test three rules against the parcels layer O must not have gaps O must not overlap O must not have duplicates 10 The rules that have been established are summarized in the Topology Rule Settings dialog box w Topology Rule Settings No layer Delete Rule MH Add Rule Rule yer 1 Layer 2 Tolerance f must not have gaps parcels No layer No tolerance 2 must not overlap parcels No layer No tolerance 3 must not have duplicates parcels No layer No tolerance 11 A rule can be deleted by selecting it and choosing Delete Rule Delete Rule 12 Click on OK when the topological rules have been defined As of version 0 1 of the Topology Checker plugin the X Tolerance setting is not operational If it were it would allow a tolerance to be set in map units
292. mmetrical Difference tools by showing example inputs and outputs for each Intersect Symmetrical Difference 122 Chapter 4 Overlaying polygon layers with Union The Union tool overlays two polygon layers and preserves all the features of both datasets whether or not they intersect The following figure displays an example union using the North_PDX_parks shp and Kenton shp shapefiles from the sample data Attributes from both datasets are contained in the output attribute table Output of the Union tool Creating buffers The Buffer tool is a commonly used tool that produces a new vector polygon layer that represents a specific distance from the input features It can be used to identify proximity to a feature Here we will buffer the Portland fire departments by one mile this will identify all the areas within the one mile buffer distance To buffer we will perform the following steps 1 The Input vector layer field will be set to Portland _FireStations 123 Preparing Vector Data for Processing 2 Next we ll specify the buffer distance The tool will use the units of the coordinate reference system of the input vector layer as the distance units This data is in State Plane Oregon North with the unit as feet Enter 5280 to produce a one mile buffer You also have the option of specifying an attribute column containing buffer distances This allows you to specify different buffer distances for different features
293. mple color button The following screenshot shows the color sampler with Sample average radius of 5 px pixels and a preview of the green color currently under the mouse cursor 4 0 H A Sample average radius 5 px Press space fo sample a color from under ie mouse ELSO Now that the color picker dialog has been toured and you know how to select colors and manage them in palettes we will look at how to create and manage color ramps Managing color ramps Color ramps are used in multiple applications when styling data A color ramp is a series of continuous or discrete colors that can be applied to raster or vector data values QGIS contains a number of color ramps that are ready to use and also allows you to add new color ramps In this section we will first demonstrate how to manage the QGIS color ramp collection and then how to add new color ramps 57 Styling Raster and Vector Data Managing the QGIS color ramp collection Color ramps can be managed and created using the Style Manager window The Style Manager window provides an interface to manage the marker line fill and color ramps that are available in the Style tab of the layer property window To open the Style Manager navigate to Settings Style Manager and then click on the Color ramp tab The Style Manager window is shown in the following screenshot Smart Groups ri Marker ee Line l Fill br Color ramp BrBG BuGn GnBu Greens Greys Oranges Help
294. myFeature geometry bbox oldGeom boundingBox newGeom QgsGeometry fromRect bbox myVector startEditing myVector changeGeometry myFeature id newGeom myVector commitChanges In this case it s not necessary to refresh the canvas because a canvas refresh is triggered by events generated during a commit After the commit the interface will appear similar to the image shown in the Adding a feature section 358 Chapter 10 In the preceding code you can see that the changing geometry has a different API using the data provider In this case we can change a feature each time Changing a feature s attributes After we get feature 599 we can change its attributes in a way similar to how we changed the geometry of the feature in the previous section The following snippet will change the area value to 0 for column 2 of feature 599 columniIndex myVector pendingFields fieldNameIndex AREA MI myVector startEditing myVector changeAttributeValue myFeature id columniIndex 0 myVector commitChanges You can check whether the value of the parameter has been changed by navigating to View Identity Features In the preceding code notice that the changing attribute has a different API using the data provider In this case we can change features one at a time Adding and removing a feature Since the procedure is really similar to those applied using the data provider you can test these actions b
295. n appropriate value for the Column field to use the attributes for classification Optionally an expression can be created for classification by clicking on the Expression button to open the Expression dialog 2 Choose the number of classes and the classification mode The following five modes are available for use O Equal Interval In this mode the width of each class is set to be the same For example if input values ranged between 1 and 100 and four classes were desired then the class ranges would be 1 25 26 50 51 75 and 76 100 so that there are 25 values in each class 83 Styling Raster and Vector Data O Quantile Equal Count In this mode the number of records in each class is distributed as equally as possible with lower classes being overloaded with the remaining records if a perfectly equal distribution is not possible For example if there are fourteen records and three classes then the lowest two classes would contain five records each and the highest class would contain four classes Natural Breaks Jenks The Jenks Breaks method maximizes homogeneity within classes and creates class breaks that are based on natural data trends Standard Deviation In this mode classes represent standard deviations above and below the mean record values Based on how many classes are selected the number of standard deviations in each class will change Pretty Breaks This creates class boundaries that are roun
296. n streams on the Tree panel and then choose Add to canvas 34 Chapter 2 parang a shapefile into SpatiaLite Open DB Manager by clicking on DB Manager under Database Expand SpatiaLite and select GiffordPinochet sqlite on the Tree panel 2 Navigate to Table Import layer file to open the Import vector layer dialog as shown in the following screenshot 3 Click on the ellipsis button at the right hand side of the Input drop down box and select and open NF_roads shp from the sample dataset that is available for download on the Packt Publishing website 4 Click on the Update options button to load the remainder of the dialog box The output table name will populate as NF_roads and it will match the base name of the input file 5 Set the following options Select Source SRID and enter 26910 This is the EPSG code for NAD 83 UTM Zone 10 North Since we don t want to change the coordinate system during import we do not need to set Target SRID O Select Create spatial index 6 Click on the OK button to import the file Import vector layer Input Ins Mastering QGIS Chapter 2 G5T103_Lab2_Data Data NF_roads shp 7 Update options Output table schema P Table NF_roads Options Primary key Geometry column Source SRID 26910 Target SRID Encoding LTF 8 Drop existing table Create single part geometries instead of multi part M Create spatial index 35 Creating Spatial Databases 7 Afte
297. n style border width and X Y offsets These parameters change depending on the geometry of your layer You can also use this hierarchy to build symbol layers which are styles built from several symbols that are combined vertically 17 A Refreshing Look at QGIS Styling raster data You also have many choices when styling raster data in QGIS Desktop There is a different choice of renderers for raster datasets which are as follows Singleband gray This allows a singleband raster or a single band of a multiband raster to be styled with either a black to white or white to black color ramp You can control contrast enhancement and how minimum and maximum values are determined e Multiband color This is for rasters with multiple bands It allows you to choose the band combination that you prefer e Paletted This is for singleband rasters with an included color table It is likely that it will be chosen by QGIS automatically if this is the case Singleband pseudocolor This allows a singleband raster to be styled with a variety of color ramps and classification schemes The following is a screenshot of the Style tab of a raster file s Layer Properties showing where the aforementioned style choices are located 4 Layer Properties dem Style General W Band rendering Render type Singleband gray asa Transparency Gray band rN Pyramids Color gradient Singleband pseudocolor Min 1797 95 Ih Histogram Max 2279
298. n using a tool that requires multiple vector input data layers the layers must be in the same coordinate reference system Using the Clip and Difference tools These two tools are related in that they are the inverse of each other Data often extends beyond the bounds of your study area In this situation you can use the Clip tool to limit the data to the extent of your study area It is often described as a cookie cutter It takes an input vector layer and uses a second layer as the clip layer to produce a new dataset that is clipped to the extent of the clip layer The Difference tool takes the same inputs but outputs the input features that do not intersect with the clip layer In this example we will clip fire stations to the boundary of the city of Portland Load the fire_sta shp and PDX city_limits shp files from the sample data Select fire sta in the Input vector layer field and PDX_city limits in the Clip layer field The output shapefile will be loaded into QGIS automatically since Add result to canvas is checked This is the default setting for most tools If you have any selected features you can choose to just use them as inputs The operation creates a fire stations layer consisting of the 31 features that lie within the Portland city limits Input vector layer fire sta Use only selected features Clip layer PDX_city_limits Use only selected features Output shapefile a Portland Output Chapter4 Portland FireStation
299. nates of vertices that are separated by commas For example LINESTRING 30 10 20 20 40 30 would represent the line feature shown in the following screenshot To demonstrate how WKT can be mapped via the Add Delimited Text Layer tool we will map the Parcels WKT csv sample data file this has WKT geometries for eight parcels polygons 1 Click on Add Delimited Text Layer by navigating to Layer Add Layer 2 Select the file name by clicking on Browse and locate the delimited text file on your system In this example the Parcels_WKT csv file is being used 3 Choose an appropriate value for the Layer name field By default this will be the prefix of the delimited text file 174 Chapter 6 4 Use the File format radio buttons to specify the format of the delimited text file This is a CSV file For Record options set the Number of header lines to discard option as 1 Set the Geometry definition option to Well known text WKT Set the Geometry field option to field_1 ot amp Click on OK and the Coordinate Reference System Selector will open Use this dialog box to identify the coordinate reference system of the data For this example the data is in EPSG 2903 The following screenshot shows an example of a completed Create a Layer from a Delimited Text File tool set up to parse a WKT file Create a Layer from a Delimited Text File File Name 0 PACKT Data Chapter 6 Parcels_ WKT csv Layer name Parcels WKT
300. ncidence of crime Once they are mapped the points can be used in many ways to generate information For example they can be used as inputs to generate density surfaces or they can be linked to parcels of land and characterized by socio economic data They may also be an important component of a cadastral information system An address geocoding operation typically involves the tabular address data and a street network dataset The street network needs to have attribute fields for address ranges on the left and right hand side of each road segment You can geocode within QGIS using a plugin named MMQGIS nttp michaelminn com linux mnqgis MMQGIS is a collection of vector data processing tools developed by Michael Minn To install the plugin perform the following steps 1 Navigate to Plugins Manage and Install Plugins 2 Click on the All tab and type MMQGIS into the search bar 3 Install the plugin that manifests as the MMQGIS menu 176 Chapter 6 MMQOGIS has many useful tools For geocoding you will use the tools found in Geocode under MMQGIS There are two tools there Geocode CSV with Google OpenStreetMap and Geocode from Street Layer The first allows you to geocode a table of addresses using either the Google Maps API or the OpenStreetMap Nominatim web service This tool requires an Internet connection but no local street network data The web services provide the street network The second tool uses a local street netw
301. ncy of shorter stream segments which makes sense in this particular topography because they typically represent numerous small tributaries within a radial drainage network Longer and more continuous river segments would more likely be found in areas with less topographic variation This is illustrated in the next screenshot which highlights stream segments that are less than 1 000 meters long 266 Chapter 7 We can perform a similar analysis with raster layers by using the Raster Histogram tool under Raster Processing For example we might be interested in exploring the variation in elevation or slope within a given region for evaluating development potential or landslide susceptibility The next screenshot illustrates the resulting output after running this tool on the elevation layers Histogram 1500000 1000000 pem mi ET EL m LL 267 The Processing Toolbox The next screenshot illustrates the resulting output after running this tool on the slope layers Histogram a E Eli m T ELi La LL From these two simple histograms we can quickly surmise that although there is a lot of variation in the elevation throughout the Crater Lake National Park region the slope is rather unimodal and indicates a great occurrence of gentler slopes As previously mentioned QGIS has a built in viewer for exploring similar information but it is often less visually concise 268 Chapter 7
302. nd PyQGIS code snippet looping the layer features In this paragraph we ll introduce how to loop in Python and how to apply loops to explore the content of the layer loaded in the previous paragraph Write the following snippet in the Python Console taking special care with the code indentation for feature in layer getFeatures print Feature d has attributes and geometry feature id print feature attributes print feature geometry asPoint This will print a pattern like the following Feature with id 21 has attributes and geometry 22 u US00342 858 0 u Airport Airfield u PATL u TATALINA LRRS u Other 328415 4 71636e 06 The layer getFeatures method returns an object that can be iterated inside a for Python instruction getting a OQgsFeature instance for every loop The feature attributes method returns a list inside the brackets of the integer and unicode strings the u values The feature geometry method returns QgsGeometry that is converted in QgsPoint to be printed as a tuple inside the parenthesis of coordinates It is strongly recommended that you explore the preceding classes You can also practice by referring to the documentation at http qgis org api Start by exploring the QgisInterface and QGis classes Indentation is an important part of the Python language in fact nesting the code in Python is done using indentation as specified in the standard followed gl
303. nd references the vector processing parts of GDAL OGR is not really a standalone project as it is part of the GDAL code now however for historical reasons OGR is still used More information about GDAL and OGR can be found at http gdal org GDAL is an Open Source Geospatial Foundation http osgeo org project Loading databases QGIS supports PostGIS SpatiaLite MSSQL and Oracle databases Regardless of the type of database you wish to load the loading sequence is very similar Therefore instead of covering specific examples the general sequence will be covered First click on Add Layer under Layer and then choose the database type you wish to load This will open a window with options for adding the data stored in a database As an example the following screenshot shows the window that opens when you navigate to Layer Add Layer Add SpatiaLite Layer 8 Chapter 1 Add SpatiaLite Table s Databases test sqlite c temp test salite Connect New Delete Table Type Geometry column Sal testsgite aties counties roads LINESTRING geometry Also list tables with no geometry Keep dialog open Search options Add Set Filter Update statistics Close Help Note that the window will look the same for any database 7 you choose except for the window name To load data from a database we must first create a connection to the database To create a new connection click on the New button to
304. ndiInterface refreshLayerSymbology myVector 352 Chapter 10 Editing through QgsVectorDataProvider Each QgsMapLayer as a QgsVectorLayer instance has its own data provider that can be obtained with the dataProvider method This is shown in the following code snippet that is executed in the Python console myVector QgsVectorLayer qgis sample data shapfiles alaska shp MyFirstVector ogr QgsMapLayerRegistry instance addMapLayers myVector myDataProvider myVector dataProvider print myDataProvider This will print something similar to lt qgis _core QgsVectorDataProvider object at Oxaabdc20 gt on the Python console This is the instance of the QgsVectorDataProvider class The data provider will directly access the stored data avoiding any control by QGIS This means that no undo and redo options will be available and no events related to the editing actions will be triggered The code snippets that follow describe how to interact with vector data directly using the data provider These code samples will modify the Alaska shapefile set of files so it s better to have a copy of the original files to restore them after you apply the following examples Some of the following examples will work directly on features of myVector We can get a feature directly using the QgsFeatureRequest For example to get the feature with ID 599 we can use the following code snippet features myVector getFeatures QgsFe
305. ne These can interfere with network analysis The features with pseudo nodes will be flagged as errors 201 Advanced Data Creation and Editing Rules for polygon features The rules for polygon features are as follows must contain This checks whether the target polygon layer contains at least one node or vertex from the second layer If it doesn t it is flagged as an error must not have duplicates This flags additional duplicated stacked polygons as errors must not have gaps This flags adjacent polygons with gaps as errors Watersheds or parcel boundaries would be suitable for this test must not have invalid geometries This checks whether the geometries are valid Some of the rules that define a valid geometry are as follows O Polygon rings must close Rings that define holes should be inside rings that define exterior boundaries Rings should not self intersect they may neither touch nor cross one another Rings should not touch other rings except at a point must not have multi part geometries This flags all multi part polygons as errors must not overlap This flags adjacent polygon features in the same layer that overlap one another as errors Watersheds or parcel boundaries would be suitable for this test must not overlap with This relationship test evaluates how polygon features from the target layer interact with polygon features from a second polygon layer Those that do will be flagged as
306. nee 100 WA Trees WA Riparian Trees MT Rivers iii 100 MT Trees iai MT Riparian Trees Add row Delete row Converting a model into a Python script In QGIS version 2 4 it was possible to convert a model tool to a Python script It was done by right clicking on the model in the Processing Toolbox panel and choosing Save as Python script During the development of QGIS 2 6 the underlying code for the graphical modeler was completely rewritten As a result there are many changes and improvements in the new modeler For example mode1 files are now stored as JSON instead of XML there is no depth limit to nesting models within models and algorithms and inputs can be dragged and dropped onto the modeler canvas The modeler is also more stable and less buggy Unfortunately the ability to export a model as a Python script has not yet been implemented in the updated modeler There simply was not enough time to get this piece completed before the release of QGIS 2 6 This functionality is expected to be implemented in a future QGIS release However since models are algorithms they can be executed from the Python Console This topic is covered in Chapter 10 PyQGIS Scripting 299 Automating Workflows with the Graphical Modeler Summary In this chapter we covered automating workflows with the QGIS graphical modeler We showed you how to set up edit document and run a model You learned how to add inputs and algorithms to mo
307. nformation printout note that a warning is displayed stating that no spatial index has been defined shown in following figure To improve access speed it is best that a spatial index be set Click on create it and then click on the Yes button on the pop up GiffordPinochet sqlite Ye MF roads 2 Peaks SihverCreek Spatiallndex Waterfalls geom_cols_ref_sys geometry_columns geometry_columns_auth geometry_colurmns_field_ geometry_colurmns_statis geometry_columns_time idx_NF_roads_geom idx_NF_roads_geom_node idx_NF_roads_geom_parent idx_MF_roads_geom_rowid idx_Peaks_geom idx_Peaks_geom_node idx Peaks parent Waterfalls General info Relation type Table Rows 100 SpatiaLite Column geom Geometry POINT Dimension KY Spatial ret NADSS UTM zone 10N 26910 Extent unknown find out 7 To preview the attribute table click on the Table tab on the information panel To preview the geometry click on the Preview tab on the information panel To view the newly created SpatiaLite layer in QGIS Desktop right click on NF_roads in the tree and then choose Add to canvas Exporting tables out of SpatiaLite as a shapefile To export a table as a shapefile perform the following steps 1 Open DB Manager by clicking on DB Manager under Database Expand SpatiaLite and select the database from which you wish to export a table in the Tree panel In the Tree panel select the table that you w
308. ng Toolbox Before using any shapefile to calculate upstream contributing area it is worthwhile to ensure that every point is located on a grid cell representing the stream network otherwise the algorithm won t be Q able to accurately characterize surface flow To make sure each point is located on the network we can use the Move Outlets To Streams tool which will move each point to the nearest cell representing the network C Mastering_QGIS TauDEM dnp_d ca tif a 258 Chapter 7 Calculating a stream network raster grid The resulting output looks similar to a watershed boundary and could easily be converted to a polygon using the r to vect tool However in addition to identifying the contributing watershed area we can also model the potential stream network that drains this watershed If we make the provided River hyd1 shp visible we can see that it has relatively low resolution and for a watershed of this size there are likely to be other smaller tributaries that we can extract from the topography To accomplish this we are going to use the Stream Definition By Threshold tool and input the D8 contributing area for Sun Creek as illustrated in the next screenshot Q Stream Definition By Threshold Accumulated Stream Source Grid D8 Contributing Area Grid Sun Creek EPSG 26710 Threshold 100 Mask Grid Not selectec Stream Raster Grid C Mastering_OGIS TauDEM dnp_sdbt tif Open output file after running algorit
309. nvas event xyCoordinates QgsPoint and it can be coded using the following code snippet def showCoordinates currentPos o print move coordinate d d currentPos x currentPos y After creating the handler we have to attach it to the canvas event with the following code iface mapCanvas xyCoordinates connect showCoordinates If you want to remove the handler use the following code iface mapCanvas xyCoordinates disconnect showCoordinates Ensure that you write the function name as showCoordinates because if you don t pass any parameters to the disconnect call then all handlers attached to the xyCoordinates event will be removed Creating a Map Tool event handler An event handler will be attached to the QgsMapToolEmitPoint event canvasClicked QgsPoint Qt MouseButton and then it can be coded using the following code snippet import the Qt module that contain mouse button definitions like Qt LeftButton or Qt RightButton used later from PyQt4 QtCore import Qt create handler def manageClick currentPos clickedButton if clickedButton Qt LeftButton print Clicked on d d currentPos x currentPos y if clickedButton Qt RightButton reset to the previous mapTool iface mapCanvas setMapTool previousMapTool clean remove myMapTool and relative handlers myMapTool deleteLater 371 PyQGIS Scripting After creating the handler we have to attach it
310. nverting polygons to points to create label points The following tools can be found on the Geometry Tools menu under Vector Vector Raster Database Processing Help OpenStreetMap Ba O gt A Analysis Tools ea G7 sg Research Tools 5 a z Geoprocessing Tools vy Geometry Tools mq Check Geometry Validity Data Management Tools Export Add Geometry Columns Polygon Centroids Delaunay Triangulation KE Voronoi Polygons Simplify Geometries Densify Geometries gt Multipart to Singleparts Singleparts to Multipart Polygons to Lines Lines to Polygons Z Extract Nodes 1097 Preparing Vector Data for Processing Creating polygon centroids With the Polygon Centroids tool that can be found by navigating to Vector Geometry Tools you can generate points that will be located at the center of polygons Simply provide the input polygon layer and name the output In the following example centroids of the Neighborhoods_pdx shp shapefile have been generated This tool preserves all the attributes during the conversion With the data in point form you can generate a heat map compute densities or measure distances Converting polygons to lines and lines to polygons With the Polygons to Lines and Lines to Polygon tools that can be found by navigating to Vector Geometry Tools you can convert between these two geometry types There are many reasons why this
311. o we will only cover a quick refresher that is relevant to this chapter SQL provides functions to select insert delete and update data Four commonly used SQL data functions are discussed as follows e SELECT This retrieves a temporary set of data from one or more tables based on an expression A basic query is SELECT lt field s gt FROM lt table gt WHERE lt field gt lt operator gt lt value gt where lt fields is the name of the field from which values must be retrieved and lt table gt is the table on which the query must be executed The lt operator gt part checks for equality such as gt LIKE and lt value gt is the value to compare against the field 29 Creating Spatial Databases e INSERT This inserts new records into a table The INSERT INTO lt table gt lt fieldils lt field2s lt field3 gt VALUES lt valuels lt value2s lt value3 gt statement inserts three values into their three respective fields where lt valuels lt value2 gt and lt value3 gt are stored in lt field1 gt lt field2 gt and lt field3 gt of lt table gt e UPDATE This modifies an existing record in a table The UPDATE lt table gt SET lt field gt lt value gt statement updates one field s value where lt value gt is stored in lt field gt of lt table gt DELETE This deletes record s from a table The following statements deletes all records matching the WHERE clause DELETE FROM lt table g
312. oad The second way is to open the Browser panel navigate to the data you wish to load and then drag the data onto the map display or onto the Layers panel The third way to load data is to enable the Manage Layers toolbar and click on the button representing the data type that you wish to load The fourth way is to locate the data in QGIS Browser drag the data and drop it onto the QGIS Desktop map display or the Layers panel Loading vector data To load vector files click on Add Vector Layer by navigating to Layer Add Layer This will open the Add Vector Layer dialog that will allow us to choose the source type and source of the dataset that we wish to load 6 Chapter 1 The source type contains four options File Directory Database and Protocol When you choose a source type the source interface will change to display the appropriate options Let s take a moment to discuss what type of data these four source types can load File This can load flat files that are stored on disk The commonly used flat file types are as follows ESRI shapefile shp AutoCAD DXF dx Comma separated values csv GPS eXchange Format gpx Keyhole Markup Language km1 6SQLite SpatiaLite sqlite db Directory This can load data stored on disk that is encased in a directory The commonly used directory types are as follows U S Census TIGER Line Arc Info Binary Coverage Database This can l
313. oad databases that are stored on disk or those available through service connections The commonly used database types are as follows ODBC ESRI Personal GeoDatabase MSSQL MySQL PostgreSQL Protocol This can load protocols that are available at a specific URI QGIS currently supports loading the GeoJSON protocol Loading raster data To load raster data into QGIS click on Add Raster Layer by navigating to Layer Add Layer This will open a file browser window and allow you to choose a GDAL supported raster file The commonly used raster types supported by GDAL are as follows ArcInfo ASCII Grid asc Erdas Imagine img 7 A Refreshing Look at QGIS GeoTIFF tif tiff JPEG JPEG 2000 jpg or jpeg jp2 or 42k Portable Network Graphics png Rasterlite sqlite USGS Optional ASCII DEM dem To add an Oracle GeoRaster click on Add Oracle GeoRaster Layer by navigating to Layer Add Layer then connect to an Oracle database to load the raster More information about loading database layers is in the following section The Geospatial Data Abstraction Library GDAL is a free and open source library that translates and processes vector and raster geospatial data formats QGIS as well as many other programs use GDAL to handle many geospatial data processing tasks You may see references to OGR or GDAL OGR as you work with QGIS and GDAL OGR is short for OGR Simple Features Library a
314. obally You can find this at http legacy python org dev peps pep 0008 Exploring iface and QGis The iface class used in the preceding snippets is important in every PyQGIS code it is used to access most graphical QGIS components from displayed layers to the toolbar buttons 308 Chapter 9 The iface class is a Python wrapper for the C class QgisInterface which is documented at http qgis org api classQgisInterface html Most QGIS classes have a Qgs prefix Some special classes can have the Qgis or QGis prefixes The prefix Qgs is the Qt namespace registered by Gary Sherman the QGIS creator so Q stands for Qt and gs stands for Gary Sherman The most common use of the iface class is to get a reference of the canvas where maps are displayed canvas iface mapCanvas The class can also be used as a shortcut to load raster or vector layers for example loading the raster path to my raster tif and naming it myraster in the legend panel This can be done by typing the following command iface addRasterLayer path to my raster tif myraster Pay attention to writing paths with Windows A path string such as C path to raster tif has the special escape character so rewrite it by double escaping C path to raster tif or using the Unix Jp notation C path to myraster tif or notify Python with a raw string adding an r as in r C path to raster tif Generally it s good practice to create path s
315. ocessing module is not imported every processing command will generate an error such as the following one Traceback most recent call last File lt input gt line 1 in lt module gt NameError name processing is not defined Looking for an algorithm The processing toolbox contains a huge list of algorithms that can be searched for using keywords Similar to how we can look for an algorithm in the processing toolbox GUL it s possible to search for an algorithm using the PyQGIS commands For example to look for commands to convert something we can execute the following command at the Python console import processing processing alglist convert This will generate some output lines and among them we will find the command that we are looking for Convert format gt gdalogr ogr2ogr If it s not the previous command it can be this depending on the processing toolbox version Convert format 7 7 gt gdalogr convertformat 360 Chapter 10 If the version of the processing toolbox is greater than 2 2 substitute the gdalogr ogr2ogr string with gdalogr convertformat To b find out the version of the processing toolbox look for it in the QGIS plugin manager This string is composed of two parts Convert format is the command as shown in the processing toolbox GUI and gdalogr ogr2ogr is the command recognized by the processing comm
316. ocolor table from the first image option this option assumes that the same color table is used for all the input rasters 59 A Chapter 5 Notice that the syntax equivalent to the gdal_merge command line is displayed as you choose your merge options If you are familiar with the GDAL command line syntax you can use the edit button to set the tool options by editing the command directly For example you could specify the output image format as a 16 bit integer by using the ot Int16 parameter You could s gt also specify the output pixel size with the ps parameter The GDAL help page for this command can be found at http www gdal org gdal_merge html The Creation Options allow you to add your own command line options and set parameters such as the compression profile to be used on the output image The following screenshot shows the Merge command that is configured to mosaic the collection of input rasters The equivalent command line syntax is displayed in the window Choose input directory instead of files Input files ater 5 35106 H5 tif Select Output file hapter 5 Mosaic tif Select No data value O Layer stack Use intersected extent Grab pseudocolor table from the first image b Creation Options Load into canvas when finished odal_merge bat of GTiff o D PACKT Data Chapter f Mosaic tif D PACKT Data Chapter 5 35106 G4 tf D PACKT Dat
317. ode Showing and running the dialog We don t have to write any code here because it will be generated by the Plugin Builder The action to show how a dialog is saved in the self dl1g instance variable is shown here show the dialog self dlg show Run the dialog event loop result gelf dlg exec See if OK was pressed if result Do something useful here delete the line containing pass and substitute with your code pass This code is self explanatory by the comments generated by the Plugin Builder Some improvements As you can see for yourself the plugin doesn t work well if it is run more than one time The content of comboboxes grow on every run of the plugin It s left to the reader to find a solution how to avoid this behavior 324 Chapter 9 More detail of the code The complete code of this example as usual can be obtained from the source code of the book Here we ll give a bird s eye view in the test_plugin py source code that contains the TestPlugin class This class has other methods than run sel these are as follows init self iface tr self message initGui self add _ action self icon path text callback enabled flag True add to menu True add to toolbar True status tip None whats __ this None parent None unload self The following provides a brief description of these methods The init self iface method is always present in every Python class and it
318. ode uses the PyQGIS get Features statement that doesn t return features directly but instead returns an iterator In fact executing the code myVector getFeatures produces an output similar to the following one lt qgis core QgsFeatureIterator object at 0xd023c20 gt An iterator is a Python object that gets the record every time it is asked to get one in our case the for statement requests a record after every iteration The iterator works like a proxy it doesn t load all the features in memory but gets them only when it is necessary In this way it is possible to manage big vectors without memory limitations Editing features After being able to parse all the features it s necessary to learn how to modify them to satisfy our processing needs Features can be modified in two ways e Using the data providers of the vector Using the methods of QgsVectorLayer The difference that exists between these two ways is the ability to interact with some editing features of the QGIS framework Updating canvas and symbology We will now modify the Alaska shapefile in the following subsections If we modify some geometry of the legend classification it will be necessary to refresh the canvas and or layer symbology The canvas can be refreshed with the following command iface mapCanvas refresh The symbology of a modified QgsVectorLayer instance saved in the myVector variable can be updated with the following code iface lege
319. of the same name as the layer on disk 102 Chapter 3 Summary In this chapter we provided you with steps on how to style vector and raster data in QGIS We first covered how to pick colors using the new color picker then we covered how to create and manage color ramps using the style manager Next we reviewed the different ways to style single band and multiband rasters create a raster composite as well as how to overlay rasters using renderers Vector styling was reviewed next and we covered the six different style types We also looked at how to use vector renderers for layer overlays Next we toured the three diagram types that can be visualized on top of vector datasets We finished the chapter with instructions on how to save and load the styles for use in other QGIS projects In the next chapter we will move from viewing data to preparing data for processing Preparation topics will range from spatial and aspatial queries and converted geometry types to defining new coordinate reference systems 103 Preparing Vector Data for Processing Typically raw data obtained for a GIS project needs to be massaged for use in the specific application It may need to be merged converted to a different geometry type saved to the coordinate reference system of the project subset to the extent of the study area or subset by attribute values While QGIS provides a powerful set of tools that can handle many types of vector prepara
320. olation QGIS supports surface interpolation from vector points to a raster using the Interpolation plugin The Interpolation tool supports Inverse Distance Weighted IDW and Triangular Interpolation TIN To enable the Interpolation plugin click on Manage and Install Plugins under Plugins As an example of how to use the Interpolation tool let s use the Pecos DEM Points shp sample file to interpolate a surface using IDW 1 Add Pecos DEM Points shp to the map canvas 2 Open the Interpolation plugin tool by navigating to Raster Interpolation Interpolation The tool is shown in the following figure and uses the Pecos DEM Points shp sample file as the input vector layer 164 Chapter 5 v Interpolation plugin Input Output Vector layers Pecos DEM Points Interpolation method Inverse Distance Weighting IDW v Interpolation attribute value Number of columns 300 Number of rows 300 Yv st Cellsize X 0 00003 Cellsize Y 0 00002 Add Remove Xmin 102 662 Xmax 102 652 Vector layer Attribute Type Ymin 30 2218 Ymax 30 2286 Pecos DEM P value Points Set to current extent Output file ublications Mastering QGIS Chapter 5 Data Pecos DEM Pecos_Surface_IDW tif Add result to project The interface of the Interpolation tool is identical whether IDW or TIN is chosen as the interpolation method The only exceptions are the interpolation method parameters which can be set by clicking on the configure but
321. ollowing screenshot will be the basis for explaining the parameters w Band rendering Render type Singleband pseudocolor v Band Band 1 Gray Generate new color map Color interpolation Linear Ep OD wiz OOO Mode Continuous Cases 5 F Min 105 Max 207 Value Color Label 105 000000 105 000000 Classify 108 519000 108 519000 112 038000 112 038000 iA ee 115 546800 115 546800 119 065800 Exact cumulative cut of full extent 122 584800 126 103800 129 622800 Load min max values 133 141800 133 141800 136 650600 136 650600 Cumulative 55 140 169600 140 169600 count cut 1 First the band should be selected For the Band field choose Band 1 Gray If this were a multiband raster more bands would be available for selection 2 Next we should choose the color ramp to apply to the raster As none of the default color ramps are suitable for our example click on the color ramp combobox to open it scroll to the bottom and choose New color ramp as shown in following screenshot 70 Chapter 3 Generate new color map gg Greys YelloGreen iGn lGnBu lOrBr lOrRd elevation spectrum dark wiki 2 Random colors New color ramp 3 When prompted choose cpt city as the color ramp type then click on OK This will open the cpt city color ramp window In the Topography bathymetry group select the wiki 2 0 color ramp and add it Optionally the color ramp can be inverted
322. ollows e Cell value selection Selects cell values to include in the Min and Max value determination Rasters may have cell values that are outliers which may affect the rendering of the image For instance if only a few cells have an abnormally high value then the gradient will stretch all the way to these high values which will cause the raster to look overly gray and bland To combat this grayness some cell values can be excluded so that the gradient is not skewed by these outliers Three methods are available to select cell values and it is recommended that you experiment with these values to achieve the most desirable selection of cell values Cumulative count cut This includes all values between the two parameters In the preceding screenshot all values between 2 and 98 of the cell data range were included In general this will remove the few very high and very low values that may skew the gradient Min max This includes all values Mean standard deviation This includes all values within the specified number of standard deviations about the mean of all values e Extent The extent of the raster to sample for cell values Either the Full extent of the raster or the Current canvas extent can be used e Accuracy This determines the accuracy of the min max calculation The calculation can either be an Estimate faster or an Actual slower option In general Actual slower is the preferred option however for
323. ome page Repository Tags Flag the plugin as experimental Help Cancel i Each parameter is self explanatory through tooltips and can be seen by moving the cursor on each parameter line 312 Chapter 9 Setting mandatory plugin parameters There is a set of mandatory parameters that are always checked by the QGIS plugin repository when a plugin is uploaded These parameters are also manually checked by QGIS members to approve the plugin officially in the central repository The parameters are as follows Class name This is the name of the class that will contain the plugin business logic It will be named in the CamelCase format refer to http en wikipedia org wiki CamelCase to be aligned with the Python standard This name will be used by Plugin Builder to generate a directory that will contain the generated code Edit the value TestPlugin Plugin name This refers to the colloquial name of the plugin and is what will be shown in the Plugin Manager and in the QGIS Plugins menu Enter the value My First Test Plugin Description This is a string containing the description or the plugin scope Enter the value This is the description of the plugin Module name In Python a group of classes can be addressed and imported as a module The module name should be in lowercase and if necessary with underscores to improve readability We shall insert the value test plugin Version number This is the version
324. on and filename for it Here the data is being saved as a shapefile Click on Run to execute the model Riparian trees Parameters Log Help Rivers AKRivers EPSG 2964 Buffer distance 100 Trees Riparian Trees EPSG 2964 Riparian Trees D PACKT Data Chapter 3 Solution RiparianTrees shp Open output file after running algorithm 285 Automating Workflows with the Graphical Modeler As the model runs the dialog will switch to the Log tab which provides output as it runs Riparian trees Parameters Log Help Algorithm Riparian trees starting Prepare algorithm QGISFIXEDDISTANCEBLIFFER_ 1 Running Fixed distance buffer 1 2 Parameters INPUT D PACKT Data Chapter 8 AKRivers shp DISTANCE 100 SEGMENTS 20 DISSOLVE True Converting outputs OK Execution took 0 528 ms 1 outputs Prepare algorithm QGISCLIP_1 Running Clip 2 2 Parameters INPUT C Wsers KURTME 1 ApoData Local Temp processing eSbf0df6 5c4b4a60b0b2985275e le5e a OUTFUTALGOGISCLIP 1 shp OVERLAY C Users KURTME 1 ApoData Local Temp processing 47c50d 309b8a49 5c9a 73c32c38560cd 4 OUTPLUT shp All the model files and data inputs discussed in this chapter j are included with the Mastering QGIS sample data Editing a model Existing QGIS models can be modified as needed Right clicking on a model in the Processing Toolbox panel opens a context menu Choosing Edit model will open the model in the Processing modeler w
325. onversely if you have an integer raster with values between 10 and 500 and want to convert it to floating point you will need to set up a rule text file with this 10 500 0 1 5 0 This will result in a raster with cell values ranging from 0 1 to 5 0 Resampling rasters When an analysis requires that multiple raster datasets be combined or overlaid their pixel resolutions should be equal The spatial resolution or cell size of a raster can be increased or decreased by a process known as resampling Although you can increase or decrease the resolution of a raster it is considered a better practice to decrease the resolution of the finer datasets to match the resolution of the coarsest raster As an example let s say you have two rasters a 90 meter resolution elevation raster and a 30 meter vegetation raster In this scenario it would be best to resample the vegetation data to a 90 meter resolution to match the elevation data This way all the data will be matched accurately Conversely if you resample to match the finest resolution raster you will introduce false accuracy The elevation data has a 90 meter resolution because that was the smallest unit that could be differentiated from the neighboring pixels If you increase the spatial resolution each elevation pixel is converted to nine 30 meter pixels You cannot assume that all nine resulting pixels actually have the same elevation value in the real world It is more likely that only th
326. ools to convert between raster and vector data models In this section we will convert between the two data models using a national land cover dataset of 2006 for the Dallas area in Texas Converting from raster to vector To convert a raster to a vector format QGIS provides the Polygonize tool The Polygonize tool converts an input raster file into any supported type of vector file and writes the raster cell values to a field in the vector file When the raster is polygonized adjacent cells of the same value are aggregated to a single larger polygon To access the Polygonize tool click on Polygonize Raster to Vector by navigating to Raster Conversion The Polygonize tool is shown in the following screenshot and uses the sample DFW Land Cover tif file as input Polygonize Raster to vector Input file raster DFW Land Cover Y Output file for polygons shapefile 3FVW_Land_Cover shp X Field name LandCov Use mask Pecos 3 Load into canvas when finished gdal_polygonize bat D Users Rick OneDrive Publications Mastering QOGIS Chapter 5 Data nied2006_TX_landcover OFW Land Cover tif f ESRI Shapefile D Users Rick OneDrive Publications Mastering OGIS Chapter 5 Data nied2006 TX_landcover OFW Land Covershp DFW Land Cover LandCov 161 Preparing Raster Data for Processing To convert a raster to a vector polygon the following options are available Input file raster Input file to be polygonized Output
327. op right corner of the following figure Instead the Hard Light or Multiply Layer blending methods illustrated at the bottom left and bottom right corners respectively in the following figure can be used to maintain strong color and include the hillshade 92 Chapter 3 Layer rendering can really improve the look of your map So experiment with the layer rendering methods to find the ones that work best for your overlays Using diagrams to display thematic data QGIS supports the addition of three diagram types as overlays on top of vector data The three diagram types are pie chart text diagram and histogram The underlying vector data can still be styled to provide a nice base map To add a diagram open the vector s Layer Properties window and then click on the Diagrams tab The Diagrams tab shown in the following screenshot is split into two sections The top section contains the Appearance and Options tabs which contain parameters that are unique to each selectable Diagram type value It also contains the Size and Position tabs which contain parameters shared by all the Diagram types value The bottom section Attributes is common to all diagram types and provides the mechanism for adding attributes to diagrams X Display diagrams Diagram type Pie chart bd Priority Low J Appearance Size Position Options an v Size units mm v Scale linearly between 0 and the following attribute value diagram size
328. or Map Tools is QgsMapTool which has a set of specializations that can be useful for most of the user interaction with the canvas These derived tools are listed as follows e QgsMapToolEmitPoint This is focused on intercepting point clicks on canvas and returning the map coordinates e QgsMapToollidentify This is focused on getting layer values at specified point clicked coordinates e QgsMapToollIdentifyFeature This is similar to the previous tool but results also reference the pointed features e QgsMapToolPan This is focused on managing panning and its events e QgsMapToolTouch This is focused on managing touch events on the canvas e QgsMapToolZoom This is focused on managing zoom events 368 Chapter 10 Setting the current Map Tool For each map canvas only one map tool runs at a time When it s necessary to set a Map Tool it s good practice to get the previous one and set it back again at the end of use of the new Map Tool The following code snippet shows you how to get an old Map Tool set a new one and restore the previous one import the map tool to use from gqgis gui import QgsMapToolZoom get previous map tool and print it OldMapTool iface mapCanvas mapTool print Previous map tool is a oldMapTool create a zoom map tool pointing to the current canvas the boolean parameter is False to zoom in and True to zoom out newMapTool QgsMapToolZoom iface mapCanvas False set the curr
329. or ramp click on the Add item button and then choose Gradient This will open the Gradient color ramp window A Gradient color ramp uses two colors which are specified as Color 1 and Color 2 to set the start and end colors QGIS applies an algorithm to create a gradation between the two colors Additional colors can be added to the gradient by clicking first on Multiple stops and then on Add stop this will open the color picker Once a color is chosen you can enter the percentage along the gradient to apply the stop s color Additional stops can be added by again clicking on Add stop As an example in the following screenshot a gradient is created between red and green with a yellow stop at the 50 percent location a Gradient color ramp oo D Color 2 E M Type Continuous Multiple stops Offset Add stop Remove stop Preview Information When all the gradient parameters have been set click on OK to save the gradient QGIS will prompt you to name the gradient Once it is named the gradient will appear in the Style Manager s list of color ramps after the default color ramps 61 Styling Raster and Vector Data Adding a Random color ramp To add a Random color ramp click on the Add item button and then choose Random This opens the Random color ramp window shown in the following screenshot A Random color ramp generates a number of randomly generated colors that fa
330. ords between tables The benefit of relating tables is reducing data redundancy and increasing data integrity In order to relate two tables together each table must contain an indexed key field The process of organizing tables to reduce redundancy is called normalization Normalization typically involves splitting larger tables into smaller less redundant tables followed by defining the relationship between the tables A field can be defined as an index A field set as an index must only contain values that are unique for each record and therefore it can be used to identify each record in a table uniquely An index is useful for two reasons Firstly it allows records to be quickly found during a query if the indexed field is part of the query Secondly an index can be set to be a primary key for a table allowing for table relationships to be built A primary key is one or more fields that uniquely identify a record in its own table A foreign key is one or more fields that uniquely identify a record in another table When a relationship is created a record s from one table is linked to a record s of another table With related tables more complex queries can be executed and redundancy in the database can be reduced Structured Query Language Structured Query Language SQL is a language designed to manage databases and the data contained within them Covering SQL is a large undertaking and is outside the scope of this book s
331. ork dataset with address range attributes to geocode the address data Processing MMQGIS Help Animate Combine Create E lew G Geocode CSV with Google OpenStreetMap search Select 1 Geocode from Street Layer Import Export gt Modify 3 Geocode How address geocoding works The basic mechanics of address geocoding are straightforward The street network GIS data layer has attribute columns containing address ranges on both the even and odd side of every street segment In the following example you see a piece of the attribute table for the Streets shp sample data The columns LEFTLOW LEFTHIGH RIGHTLOW and RIGHTHIGH contain the address ranges for each street segment We Attribute table Streets Features total 41825 filtered 41825 selected 0 LEFTLOW 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14301 00000000 14297 00000000 1422700000000 LEFTHIGH 14305 00000000 14323 00000000 14309 00000000 14339 00000000 14321 00000000 14321 00000000 14335 00000000 14331 00000000 14317 00000000 14331 00000000 14233 00000000 RIGHTLOW 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14300 00000000 14296 00000000 14226 00000000
332. ors from the layer below the raster will show through otherwise no colors will be mixed 76 Chapter 3 Lighten In this mode for each raster cell in the or the raster below the maximum value for each color component that is found in either raster is used Screen In this mode lighter cells from the raster below are displayed transparently over the raster while darker pixels are not Dodge This mode increases the brightness and saturation of the raster cells below based on the brightness of the raster s cells Addition This mode adds the color components of each cell of this raster and the raster below together If the color component value exceeds the maximum allowed value then the maximum value is used Darken In this mode for each raster cell in the raster or the raster below the minimum value of each color component found in either raster is used Multiply This mode multiplies the color components of each cell of the raster and the raster below together This will darken the raster Burn In this mode the raster below is darkened using the darker colors from this raster Burn works well when you want to apply the colors of this raster subtly to the raster below Overlay This mode combines the multiply and screen methods When this is used for the raster below lighter areas become lighter and darker areas become darker Soft light This mode combines the burn and dodge methods Hard light This mode is the
333. oth QGIS Desktop and QGIS Browser QGIS Desktop The QGIS interface is divided into four interface types menu bar toolbars panels and map display The following screenshot shows QGIS Desktop with all four interface types displayed a QGIS 4ce2ade Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help Menu Bar leaRaSs Toolbars P R amp amp amp 95 4a be S S P G4 my amp Fal a omy Browser mS Cla PF a H yb Home Favourites Panel Map Display QS Coordinate 1 487 1 635 Scale 1 1 782 570 v Render EPSG The map display shows the styled data added to the QGIS project and by default takes up the majority of the space in QGIS Desktop The menu bar displayed across the top provides access to most of QGIS Desktop s functionality The toolbars provide quick access to QGIS Desktop functionality The toolbars can be arranged to either float independently or dock at the top bottom left or right sides of the application The panels such as Browser and Layers provide a variety of functionality and can be arranged to either float independently or dock above below right or left of the map display 4 Chapter 1 There are four toolbars that are particularly useful and it is recommended that you enable them The File toolbar provides quick access to create open and save QGIS projects and create and manage print composers The Manage Layers toolbar
334. ou can find this information in the plugin manager as shown in the following screenshot Search JF PostGIS Topology E 4 z ag 5 RP postgisqueryBuilde PS Time Series Viewer AS installed a Postgres 91 plus Au Computation and visualization of time series of speed F PPConverter for Permanent Scatterers derived from satellite lt Invalid O prepair interferometry v amp Processing y i Sula 7 i Processing LWGEON SF ProcessingPermacli frir r r fr 9 rating vote s 2652 downloads S Profile tool PS Time Series Vie dc Oe a pyArchInit More info homepage tracker code repository 4 Paiz Author Giuseppe Sucameli Luigi Pirelli Faunalia ainage pa QConsolidate Installed version 0 2 2 in home ginetto qgis2 python plugins pstimeseries O Q QGIS Cloud Plugin Available version 0 2 2 in QGIS Official Plugin Repository QGIS Remote CN C CO amp Qgis Web Connecto O amp agis2leaf v Uninstall plugin Reinstall plugin Help _ Close If you are looking for the link tracker it will be found in the area marked by the red box in the second link tracker Other useful links can be found in the area marked in red The first link points to the homepage plugin where the plugin is described The third link points to the the code repository of the plugin In the QGIS plugin central repository every plugin is accepted only if it has a lease tracker and a code repository 305 Creating
335. p To explore the functionality of TauDEM we will characterize the watershed of Sun Creek upstream to the town of Fort Klamath California To accomplish this we will perform the following tasks 1 Remove pits from the DEM 2 Calculate flow directions across the landscape 3 Calculate the upstream area above Fort Klamath 254 Chapter 7 4 Calculate a stream network raster grid 5 Create a watershed specific vector stream network Reminder about installing TauDEM al Using TauDEM requires you to carefully follow the instructions for your particular operating system as described at http docs qgis org 2 6 uk docs user_manual processing 3rdParty html This particular library is easier to install and run on Windows and Linux than Mac OS X TauDEM requires the initial input to be a tif file rather than the dem file that we used throughout this chapter So our first task is to export our original Crater Lake elevation layer to a tif file We can accomplish this by right clicking on the DEM and clicking on Save As By default QGIS offers GTiff as the export option We just need to specify a new output file and we can begin using the TauDEM tools Reminder about accessing additional libraries E Remember that in order to access any of the algorithms available through additional providers we need to make sure that the processing toolbox is set to the Advanced Interface Removing pits from the DEM Before u
336. patial analyses and geoprocessing tasks for both raster and vector formats To illustrate how these processing tools might be applied to real world questions two hypothetical scenarios are illustrated by relying heavily on GRASS and SAGA tools x Preface Chapter 8 Automating Workflows with the Graphical Modeler covers the purpose and use of the graphical modeler to automate analysis workflows In the chapter you will develop an automated tool model that can be added to the Processing Toolbox Chapter 9 Creating QGIS Plugins with PyQGIS and Problem Solving covers the foundational information to create a Python plugin for QGIS Information about the API and PyQGIS help will be covered first followed by an introduction to the iface and QGis classes Next the steps required to create and structure a plugin will be covered The chapter will be wrapped up after providing you with information on creating graphical user interfaces and setting up debugging environments to debug code easily Chapter 10 PyQGIS Scripting provides topics for integrating Python analysis scripts with QGIS outside of the Processing Toolbox Layer loading and management are first covered followed by an exploration of the vector data structure Next programmatic launching of other tools and external programs are covered Lastly the QGIS map canvas is covered with respect to how a script can interact with the map canvas and layers within What you need for this
337. pecify the vertex using negative numbers So to retrieve the x coordinate of the last vertex the command will be xat 1 136 Chapter 4 4 The following figure shows the completed calculations for xat 0 and xat 1 for a line geometry type XCoord1 XCoord 7644082 26869565 7644085 7078612 7626380 5004895 7626208 0759656 The third example calculating a population dependent label string This third example populates a new field with a string that is used for labeling states States that have a population of over five million will have a label with the state name and population All other states will simply have a label with the state name The basic logic of our calculation is If a state has a population of over five million then create a label that lists the state name and population otherwise create a label that lists the state name Since we have two cases of possible labels we will need to use the CASE ELSE conditional function The purpose of the CASE ELSE function is to direct the field calculator to a calculation block when a condition is met So we will have one calculation block for states over five million in population and one for all other states For this example the states48 shp sample data is being used The PoP1996 field contains the states population values as of 1996 and is the field used to determine whether a state s population is over or under five million 1 Open the Field calcu
338. plore the various types of input options and available algorithms and develop an example model that we can add to the toolbox as a reusable tool You will also learn how to export models to Python in preparation for the last chapter which will explore Python scripting within QGIS 269 Automating Workflows with the Graphical Modeler This chapter will provide you with an overview of the graphical modeler GM First we will introduce the modeler and explore the various inputs and algorithms available for models Then we will demonstrate via step by step examples how to develop a model that can be added to the Processing Toolbox We will also cover more advanced topics including nesting models and executing models iteratively The specific topics that we will cover in this chapter are as follows e An introduction to the graphical modeler e Opening the graphical modeler e Configuring the modeler and naming a model e Adding inputs e Adding algorithms e Running a model e Editing a model e Documenting a model e Saving loading and exporting models e Executing model algorithms iteratively e Nesting models e Using batch processing with models e Converting a model into a Python script 271 Automating Workflows with the Graphical Modeler An introduction to the graphical modeler A typical spatial analysis involves a series of GIS operations with the output of one operation as the input for the next one until the fin
339. polygon vector layer Hawai counties Unique ID field NAME 10 Output shapefile D Hawaii Counties shp Add result to canvas I 0 Adding geometry columns to an attribute table To add geometry values to new attribute columns in a vector layer s attribute table click on Export Add Geometry Columns by navigating to Vector Geometry Tools As shown in the following screenshot the tool requires an input vector layer a determination of which CRS will be used for the calculations Calculate using and a choice of whether you wish to create the columns in the existing file or Save to new shapefile with the added geometry columns Export Add geometry columns Input vector layer Hawali_countes Calculate using Layer CRS T Save to new shapefile Project CRS Elipsoid 117 Preparing Vector Data for Processing Depending on the geometry type of the input vector layer different geometry columns will be created as follows e Point The xcooRD and YCooRD columns will contain the x and y coordinates of the point Line The SHAPE LEN column will contain the length of the record s line s e Polygon The AREA and PERIMETER columns will contain the area and perimeter of the record s polygon s Using basic vector geoprocessing tools This section will focus on geoprocessing tools that use vector data layers as inputs to produce derived outputs Geoprocessing tools are part of the fTools plugin that
340. pport and register to have the files e mailed directly to you Downloading the color images of this book We also provide you with a PDF file that has color images of the screenshots diagrams used in this book The color images will help you better understand the changes in the output You can download this file from https www packtpub com sites default files downloads 86820S ImageBundle pdf Errata Although we have taken every care to ensure the accuracy of our content mistakes do happen If you find a mistake in one of our books maybe a mistake in the text or the code we would be grateful if you could report this to us By doing so you can save other readers from frustration and help us improve subsequent versions of this book If you find any errata please report them by visiting http www packtpub com submit errata selecting your book clicking on the Errata Submission Form link and entering the details of your errata Once your errata are verified your submission will be accepted and the errata will be uploaded to our website or added to any list of existing errata under the Errata section of that title xiii Preface To view the previously submitted errata go to https www packtpub com books content support and enter the name of the book in the search field The required information will appear under the Errata section Piracy Piracy of copyrighted material on the Internet is an ongoing problem across
341. r 162 164 raster converting to 161 162 vector data models and raster converting 161 vector data topology checking 200 rules 200 Topology Checker installing 200 vector data topology rules for line features 201 for point features 201 for polygon features 202 Topology Checker using 203 vector formats OGR URL 342 vector geometries converting 109 features densifying 113 115 features simplifying 113 115 geometry columns adding to attribute table 117 lines converting to polygon 110 multipart and singlepart feature converting 115 116 nodes extracting from lines 112 nodes extracting from polygons 112 polygon centroids creating 110 polygon converting to lines 110 polygons around individual pints creating 111 112 vector layer rendering 91 93 rendering parameters 92 vectors styling 80 387 vector structure elevation layer clipping to park boundary basic vector methods 345 with GDAL 237 describing 345 346 evaluating 236 header describing 347 for towers calculating with rows describing 348 349 r viewshed tool 238 239 vector structure describing 346 raster statistics calculating vector styling with r stats 242 244 about 80 categorized vector style 81 82 W graduated vector style 83 84 inverted polygons vector style 90 91 point displacement vector style 88 90 rule based vector style 85 88 single symbol vector style 80 81 Version Control System VCS Windows Git 314 QGIS installing on 2
342. r Data for Processing 9 Set the Cellsize Y value to 0 0005 to match the Cellsize X value Again note that as you type in the value the Number of rows value changes to 127 The extent values can be manually changed by typing in the desired values or they can be automatically filled with the current extent of the map canvas by clicking on Set to current extent The default extent is set to the extent of the input vectors 10 Set the output file to Pecos_IDW tif 11 Select Add result to project to automatically add the interpolated raster to the map canvas Click on OK to execute the IDW interpolation The following figure shows the resulting raster from the IDW interpolation with the Pecos DEM Points shp input points on top a r oo oe ee a EEJ 5 Pier E E 600 866 amp g o p oa a a ith There are additional interpolation tools available via the Processing Toolbox In the GRASS commands there are v surf bspline v surf idw andv surf rst which interpolate using the bicubic or bilinear spline inverse distance weighted and regularized spline respectively In the SAGA commands there are many interpolation methods under the Grid Gridding Grid Spline and Kriging sections in the advanced interface of the Processing Toolbox 9G Chapter 5 Summary This chapter provided steps and examples showing how to prepare raster data for further processing This chapter covered common raster operations such as r
343. r afew moments you will be notified that the import is complete To view the newly created table you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then click on Refresh under Database or press the F5 key on your keyboard The NF_roads table should now appear and have the polyline icon next to it To preview the attribute table click on the Table tab on the information panel To preview the geometry click on the Preview tab on the information panel To view the newly created SpatiaLite layer in QGIS Desktop right click on NF_roads in the tree and then choose Add to canvas Importing tables into SpatiaLite To import a table file into a SpatiaLite database complete the following steps 1 Open DB Manager by clicking on DB Manager under Database Expand SpatiaLite and select GiffordPinochet sqlite on the Tree panel Navigate to Table Import layer file to open the Import vector layer dialog Click on the ellipsis button to the right hand side of the Input drop down box and select and open Waterfalls xls from the sample dataset that is available for download on the Packt Publishing website Click on the Update options button to load the remainder of the dialog box The output table name will populate as Waterfalls and it match the base name of the input file Note that all options related to spatial datasets are not modifiable and are grayed out as shown in in the following screenshot This
344. r offered you a simplified way to interact with QGIS and it is more oriented towards a GIS chapter is also oriented analyst than a GIS programmer or a computer scientist The to GIS companies that are interested in reusing code that is already developed but was probably developed for GIS platforms that are not free This chapter gives basic knowledge on how to interact with QGIS using the PyQGIS programming language 376 Chapter 10 We saw how to programmatically load different kinds of layers from raster to vector We also explained vectors and how to manage different kinds of vector resources from filesystems to remote database connections We also explored the vector structure in more detail and you learned how to browse and edit its records Different kinds of editing workflows were proposed in the chapter so that you can interact with the QGIS framework in a better manner You learned how to launch processing toolbox algorithms and user developed processing toolbox scripts to enhance QGIS with new functionalities You also learned how to launch external commands or scripts to offer a way to integrate already developed code and thereby reduce developing and testing costs Lastly we explored the QGIS map canvas and how to interact with that using Map Tools We obtained canvas coordinates and created new Map Tools to draw canvas objects This will give you the basic skills needed to create new plugins that interact with layers d
345. r remove rows remember that you can also use the GRASS r recode tool after creating a recode rule file This might be a good exercise to work through to make sure you understand the formatting requirements for GRASS recode rule files For a more in depth explanation visit http grass osgeo org grass65 manuals r recode html To use the SAGA Reclassify grid values tool we need to provide an input grid which in this case is the clipped land use layer lulc_clip tif and set Method to 2 simple table The values that need to be reclassified shown in the previous screenshot can be entered by clicking on the Fixed table 3x3 button Make sure you provide a name for the new reclassified grid for example lulc_rec tif 251 The Processing Toolbox Combining raster layers using the SAGA Raster calculator Now we have all the necessary layers to finalize our simplistic model of Northern Spotted Owl habitat Since we have zero values that need to be preserved that is places where owls will never roost we will multiply the three layers together using the SAGA Raster calculator tool The next screenshot illustrates how to populate the raster calculator by selecting the reclassified elevation layer elev_1800 tif as the main input layer and the reclassified water proximity buf_water tif and land use lulc_rec tif layers as the two additional reclassified layers Parameters log Help Main input layer Elevation Recass
346. r the Linear resampling Cubic Spline This algorithm is based on a spline function and produces smooth output Lanczos This algorithm produces sharp output It must be used with caution because it can result in output values that are both lower and higher than those in the input The following figure shows setting the Resampling method setting within the Transformation settings window Transformation settings Transformation type Polynomial 2 Resampling method Compression linear Cubic Cubic Spline Lanczos p 193 Advanced Data Creation and Editing As is the case with Transformation type there is no best RS Resampling method Choosing the most appropriate algorithm Q depends on the nature of the data and how that data will be used after it has been georeferenced Nearest neighbour Linear and Cubic are the most frequently used options 2 Since raster data tends to be large it may be desirable to choose a compression algorithm There are four choices for Compression Some choices offer better reductions in file size while others offer better data access rates For this example use None The choices are as follows None This offers no compression O LZW This offers the best compromise between data access times and file size reduction Packbits This offers the best data access times but the worst file size reduction Deflate This offers the best file size reduction Name the output ras
347. ramp adding 62 removing 59 renaming 59 colors managing 52 selecting 52 common parameters diagrams position parameter 94 95 size parameter 94 complex aspatial queries 140 143 complex spatial queries 140 143 convex hulls generating with Convex Hull tool 125 coordinate data points creating 169 173 well known text WKT representations of geometry mapping 174 176 coordinate reference system CRS about 184 defining 127 129 Proj 4 definition format 127 128 working with 10 11 cpt city color ramp adding 64 65 URL 65 custom coordinate reference system defining 129 130 custom processing script running 363 365 test processing toolbox script creating 364 viewing 364 D data databases loading 8 9 editing 14 15 importing into SpatiaLite database 33 loading 6 raster data loading 7 8 vector data loading 6 7 web services 9 database concepts 27 data types 28 geometry types 28 SQL 29 30 table relationship 29 tables 28 database management system DBMS 30 data editing about 14 15 blending modes 20 22 contrast enhancement 19 20 raster data styling 18 snapping 15 vector data styling 16 17 debugger 326 debugging environment Aptana installing 327 debugger 326 Pydevd server starting 328 PYTHONPATH setting up 327 QGIS connecting to Pydevd server 329 setting up 326 default algorithm settings adjusting 227 diagrams attributes adding to 95 common parameters 94 histogram chart diagram creating 100 101 p
348. ramp is selected the Selection and preview and Information tabs are populated There are two ways to add a cpt city color ramp to the list in the Style Manager as a cpt city or as a standard gradient color ramp To save the color ramp as a cpt city color ramp click on OK with a color ramp selected This will keep the link between the added color ramp and the cpt city color ramp list The color ramp cannot be modified if it is added as a cpt city color ramp 64 Chapter 3 To save the color ramp as a standard gradient color ramp check Save as standard gradient and then click on OK This will save the color ramp as a gradient color ramp and the color ramp will not link back to the cpt city color ramp collection The color ramp can be modified later as it has been converted to a standard gradient color ramp a Be sure to review the license information for the cpt city ay color ramps in the Information tab s License field Many Q different licenses are used and some require attribution before they can be used The cpt city archive of gradients is available at http soliton vm bytemark co uk pub cpt city The archive contains thousands of gradients The gradients that are most applicable to style geographic data have been included in the QGIS cpt city collection Editing a color ramp To edit a color ramp select the color ramp and then click on the Edit button This will open one of the four types of windows depending on whi
349. rd If the password is set to None QGIS will ask you for the password for connecting to database The setDataSource parameter refers to the schema name the table name and the geometry column where the geometry is archived Finally an optional where string could be set to directly filter data in this case mywhere 343 PyQGIS Scripting You can load a query without having it as a table Just write your KS query instead of the table name and place round brackets around it Q For example the previous setDataSource method will become uri setDatasource public lt here your query gt the geom myWhere The last line shows you the URI string that will be used to point to the vector data It will be a string similar to the following one dbname myDb host localhost port 5432 user myUserName password myPassword table public myTable the geom sql myWhere An alternative way is to create the URI string manually rather than populating the QgsDataSourceURI class but it s generally more readable and less error prone to write the previous code than a complex string If the database is on a SpatiaLite file it s necessary to substitute the setConnection method with the following code url QgsDataSourceURI uri setDatabase path to myDb sqlite uri setDataSource myTable the geom myWhere Peine Uri uri This generates the following URI string dbname path to myDb sqlite table m
350. rdinates Enter X and Y coordinates DMS dd mm ss ss DD dd dd or projected coordinates mmmm mm which correspond with the selected point on the image Alternatively dick the button with icon of a pencil and then cick a corresponding point on map canvas of QGIS to fill in coordinates of that point X East 478424 8769028668757528 1 North 71900040097534657 Snap to background layers From map canvas Cancel 6 Click on OK 7 As you enter GCP information for the source srcX srcY and destination dstX dstY coordinates they will be displayed in the GCP Table window in the Georeferencer window Z Georeferencer Scanned1990 tif File Edit View Settings Help gt aa Paaa O PLO AA gt fee Bx dstY dX pixels dY pixels residual pixels 121 45 245 21 474792 14 4410598 36 0 00 0 00 0 00 1567 70 98 25 478424 88 4410898 72 0 00 0 00 0 00 Transform Not set 399 245 187 Advanced Data Creation and Editing 8 Repeat steps 1 4 and 1 7 above to enter the remaining GCPs If you are georeferencing against benchmarks datums or other printed coordinates on the input raster you need to perform the following steps 1 Enter the X East and Y North coordinates in the appropriate boxes 2 Click on OK 3 Repeat steps 1 4 and 1 2 above to enter the remaining GCPs Now that you ve learned the basic procedure for georeferencing in QGIS we will go through two
351. riable For example to create geometry from a point that is expressed as a WKT we can use the following code myPoint QgsGeometry fromwWkt POINT 195935 165 76639002585 ey Notice that the OgsGeometry class name is used to call the fromwkt method This is because fromWkt is a static method In the same way it s possible to create myPoint from QgsPoint with the following code newPoint QgsPoint 195935 165 7663900 585 myPoint QgsGeometry fromPoint newPoint The two ways that we just described are equivalent and generate QgsGeometry in the myPoint variable Iterating over features Now it s time to discover how to get all the features or a subset of them The main way to iterate over all features or records of myVector is by using the following code that shows the ID of each feature for feature in myVector getFeatures feature id This will print a list of all the 653 record IDs as shown here OL 1L Sed COUE des 652L It s not always necessary to parse all records to get a subset of them In this case we have to set the QgsFeatureRequest class parameters to instruct getFeatures and then retrieve only a subset of records in some cases we must also retrieve a subset of columns 350 Chapter 10 The following code will get only a subset of features and columns rect QgsRectangle 1223070 695 2293653 357 9046974 211 4184988 662 myVector setSubsetString AREA MI gt 1000 request
352. right the tools in the digitizing toolbar are as follows The Current Edits tool allows you to manage your editing session Here you can save and rollback edits for one or more selected layers The Toggle Editing tool provides an additional means to begin or end an editing session for a selected layer The Save Layer Edits tool allows you to save edits for the selected layer s during an editing session The Add Features tool will change to the appropriate geometry depending on whether a point line or polygon layer is selected Points and vertices of lines and polygons are created by clicking To complete a line or polygon feature right click After adding a feature you will be prompted to enter the attributes Features can be moved with the Move tool by clicking on them and dragging them to the new position Individual feature vertices can be moved with the Node tool Click ona feature once with the tool to select it and the vertices will change into red boxes Click again on an individual vertex to select it The selected vertex will turn into a dark blue box Now the vertex can be moved to the desired location Additionally edges between vertices can be selected and moved To add vertices to a feature simply double click on the edge where you want the vertex to be added Selected vertices can be deleted by pressing the Delete key on the keyboard Features can be deleted cut copied and pasted using the Delete Selected Cut F
353. rld examples to get you started with QGIS Building Mapping Applications with QGIS ISBN 978 1 78398 466 4 Paperback 264 pages Create your own sophisticated applications to analyze and display geospatial information using QGIS and Python 1 Make use of the geospatial capabilities of QGIS within your Python programs 2 Build complete standalone mapping applications based on QGIS and Python 3 Use QGIS as a Python geospatial development environment Please check www PacktPub com for information on our titles PACKT open source PUBLISHING PostGIS Cookbook Building Web Applications with ArcGIS Hussein Nasser PostGIS Cookbook ISBN 978 1 84951 866 6 Paperback 484 pages Over 80 task based recipes to store organize manipulate and analyze spatial data in a PostGIS database 1 Integrate PostGIS with web frameworks and implement OGC standards such as WMS and WFS using MapServer and GeoServer 2 Convert 2D and 3D vector data raster data and routing data into usable forms 3 Visualize data from the PostGIS database using a desktop GIS program such as QGIS and OpenJUMP Building Web Applications with ArcGIS ISBN 978 1 78355 295 5 Paperback 138 pages Build an engaging GIS Web application from scratch using ArcGIS 1 Learn how to design build and run high performance and interactive applications with the help of ArcGIS 2 Incorporate ArcGIS for Server services to allow end users to
354. rocessing toolbox The next section will provide you with an overview of the structure and organization of the toolbox interface 218 Chapter 7 Understanding the processing toolbox In this section we will explore the organization and establish a common language for describing the various components of the toolbox Until this point we haven t actually seen the interface itself We merely configured and possibly installed the required dependencies to make the toolbox function To view the toolbox you need to click on Toolbox under Processing as illustrated in this screenshot History and log FS Options and configuration i Results viewer gt Commander The processing toolbox will appear on the right hand side of the QGIS interface however by default most algorithm providers are not visible You need to click on the Simplified interface option that is visible in the next screenshot in the lower right hand corner which will then display the option to select the Advanced interface option Processing Toolbox Search E Recently used algorithms E eki Models 3 geoalgorithms E Scripts 13 geoalgorithms Simplified interface Advanced interface 219 The Processing Toolbox After selecting the Advanced interface option you will finally have access to the full list of algorithms that the toolbox provides as illustrated in this screenshot Processing Toolbox i T i 1 Recently used algor
355. ry statistics tool select the river s layer as our input and define an output file to produce a text summary of the underlying data as illustrated in the following screenshot 262 Summary statistics C Mastering_OGIS R river_stats html Chapter 7 The following screenshot illustrates the summary produced as a result of running the Summary statistics tool Summary statistics LENGTH Sum 1 2609272e d06 b Count 8 280000et02 Unique values 8 280000e t02 Minimum value 5 627340e 00 Maximum value 1 412734e104 Range 1 412172e 04 Mean value 1 532937e 03 Median value 335031e 02 Standard deviation 1 978282e 03 263 The Processing Toolbox However often a more informative way of evaluating this type of data is through a histogram to see how frequently a feature or characteristic of a feature occurs In this case we can use the Histogram tool under Vector Processing to produce a visual that summarizes the most and least frequently occurring stream length This is one parameter that is often used to characterize the surface hydrology of a given watershed The following screenshot illustrates the input parameters Histogram C Mastering_QGIS R river_histo html 264 Chapter 7 The following screenshot illustrates the resulting histogram Histogram of LENGTH i E mi a aT EL L LL LENGTH 265 The Processing Toolbox For this particular case we see a higher freque
356. s No enhancement except that all cell values below the Min value and all values above the Max value are assigned no color and they are transparent The following figure shows the effects of the four different Contrast enhancement methods on the Gray_50M_SR_W tif sample file when the Color gradient field is set to Black to white Min is set to 107 and Max is set to 207 A Min value of 107 is selected to exclude the cell value of 106 that is associated with the oceans g Stretch and clip toM 69 Styling Raster and Vector Data Singleband pseudocolor raster band rendering The singleband pseudocolor band renderer stretches a color ramp to a single raster band Additionally three Color interpolation methods are available to adjust the way the color ramp is stretched across the raster band s values with respect to the min and max cell values for a discussion on determining min and max values see the preceding section Let s apply a singleband pseudocolor renderer to the GRAY_50M_SR_W tif sample data raster file that represents shaded relief hypsography and flat water for Earth Add GRAY_50M_SR_W tif to the QGIS canvas and open its Style tab from Layer Properties For the Render type field choose Singleband pseudocolor The singleband pseudocolor render type has many interworking parameters that are best explained as a whole through the lens of a workflow instead of explaining them as separate parts The example shown in the f
357. s are duplicated For example the GRASS commands SAGA tools and QGIS geoalgorithms all include a buffer tool At the bottom of the Processing Toolbox there is a toggle between the default simplified interface and the advanced interface The advanced interface organizes the tools by the source software package With the default installation of QGIS 2 6 the toolbox contains almost 400 tools so the search box is a convenient way to locate tools within the toolbox For a more detailed look at the Processing Toolbox refer to Chapter 7 The Processing Toolbox Processing Toolbox Geoalgorithms Domain specific g6 Images Raster age Raster vector Vector alt Models 0 geoalgorithms Scripts 0 geoalgorithms Simplified interface Spatial overlay tools Spatially overlaying two data layers is one of the most fundamental types of GIS analysis It allows you to answer spatial questions and produce information from data For instance how many fire stations are located in Portland Oregon What is the area covered by parks in a neighborhood This series of spatial overlay tools compute the geometric intersection of two or more vector layers to produce different outputs Some tools identify overlaps between layers and others identify areas of no overlap The spatial overlay tools include Clip Difference Intersect Symmetrical Difference and Union 119 Preparing Vector Data for Processing 1 gt Whe
358. s best for cases where you want to reduce the number of colors that will be used to render the raster 71 Styling Raster and Vector Data O Linear This assigns a unique color to each unique raster value Values between values listed in the Value column are assigned a unique color that is calculated linearly and is based on its location between the surrounding listed values In other words if there are say 164 unique values in the raster and 15 colors listed in the classification list the raster will be rendered with the 164 unique colors that appear as a nice linear progression through the 15 listed colors This method is best for raster data that represents continuous information for example elevation or temperature data where you want a smooth progression of color that is stretched across the raster values Exact This assigns a unique color to only the values listed in the Value column of the classification list In other words if there are say 164 unique values in the raster and 15 colors and 15 associated values listed in the classification list only the 15 raster values that are listed will be rendered with their associated colors No other values will be assigned a color This method is best for raster data that represents discrete data classes where you do not want non listed values to be assigned any color The following figure shows the effects of the three Color interpolation methods on our sample data as confi
359. s focus on specialization This book is part of the Packt Open Source brand home to books published on software built around open source licenses and offering information to anybody from advanced developers to budding web designers The Open Source brand also runs Packt s Open Source Royalty Scheme by which Packt gives a royalty to each open source project about whose software a book is sold Writing for Packt We welcome all inquiries from people who are interested in authoring Book proposals should be sent to author packtpub com If your book idea is still at an early stage and you would like to discuss it first before writing a formal book proposal then please contact us one of our commissioning editors will get in touch with you We re not just looking for published authors if you have strong technical skills but no writing experience our experienced editors can help you develop a writing career or simply get some additional reward for your expertise PACKT open source PUBLISHING Learning QGIS Second Edition Anita Graser Building Mapping Applications with QGIS Erik Westra Learning QGIS Second Edition ISBN 978 1 78439 203 1 Paperback 150 pages Use QGIS to create great maps and perform all the geoprocessing tasks you need 1 Load visualize and edit vector and raster data 2 Create professional maps and applications to present geospatial data 3 A concise guide packed with detailed real wo
360. s in the progress bar of the processing toolbox and the output of the command in the Python console Interacting with the map canvas A plugin will commonly interact with the map canvas to get some useful information This information could be for example point coordinates or features identified by these coordinates We can use them to draw geometry entities like points lines or polygons 367 PyQGIS Scripting Getting the map canvas The QgsMapCanvas class is the class that represents a QGIS canvas There can be different canvas instances but the main canvas instance can be referenced with the following code snippet mapCanvas iface mapCanvas The QgsMapCanvas class generates some useful events to support location based plugins For example xyCoordinates sends point locations based on canvas coordinates and the keyPressed event allows us to know which mouse button has been clicked on the canvas Explaining Map Tools The most powerful method to interact with a map canvas class is to set one of the predefined Map Tools or create a custom one that is derived from the predefined Map Tools classes In this chapter we will not create custom Map Tools using inheritance and overloading which are two basic concepts of the object oriented programming paradigm The following paragraphs are focused on using Map Tools that already exist and customizing them without deriving classes and overloading methods The base class f
361. s of the format_number function enter POP1996 o where POP1996 is the first parameter containing the population value the comma separates the function parameters and 0 is the number of decimal places to round the number The following figure shows the completed expression that is formatted across multiple lines for easy reading WHEN POP1996 gt 5000000 THEN STATE NAME I nPopulation format_number POP1996 0 ELSE STATE NAME END tata EEE Popubton 5 629 613 Click on OK to perform the field calculation This will enable editing on the layer and calculate the field values 139 Preparing Vector Data for Processing 14 Save the edits to the layer and disable the editing mode The calculated values are now stored in the layer The following figure shows a sample of labels calculated for states with populations greater and less than five million Complex spatial and aspatial queries QGIS provides powerful spatial and aspatial query tools that allow for the easy creation of data subsets In this section a series of spatial and aspatial queries will be used to determine which elementary schools are within the Southeast Uplift Neighborhood Coalition SEUL and Southwest Neighborhoods Inc SWNI coalitions in Portland Oregon This example uses the Neighborhoods _pdx shp and schools shp sample data First we will select the SEUL and SWNI coalition neighborhoods from Neighborhoods pdx shp To do this
362. s own display pixel the raster renders at full resolution also known as 1 1 However since screen sizes are limited and we may wish to enlarge or reduce the size of the raster as we work at different map scales the raster cells must be mapped to more than one pixel or a number of raster cells must be combined or dropped to map to a single pixel As some raster cells cannot be shown at different resolutions QGIS must determine how to render the raster and still maintain the character of the full resolution raster This section will discuss the parameters available for determining how the raster will be resampled for display The Resampling section of the raster Style tab has three parameters Zoomed in Zoomed out and Oversampling The Resampling section with its default parameters is shown in the following screenshot Resampling Zoomed in Nearest neighbour out Nearest neighbour gt Oversampling 2 00 The Zoomed in parameter sets the resampling method when zoomed in on the raster Three resampling methods are available for selection Nearest neighbour Bilinear and Cubic The Zoomed out parameter sets the resampling method when zoomed out from the raster Two resampling methods are available for selection Nearest neighbour and Average The Oversampling parameter determines how many subpixels will be used to compute the value when zoomed out The four resampling methods that can be selected for use are as follows
363. s shp M Add result to canvas I 0 120 Chapter 4 Using the Difference tool with the same inputs that were used with Clip results in the output fire stations that lie beyond the limits of the clip layer The outputs from these two tools contain only attributes from the input vector layer Output Difference Inputs and outputs from the Clip and Difference tools Using the Intersect and Symmetrical Difference tools The Intersect tool preserves only the areas common to both datasets in the output Symmetrical Difference is the opposite only the areas that do not intersect are preserved in the output Unlike Clip and Difference the output from these two tools contains attributes from both input layers The output will have the geometry type of the minimum geometry of the inputs For example if a line and polygon layer are intersected the output will be a line layer The following is an example of running the Intersect tool After that you will see a figure that demonstrates the output from each tool 121 Preparing Vector Data for Processing The following examples uses the North_PDX_parks shp and Kenton shp shapefiles from the sample data Input vector layer North _PDX_parks Use only selected features Intersect layer Kenton Use only selected features Output shapefile Data Portland Output Chapter4 Kenton_Int_Parks shp M Add result to canvas The following figure compares the Intersect and Sy
364. s to perform spatial analyses We need to search the toolbox for least cost paths and identify the relevant point in this example there will be only two point layers so SAGA finds them by default specify the cost erid and then define an output for the resulting least cost path as illustrated in the following screenshot J Least cost paths Parameters Log Help Source Points End EPSG 26710 Accumulated cost Cost EPSG 26710 Values optional 0 elements selected Profile points Save to temporary file Open output file after running algorithm Profile lines CilMastering_OGT5 GR 455 costpath shp Open output File after running algorithm 234 Chapter 7 Now we just need to organize the relevant layers as shown in the next screenshot to inform the Crater Lake search and rescue team about the least cost approach to the injured hiker In this exercise we made use of both GRASS and SAGA algorithms to calculate a least cost path These algorithms allowed us to calculate a hillshade and slope reclassify raster layers combine raster layers create a cost grid and calculate a least cost path from this cost grid Although this exercise was clearly hypothetical and limited in the number of parameters used to evaluate cumulative cost it hopefully demonstrates how easy it is to perform this type of analysis Least cost path analyses have been used to model historical trade routes Howey 2007 wildlife migration
365. s tool is used when reclassifying integer and _ Categorical rasters while r recode will reclassify floating point and decimal value rasters Both tools use the same format for the J rules text file More complete documentation for these tools can be found on the GRASS GIS help pages at http grass osgeo org grass65 manuals r reclass html and http grass osgeo org grass65 manuals r recode html 147 Preparing Raster Data for Processing The following figure shows the result of the reclassification The original elevation raster with the original elevation values is on the left and the reclassified raster with three values is on the right Converting datasets from floating point to integer rasters Raster datasets may have integer values or floating point values with decimal points The r recode tool can also be used to convert raster datasets between these formats The elevation tif sample data is a floating point raster with values ranging from 1502 1 to 2898 49 meters above sea level To see the full range of values in a raster navigate to Layer gt lt Properties Style Under Load min max values select the Q Min Max setting For Accuracy choose Actual slower Then click on Apply To convert this raster to an integer raster you will need to set up a rule text file with the following text 1502 1 2898 49 1502 2898 input value _ low input value high output value low output _value_high 148 Chapter 5 C
366. same as the overlay method however this raster and the raster below are swapped for inputs Difference This mode subtracts this raster s cell values from the cell values of the raster below If a negative value is obtained then the cell value from the raster below is subtracted from this raster s cell value Subtract This mode subtracts this raster s cell values from the cell values of the raster below If a negative value is obtained a black color is displayed Brightness This changes the brightness of the raster Brightness affects how bright or dark the raster appears Brightness affects all cells in the raster in the same way 77 Styling Raster and Vector Data Contrast This changes the contrast value of the raster Contrast separates the lightest and darkest areas of the raster An increase in contrast increases the separation and makes darker areas darker and brighter areas brighter For example a large negative contrast of 75 would produce a mostly gray or monotone image since the bright and dark colors are not separated very much at all Saturation This changes the saturation value of the raster Saturation increases the separation between colors An increase in saturation makes the colors look more vibrant and distinct while a decrease in saturation makes the colors look duller and more neutral Grayscale This renders the raster using a grayscale color ramp The following three rendering methods are available
367. selected Cancel 282 Chapter 8 Next we ll add the Clip tool to the model using the following parameters e Set the Input layer field to Trees e Set the Clip layer field to Buffer from algorithm Fixed distance buffer e Type Riparian Trees under Clipped lt OutputVector gt e Finally click on OK Parameters Help Description Clip Input layer Trees Clip layer Buffer from algorithm Fixed distance buffer Parent algorithms 0 elements selected re Automating Workflows with the Graphical Modeler The final model looks like the following screenshot The connecting lines show how elements are connected in the workflow The input output and algorithm elements have different colored boxes so that they can be distinguished The algorithm boxes will also include an icon representing the source library For example the Fixed distance buffer and Clip tools are QGIS algorithms and have the Q icon within the element box 4 Processing modeler gt e A A B dip Riparian trees Landcover c g GDAL OGR GDAL Extraction se Clip raster by extent Clip raster by mask layer OGR Miscellaneous se Clip vectors by extent Clip vectors by polygon E QGIS geoalgorithms Vector overlay tools cP Buffer distance Clip Fixed distance buffer E SAGA 4 Shapes Grid Out Clip grid with polygon Shapes Points S Clip points with polygons X Tools for LIDAR data Fusion X
368. settings for Feature Creation Rubberband Snapping Vertex markers and Curve Offset Tool There is also an Advanced Digitizing toolbar which is covered in Chapter 6 Advanced Data Creation and Editing Styling vector data When you load spatial data layers into QGIS Desktop they are styled with a random single symbol rendering To change this navigate to Layer Properties Style There are several rendering choices available from the menu in the top left corner which are as follows e Single Symbol This is the default rendering in which one symbol is applied to all the features in a layer e Categorized This allows you to choose a categorical attribute field to style the layer Choose the field and click on Classify and QGIS will apply a different symbol to each unique value in the field You can also use the Set column expression button to enhance the styling with a SQL expression e Graduated This allows you to classify the data by a numeric field attribute into discrete categories You can specify the parameters of the classification classification type and number of classes and use the Set column expression button to enhance the styling with a SQL expression 16 Chapter 1 e Rule based This is used to create custom rule based styling Rules will be based on SQL expressions e Point displacement If you have a point layer with stacked points this option can be used to displace the points so that they are all v
369. short form for System for Automated Geoscientific Automation environment contains powerful tools some of which have very specific applications for example geostatistical analyses and fire or erosion modeling However we will explore some of the SAGA tools that have broader applications and often dovetail nicely with tools from other providers Similar to GRASS integrating the SAGA algorithms within the processing toolbox provides access to powerful tools within a single interface To explore some of the SAGA algorithms available through the toolbox we will work through a hypothetical situation and perform the analysis to evaluate the potential roosting habitat for the Northern Spotted Owl We are going to continue using data from the provided ZIP file and we will need the following files e Elevation file dems_10m dem available in the GRAss data folder e Hillshade file hillshade tif created in the GRASS section e Boundary file crlabndyp shp e Surface water file hydp shp e Land use file lulc_clnp tif available in the cRass data folder 244 Chapter 7 Evaluating a habitat GIS has been used to evaluate potential habitat for a variety of flora and fauna in diverse geographic locations Most of the habitats are more sophisticated than the approach we will take in this exercise but the intention is to demonstrate the available tools as succinctly as possible However for simplicity s sake we are going to assume that t
370. sing any hydrologic algorithms regardless of the algorithm provider we need to make sure that the DEM is hydrologically corrected This means that we need to ensure that it behaves like the natural landscape where surface flow moves across the landscape and does not get trapped in pits or depressions in the DEM To accomplish this we are going to use the Pit Remove tool on our new DEM file as illustrated in the next screenshot 255 The Processing Toolbox Note that the DEM won t necessarily look any different but the cells will behave in a more appropriate fashion for modeling surface flow J Pit Remove Parameters Log Help Elevation Grid C Mastering_QGIS TauDEM dem tif Pit Removed Elevation Grid C Mastering_QGIS TauDEM cdnp_pit tif Open output file after running algorithm Calculating flow directions across the landscape This is the elevation grid that we will use to perform the remaining calculations Our next step is to use the D8 Flow Directions tool that creates two grids a D8 flow direction grid which calculates what direction the data would flow in each grid cell and a D8 slope grid which calculates a slope value for each grid cell For illustration purposes we are only going to use the D8 tools but there are additional options for using the D Infinity algorithms The next screenshot illustrates how to populate this tool with the new pitremoved elevation layer 256 Chapter 7 J DS Flow D
371. ss 314 class name 313 382 description 313 minimum QGIS version 313 module name 313 plugin name 313 text for the menu item 314 version number 313 map canvas current Map Tool setting 369 Map Tools 368 obtaining 368 point click values obtaining 370 point click values using 373 374 OQgsRubberBand class exploring 374 376 maps composing 23 Merge Shapefiles to One tool 106 MMOQOGIS plugin about 176 URL 176 model algorithms executing iteratively 292 293 batch processing using 298 299 converting to Python 299 documenting 288 290 editing 286 288 exporting 290 291 loading 290 291 naming 274 275 nesting 294 298 running 284 286 saving 290 291 MOOC URL 337 multi band rasters styling 73 74 O Open Source Geospatial Foundation OSGeo about 1 URL 11 127 optional plugin parameters setting bug tracker 314 home page 314 repository 314 tags 314 overlaps repairing between polygons 211 212 editing parameters setting 209 210 P paletted raster band renderer 66 pie chart diagram creating 96 97 Plugin Builder installing 311 plugins locating 312 URL 314 used for creating plugin structure 310 plugin file structure about 317 main plugin files exploring 318 Plugin Builder generated files 319 plugin logic code 325 combo box populating 323 dialog displaying 324 dialog showing 324 improvements 324 layers classifying 323 modifying 322 323 self keyword 324 plugins used for adding function
372. ssible to find more documentation on PyDev at http pydev org manual_adv_debugger html Summary In this chapter we approached three important topics on developing plugins First how to get help to solve programming problems second how to create a basic plugin as a template to develop more complex plugins and finally how to debug it These topics demonstrate basic skills to manage plugin development that can become complex during their design and development The chapter focused on creating a basic infrastructure to easily solve problems that could be found during the development of a working plugin In the next chapter we will explore the PyQGIS programming in depth and learn to manage raster vector algorithm and QGIS interface interactions These skills will be useful to add specific business functions to QGIS to solve practical processing problems 335 10 PyQGIS Scripting This chapter is focused on a specific use case or user QGIS can be used in many different ways and the GIS user is an eclectic user who has many different ways to interact with data and QGIS instruments The main focus of this chapter is a user who has an algorithm and wants to integrate it with QGIS An algorithm can be an external program such as a water modeling tool or a processing toolbox s set of instructions It s possible to interact with QGIS in many different ways from experimenting with PyQGIS in the Python console to creating pl
373. st He earned a master s degree MA in geography from the University of New Mexico in 2000 That same year he founded Bird s Eye View www BirdsEyeViewGIS com to apply his expertise with the GIS technology to the world s mounting ecological and social problems Toward this end Mr Menke s work focuses largely on wildlife conservation and public health His specialties are spatial analysis modeling and cartography He is a longtime advocate of FOSS4G He began writing MapServer applications in 2001 and has been using QGIS since 2007 He is one of the coauthors of the curriculum at the FOSS4G Academy http foss4geo org and has been teaching FOSS4G college courses since 2009 In 2014 Kurt began authoring an award winning blog on FOSS4G technologies and their use in community health mapping http communityhealthmaps nlm nih gov A special thanks goes to Phil Davis for his leadership in the development of the FOSS4G Academy and for his continuing efforts to promote FOSS4G in the U S educational system I would like to thank Rick Smith for being such a joy to work with I d also like to acknowledge Karl Benedict for introducing me to the world of FOSS4G and Jeffery Cavner for his ongoing camaraderie Finally I d like to thank my beautiful wife Sarah for her steady support and encouragement www allitebooks com Dr Richard Smith Jr is an assistant professor of geographic information science at the School of Engineering and Comp
374. st of the space on the application contains four tabs that provide different views of the selected data in the data tree listing and they are as follows e Param This tab displays details of data that is accessed through connections such as a database or WMS e Metadata This tab displays the metadata if any of the selected data e Preview This tab renders the selected data You can zoom into the data using your mouse wheel and pan using the arrow keys on your keyboard e Attribute This tab displays the attribute table associated with the selected data You can sort the columns by clicking on the column headings The toolbar provides access to four functions The Refresh function reloads the data tree view while the Manage WMS function opens the WMS management screen allowing you to manage the WMS connections The New Shapefile function opens the new vector layer dialog allowing new shapefiles to be created Finally the Set layer CRS function allows you to define the coordinate reference system of the geospatial data file that is selected in the data tree view Loading data One strength of QGIS is its ability to load a large number of data types In this section we will cover loading various types of data into QGIS Desktop In general data can be loaded in four ways The first way which will be covered in detail in this section is to use the Add Layer menu under Layer and select the appropriate type of data that you wish to l
375. st raster formats that are managed by QGIS can be loaded thanks to the GDAL library that is documented at http gdal org To code the loading of our first raster named landcover img that is available in the qgis_sample_data folder execute the following code snippet in the QGIS Python console by adapting the path to Landcover img based on your operating system and data location myRaster QgsRasterLayer qgis sample data raster landcover img In this way the layer is loaded and referred to with the myRaster variable If we want the layer to be visible in the legend with the name MyFirstRaster we need to modify the preceding code snippet by adding a second parameter as follows myRaster QgsRasterLayer qgis sample data raster landcover img MyFirstRaster Two things should be noted the first is that loading a raster layer is gt usually not necessary to specify the raster provider because it is GDAL by default The second is that loading a layer is not the same as visualizing it in QGIS a layer reference is loaded in memory to be processed and it is eventually visualized One of the basic actions after loading a layer raster or vector is to ensure that it has been loaded correctly To verify this execute the following code myRaster isValid It should return True if the layer has been loaded correctly The following snippet does some recovery actions in case loading fails if not myRaster isValid lt do som
376. t WHERE lt field gt lt operator gt lt value gt where lt table gt is the table to delete records from lt field gt is the name of the field lt operator gt checks for equality and lt value gt is the value to check against the field Another SQL function of interest is view A view is a stored query that is presented as a table but is actually built dynamically when the view is accessed To create a view simply preface a SELECT statement with CREATE VIEW lt view_name gt AS and a view named lt view_ name gt will be created You can then treat the new view as if it were a table Creating a spatial database Creating a spatial database in QGIS is a simple operation QGIS supports PostGIS SpatiaLite MSSQL SOL Anywhere and Oracle Spatial databases We will cover SpatiaLite an open source project that is cross platform simple and lightweight and provides quite a bit of functionality SpatiaLite is a spatial database management system DBMS built on top of SQLite a lightweight personal DBMS SpatiaLite and thus SQLite is built on a personal architecture which makes installation and management virtually nonexistent The trade off however is that it neither does a good job of JA j supporting multiple concurrent connections nor does it support a client server architecture For a more complex DBMS PostGIS is an excellent open source option We will create a new SpatiaLite database that we will use for the remaining
377. t of the fields in the attribute table will be listed When you select a field a Field values area will appear to the right underneath the function help as shown in the following figure Click on all unique to load the Field values area with all unique values found in the selected field Then click on 10 samples to load 10 samples that are found in the selected field into the Field values area You can also load values by right clicking on the field name and selecting it from the contextual menu contextual menu is shown in following figure Double click on a value to add it to the cursor location in the expression B Fields and Values ae r Alabama lues Arizona Arkansas California Colorada POP1996 Connecticut Recent fieldcalc Load all unique values Load values all unique 10 samples Writing advanced field calculations Let s put what we learned previously to practice This section will walk you through creating three advanced field calculations The first calculation will insert the current date into a field as a formatted string The second calculation will insert a geometry value The third calculation will calculate a label string that differs depending on the state s population ee Chapter 4 The first example calculating and formatting the current date The first example of an advanced field calculation uses two functions to calculate and format the current dat
378. t possible for a point to be in two countries at once Therefore polygons in such a dataset should not overlap The Topology Checker plugin can be used to test whether there are any overlapping polygons Installing the Topology Checker Here are the steps for installing the Topology Checker plugin 1 Navigate to Plugins Manage and Install Plugins and click on the All tab 2 Inthe search bar type topology 3 Select the Topology Checker plugin and click on Install plugin 4 Once enabled the Topology Checker plugin can be found by navigating to Vector Topology Checker 5 When the Topology Checker window opens it appears as a panel in QGIS Desktop Topological rules Different sets of topological rules are available depending on the feature geometry point line or polygon Some rules test for relationships between features in a single layer and some test the relationships between features of two separate layers All participating layers need to be loaded into QGIS The following topological rule tests are available 200 Chapter 6 Rules for point features The rules for point features are as follows must be covered by This relationship test evaluates how a point layer interacts with a second vector layer Points that do not intersect the second layer are flagged as errors must be covered by endpoints of This relationship test evaluates how a point layer interacts with a line layer Points that do not inters
379. t the same time by following the list order 341 PyQGIS Scripting After the image is loaded QGIS will appear as shown in the following screenshot Project Edit View Layer Settings Plugins Vector Raster Database Web Processing Help He e eeanpepanae E 2200 F 0 RAIRA PARAR Layers awe F MyFirstRaster 1 Python Console 2Use iface to access QGIS API interface or Type help iface for more info 3 gt gt gt myRaster QgsRasterLayer mnt data PROGRAMMING GIS GISDA TA qgis_sample_data raster Landcover img MyFirstRaster 4 gt gt gt QgsMapLayerRegistry instance addMapLayers myRaster 5 lt qgis _core QgsRasterLayer object at 0x955f3b0 gt Coordinate 6832885 3289534 Scale 3 756 544 Render EPSG 2964 a The preceding screenshot displays the image loaded in QGIS with a default false color rendering palette Managing vector files Similar to the raster layer in the previous section most of the vector formats managed by QGIS are supported by the OGR library a part of the GDAL library OGR is documented at the same link of the GDAL library All vector formats managed by OGR are listed at http www gdal org ogr_formats html To read vector data it s always necessary to specify the provider because it can be provided by different sources 342 Chapter 10 To code the loading of our first shapefile named alaska shp that is available in qgis_ sample data execute the following code snippet in the
380. t the toolbox is properly configured use a variety of specialized vector and spatial algorithms from the GRASS and SAGA libraries and perform hydrologic analyses using the Terrain Analysis Using Digital Elevation Models TauDEM library We will cover the following topics in this chapter Introduction to the toolbox Configuring the toolbox Structure of the toolbox Performing spatial analyses using GRASS and SAGA Performing a hydrologic analyses with TauDEM 215 The Processing Toolbox About the processing toolbox The processing toolbox serves as a one stop shopping GUI for accessing algorithms from both native QGIS tools and many third party providers Historically the algorithms from other geospatial packages were only accessible within the native software environment or through a command line environment Algorithms from the following providers are accessible using the toolbox e QGIS geoalgorithms e GDAL OGR e GRASS e SAGA e TauDEM e LAStools e R e Orfeo Toolbox e Models e Scripts We will not make use of all the algorithm providers or explore all the available algorithms in this chapter however the last two entries in the list offer additional options for creating reusable graphical models and running Python scripts which are covered in Chapter 8 Automating Workflows with the Graphical Modeler and Chapter 9 Creating QGIS Plugins with PyQGIS and Problem Solving respectively Configuring the processing too
381. t unit are available by right clicking on a category row Additionally advanced settings are available by clicking on the Advanced button 82 Chapter 3 Graduated vector styling The graduated vector style applies one symbol per range of numeric attribute values This vector style is the best when you want a different symbol that is based ona range of numeric attribute values such as when styling gross domestic product polygons or city population points The graduated vector style works best with ordinal interval and ratio numeric attribute data The following screenshot shows the Categorized style type with parameters for polygon vector data of the populations of the countries Its properties will be very similar to that of point and line vector data a Graduated z Column postesti J E Change Wi source M 1 to 2 Symbol Classes 5 Color ramp Invert Mode Natural Breaks Jenks Legend Format Precision 0 X Trim Symbol Values 20796 00 16715999 00 Legend x LI x O xX E x E x E 16715999 00 49052489 00 49052489 00 111211789 00 111211789 00 313973000 00 313973000 00 1338612970 00 Classify Add class Delete 20796 to 16715999 16715999 to 49052489 49052489 to 111211789 111211789 to 313973000 313973000 to 1338612970 Delete all X Link class boundaries Advanced v Styling vector data with the Categorized style type is a five step process which is as follows 1 Select a
382. ta files To have a style always apply to the layer you are saving the style from save the style file as a qm1 and name it the same sl f name as the source file For example if the shapefile name was Coastlines shp then you should save the style file as Coastlines qml Creating a style file with the same name has the same effect as setting the default style Loading a style QGIS can load styles from two formats qm1 and sld To load a style open the Layer Properties window click on the Load Style button and open the style file that you wish to load This will apply the style to the layer QGIS will try and apply the styles even if the geometry of the style is not applicable to the geometry of the current layer this can create unexpected output so it is usually best to load styles that match the geometry type Setting and restoring a default style To set a layer s current style as the default style for use in other QGIS projects click on the Save As Default button in the Layer Properties window This will save a qml file with the same name as the layer on disk When the layer is added to the map canvas in this or another QGIS project the saved default style will be automatically applied If you have made changes to the style of a layer that had a default style and wish to revert back to the default style click on the Restore Default Style button Lastly to remove a default style for a layer delete the qm1 file
383. table that would essentially convert the table to a point layer Let s do this now 1 With the Waterfalls table selected in the Tree panel navigate to Table Edit Table to open the Table properties window 37 Creating Spatial Databases 2 Click on the Add geometry column button In the new window set the following options to match the following screenshot and then click on OK to create the geometry field Name geom the name of the field that will contain the geometry information Type POINT the type of geometry the field will hold Dimensions 2 the number of dimensions values the geometry field will hold for each record SRID 26910 the coordinate system of the geometry field Es Add geometry column Name geom Type POINT a 2 F ae Dimensions 2 SRID 26910 3 Close the table properties To view the newly edited table you ll need to refresh the Tree panel by selecting GiffordPinochet sqlite in the tree and then clicking on Refresh under Database or press the F5 key on your keyboard The Waterfalls table should now appear and have the point icon next to it Now that the Waterfalls table has a geometry field we need to populate it with the coordinates We will accomplish this by writing a SQL update query and using the SpatiaLite MakePoint function To do this perform the following steps 1 Inthe SQL window click on the Clear button to clear the SQL query text area 2 Enter t
384. te Spline Projective 192 Chapter 6 1 There are five choices for Resampling method During the transformation anew output raster will be generated This setting will determine how the pixel values will be calculated in the output raster Each is described here for this example choose Linear Nearest neighbour In this method the value of an output pixel values will be determined by the value of the nearest cell in the input This is the fastest method and it will not change pixel values during the transformation It is recommended for categorical or integer data If it is used with continuous data it produces blocky output Linear This method uses the four nearest input cell values to determine the value of the output cell The new cell value is a weighted average of the four input cell values It produces smooth output because high and low input cell values may be eliminated in the output It is recommended for continuous datasets It should not be used on categorical data because the input categories may not be maintained in the output Cubic This is similar to Linear but it uses the 16 nearest input cells to determine the output cell value It is better at preserving edges and the output is sharper than the Linear resampling It is often used with aerial photography or satellite imagery and is also recommended for continuous data This should not be used for categorical data for the same reasons that were given fo
385. tensity Developed medium Lowest cost intensity Developed high Moderate cost intensity o Woody wetlands 1 1 5 20 1 20 50 20 10 5 5 20 230 Chapter 7 The following screenshot illustrates how to populate the r recode algorithm Q r recode Recodes categorical raster maps Parameters Log Help Input layer Slope EPSG 26710 File containing recode rules optional C Mastering_QGIS GRASS slope_recode txt GRASS GIS 7 region extent xmin xmax ymin ymax eave lank to 26 min covering extent L e h F e Lr k LAF Laiad m ed E we Li L GRASS GIS 7 region cellsize leave 0 for defaul 0 000000 Output raster layer C Mastering _QGIS GRASS slope_rec tif Open output file after running algorithm We need to use this same tool to recode the values of the provided land use layer using the lulc_recode txt rule set Again it is worth exploring this file to evaluate the assumptions made about the costs for moving through each land use classification For example we have assumed that water has the highest cost and developed open space has the lowest cost To properly explore this layer you will need to import the lulc_palette qml QGIS style file which will categorize land use by name for example water mixed forest and so on 231 The Processing Toolbox Combining reclassified slope and land use layers Once we have created both the slope and land use cost grids we can combine them usin
386. ter 6 Scanned1990_UTM_718 tif Target SRS EPSG 26918 Generate pdf map 0 PACKT Data Chapter 6 GeorefMap pdf al o Ral Generate pdf report OSGe04W64 bin Geore Report pdf Set Target Resolution Horizontal 1 00000 Vertical 1 00000 Use 0 for transparency when needed Load in QGIS when done 10 Once the Transformation Settings values have been set the residual pixels column in the GCP table will be populated This column contains the root mean square error RMSE for each GCP The mean RMSE for the transformation will be displayed at the bottom of the Georeferencer window RMSE is a metric that indicates the quality of the transformation It will change depending on the Transformation type value E gt chosen The general rule of thumb is that the RMSE should not be larger than half the pixel size of the raster in map units However it is only an indication Another indication is how well the georeferenced imagery aligns with other datasets 195 Advanced Data Creation and Editing 11 Additionally the Link Georeferencer to QGIS tool P and Link QGIS to Georeferencer tool will be activated These will join the Georeferencer window to the QGIS Desktop map canvas and they will be synched together when you use the pan and zoom tools The following screenshot shows the image in the Georeferencer window with 8 GCP s entered Their location is identified by numbered boxes within the ima
387. ter by clicking on the Browse button 4 Choose the target SRS by clicking on the Browse button A window for choosing the target coordinate reference system will open For this example choose EPSG 26918 5 Ifan output map and output report are desired in the PDF format click on the browse button next to the Generate pdf map and Generate pdf report options and specify the output name for each The report includes a summary of the transformation setting GCPs used and the root mean square error for the transformation 6 Click on the Set Target Resolution box to activate the Horizontal and Vertical options for output pixel resolution in map units For this example leave this option unchecked 7 The Use 0 for transparency when needed option can be activated if pixels with the value of 0 should be transparent in the output For this example leave this option unchecked 8 Click on Load in QGIS when done to have the output added to the QGIS Desktop map canvas The Georeferencer tool can be configured by clicking on Configure Georeferencer under Settings in the Georeferencing window Here Q there are options for adding tips labels for the GCPS choosing residual units and specifying sheet sizes for the PDF report 194 Chapter 6 9 Click on OK to set the transformation settings Transformation settings Transformation type Resampling method Compression _ Create world file Output raster D PACKT Data Chap
388. the color picker The color picker has four changeable panels Color ramp Color wheel D Color swatches iz and Color sampler fi Each of these panels provide convenient ways to select and manage colors This section will provide details of each of the four panels Color ramp The color ramp panel is an interactive selection tool that sets the currently selected HSV or RGB parameter values based on the location of a mouse click To select the color model parameters that the color ramp will display click on one of the radio selection buttons next to the H S V R G or B values on the right half of the color picker The selected parameter can be individually modified using the thin vertical slider control on the right hand side of the color ramp panel The other two color model parameters can be set simultaneously by clicking on the large color display on the left side of the color ramp panel In the following screenshot the V Value value in the HSV color model is selected and is represented in the thin vertical slider control the H Hue and S Saturation values are combined and are represented in the large color display 54 Chapter 3 Color wheel The color wheel panel shown in the following screenshot is an interactive selection tool that sets the color value based on mouse clicks The ring contains the hue while the triangle contains the saturation and value To set the hue click on the ring To set the sat
389. the query and execute it again In addition note that you can view log messages of the query Lastly you can create a temporary layer to the map from the selected features by clicking on the Create layer with selected button 142 10 11 Chapter 4 if Spatial Query Select source features from Result feature ID s schools Result query M 123 selected geometries Where the feature Within Reference features of a ad 123 of 926 identified M 46 selected geometries zoom to item And use the result to X Log messages Create new selection Stoo U oT Neighborhoods_pdx 0 of 46 selected Selected features features ai Finish at Sat Nov 29 15 25 21 2014 E 123 of 926 selected by Create new selection E processing time 0 00 minutes a Y Click on Close to dismiss the Spatial Query window On the map the schools layer will have the selected records that fall within the two selected coalition neighborhoods The last step is to select only the elementary schools from the selection Open the attribute table of schools Click on the Select features using an expression button to open the Select by expression window Using what you have learned so far create this expression LEVEL Elementary Click on the down arrow next to the Select button and choose Select within selection This will execute the selection The remaining 32 selected records are the elementary schools that were previously selected by location
390. this information First published March 2015 Production reference 2020415 Published by Packt Publishing Ltd Livery Place 35 Livery Street Birmingham B3 2PB UK ISBN 978 1 78439 868 2 www packtpub com Cover image by NASA Earth Observatory www allitebooks com Credits Authors Kurt Menke GISP Dr Richard Smith Jr GISP Dr Luigi Pirelli Dr John Van Hoesen GISP Reviewers Paolo Corti Abdelghaffar Khorchani Gergely Padanyi Gulyas Commissioning Editor Dipika Gaonkar Acquisition Editors Richard Harvey Rebecca Youe Content Development Editor Samantha Gonsalves Technical Editors Ruchi Desai Manal Pednekar Copy Editors Sonia Michelle Cheema Jasmine Nadar Project Coordinator Kinjal Bari Proofreaders Martin Diver Maria Gould Elinor Perry Smith Indexer Rekha Nair Graphics Valentina D silva Production Coordinator Alwin Roy Cover Work Alwin Roy www allitebooks com Foreword It has been my pleasure to witness the development of both this book Mastering QGIS and the QGIS software in the past 12 months Who could have predicted the rapid development and adoption of QGIS in such a short time QGIS is now on a par in terms of its functionality and features with the best of commercial GIS application software With an aggressive code development schedule of quarterly updates the QGIS project is adding new features faster than most GIS professionals can keep pace To help
391. tion and transformation tasks this chapter will cover what we consider to be commonly used vector preparation tasks Many of the tools covered in this chapter are found on the Vector menu in QGIS however others are available in the processing toolbox Additional vector processing tasks will be covered in Chapter 7 The Processing Toolbox and Chapter 8 Automating Workflows with the Graphical Modeler The topics that we will cover in this chapter are as follows e Merging shapefiles e Creating spatial indices e Checking for geometry errors e Converting vector geometries e Adding geometry columns e Using basic vector geoprocessing tools e Advanced field calculations e Complex spatial and aspatial queries e Defining new coordinate reference systems e Raster to vector format conversions 105 Preparing Vector Data for Processing Merging shapefiles The Merge Shapefiles to One tool merges that is combines multiple input shapefiles to a new shapefile The input shapefiles must be in a common coordinate reference system and should contain common attributes For example vector data is often provided in tiles or by political jurisdiction such as counties or states In these cases the data may need to be merged to form a seamless layer covering the study area The Merge Shapefiles to One tool that can be found by navigating to Vector Data Management Tools will combine them fe Merge shapefiles Select by layers in the folder Shap
392. tiveLayer The following lines will be displayed Help on built in function activeLayer activeLayer QgisInterface activeLayer gt QgsMapLayer This shows that the activeLayer call returns data that is a QgsMapLayer data type The Python dir function gives you more detailed information showing a list of all the methods belonging to a class Q Try typing dir iface and compare it with the result of the previous help iface command Creating a plugin structure with Plugin Builder A QGIS plugin can be created manually with a simple editor but the simplest and most complete way to start to create a plugin is to use another Python plugin called Plugin Builder Plugin Builder generates the file infrastructure of the plugin thus avoiding writing repetitive code Plugin Builder creates only basic and generic plugins which can be modified to add specific user functionalities 310 Chapter 9 It is a graphical interface used to introduce the main parameters to create a plugin It will generate a generic plugin with the following interface Cancel This is an almost empty dialog with two buttons Every piece of this interface can be modified and customized to reach the plugin goal Installing Plugin Builder The first step is to install Plugin Builder using the Plugin Manager by navigating to Plugins Manage and Install Plugins It s strongly suggested you install the experimental version of the plugin as
393. to query a table and create a spatial view in SpatiaLite i Different databases support different SQL commands SQLite supports much of but not all the standard SQL For Q a complete listing of supported SQL operations visit http www sqlite org sessions lang html Creating a SQL query To create a SQL query perform the following steps 1 Open DB Manager by clicking on DB Manager under Database 2 Inthe Tree panel navigate to and select the database on which you wish to perform a SQL query 3 Navigate to Database SQL window or press F2 on your keyboard to open the SQL window 45 Creating Spatial Databases 4 Enter a SQL query in the textbox at the top Click on the Execute button or F5 on your keyboard to execute the SQL query against the database The results of the query will be displayed in the results box at the bottom and the number of affected rows and execution time will appear next to the Execute button An example of a successfully run query is shown in the following screenshot SOL window GiffordPinochetsalite 5 SOL query rfalls select_Mc Store Delete 1 select Name Type from Waterfalls 2 where quadname Mowich Lake Execute F5 1rows 0 0 seconds Clear Result NAME TE Chenuis Falls falls 1 2 Ranger Falls falls 3 Ipsut Falls falls lt Load as new layer You can store any query by
394. to the Map Tool event using the following code myMapTool canvasClicked connect manageClick Setting up the new Map Tool Now it s the time to activate the new Map Tool to pass canvas control to its event handlers This can be done with the following code iface mapCanvas setMapTool myMapTool After executing the preceding command the new Map Tool will be activated and it will print coordinates in the console as shown in the following screenshot Python Console Python Console 2Use iface to access QGIS API interface o r Type help iface for more 3 move coordinate 31012892 4 move coordinate 31012892 5 move coordinate 31012892 6 move coordinate 30932428 7 move coordinate 30932428 info 1461082 1541545 1622009 1702473 1782936 8 Clicked on 30932428 1782936 9 move coordinate 30932428 10 move coordinate 30851965 1863400 1863400 11 Clicked on 30851965 1863400 12 move coordinate 30771501 1863400 13 move coordinate 30691037 1863400 14 move coordinate 30610574 1943864 15 Clicked on 30610574 1943864 16 Clicked on 30610574 1943864 17 Coordinate Untitled 0 x from qgils gu1 import QgsMapTooLEmi tPoint previousMapTool 1face mapCanvas mapTool myMapTooL QgsMapToolEmtPoint iface mapCanvas def showCoordinates currentPos print move coordinate d sd currentPos x current iface mapCanvas xyCoordinates connect
395. to use in this section by going to the official website of each al provider e GRASS http grass osgeo org e SAGA http www saga gis org en index html e TauDEM http hydrology usu edu taudem taudem5 e LAStools http rapidlasso com LAStools The processing toolbox also provides one additional option for accessing the underlying algorithms and that is through processing commander To access this tool click on Commander under Processing and you will be presented with an interface as shown in the following screenshot Processing Toolbox Recently used algorithms Grid Network E FiA GDAL OGR 32 geoalgorithms GDAL Analysis Processing algorithm Create Grid Surface Grid Data metrics Processing algorithm lasgrid Grid Inverse distance to a Processing algorithm Regular sampling grid Processing algorithm Random sampling arid Grid Moving average Processing algorithm Contour lines from grid bal Grid Nearest neighbor Processing algorithm Change grid values w GRASS commands 167 geoalgorith ee ao E Raster r y rout gridatb Exports GRA y lresamp interp Resample y rresamp stats Resamples y terraflow Flow computati y terraflow short Flow co Vector v yw v mkgrid Creates a GRAS z L QGIS geoalgorithms 79 geoalgorit Vector creation tools create grid Vector general tools 8 Snap points to grid R R scripts 19 geoalgorithms Point pattern an
396. ton For this example we will only cover the IDW parameter distance coefficient however you must know that for TIN you can set the Interpolation method linear or cubic and have the option to export the triangulation to shapefile 3 Inthe Input area the Vector layers combo box is populated with all the valid vector layers added to the QGIS project Make sure that Pecos DEM Points is selected 4 Select value as the interpolation attribute This will be the value that is used as input for the interpolation method If the input vector supports 3D geometry we could optionally select Use z Coordinate for interpolation instead of choosing an attribute 5 Click on Add to add the vector layer and the selected attribute to the layer list At this point we could add additional input vector layers to use as inputs The inputs will be combined into a single input 6 Moving to the Output section select Inverse Distance Weighted IDW as the interpolation method 7 Click on the configure button to access the Distance coefficient P value parameter Enter 3 0 to decrease the influence of distant points for the interpolation Click on OK to set the value 8 Set the Number of columns value to 200 Note that as you type in the value the Cellsize X value changes to 0 00005 These two values are linked together and are automatically calculated based on the extent values listed X min X max Y min Y max 165 Preparing Raste
397. trings using a Python library like os path QGis is another class that contains some useful constants such as a QGIS version or some default values We can find out the QGIS version name running on our system by typing in the following command print QGis QGIS RELEASE NAME QGis QGIS VERSION INT For example if the output is Chugiak 20400 then this value represents the version name and the version integer representation which is version 2 4 This is useful to programmatically create a plugin that can run on different QGIS versions The following snippet helps to distinguish the code among them if Qgis QGIS VERSION INT lt 20400 lt here the code compatible with older version gt else lt here the code compatible with version higher or equal to 2 4 gt 309 Creating QGIS Plugins with PyQGIS and Problem Solving Exploring a QGIS API in the Python Console The QGIS APIs can be browsed in the documentation web page but if you want to access the documentation directly in the Python Console you can use some useful Python commands The help command shows a synthesis of the API information available in the web documentation Try to edit Python Console with the command help iface The console will show all the methods of the QgisInterface class and a synthetic example of how to use this in Python syntax instead of C syntax For example if you want to show the result type of the call iface activeLayer type help iface ac
398. tu Linux 2 OGIS on Windows 2 integer rasters and floating point converting 148 Internet Relay Chat IRC channel 304 interpolation method raster surfaces converting via 164 166 Intersect tool 121 122 Inverse Distance Weighted IDW 164 inverted polygons vector style 90 91 K KML importing into SpatiaLite database 33 34 knowledge sharing 304 L LAStools URL 222 URL for installing 217 layer getFeatures method 308 layers database vectors managing 343 344 loading 338 rasters managing 339 vector files managing 342 least cost path LCP calculating 228 cumulative cost raster calculating with r cost tool 233 land use raster reclassifying 230 231 new slope raster reclassifying 230 reclassified slope combining with land use layers 232 233 slope calculating with r slope tool 229 used for calculating cost path 234 236 lines converting to polygons 110 nodes extracting from 112 Linux SAGA installing on 153 154 SAGA troubleshooting on 153 Mac OS X QGIS installing on 2 SAGA installing on 153 SAGA troubleshooting on 153 mailing lists developer list 303 URL 303 user list 303 main plugin files __init__ py file 318 icon png file 318 Makefile 318 metadata txt file 318 resource_rc py file 318 resources qrc file 318 test_plugin_dialog_base ui file 318 test_plugin_dialog py file 318 test_plugin py file 318 MakePoint function 39 mandatory plugin parameters setting Author Company and Email addre
399. tude on which a map is centered y origin False easting x_0 This is the x coordinate value for the central meridian x origin For example if the central meridian for your projected map is 96 00 and the false easting is set to 0 00 then all locations along that meridian are assigned a value of 0 00 All locations to the west of the central meridian x origin are assigned a negative value and all locations to the east of the central meridian are assigned a positive value similar to a typical Cartesian plane False northing y_0 This is the y coordinate value for the latitude of origin y origin For example if the reference latitude for a conic projection is 37 00 then all locations along that parallel are assigned a value of 0 00 All locations to the south of the reference latitude y origin are assigned a negative value and all locations to the north of the reference latitude are assigned a positive value similar to a typical Cartesian plane Standard parallel s lat_1 lat_2 This is the latitude s on which a map is centered sometimes the y origin or for conic projections the parallels along which the cone touches the earth No defaults no_defs This is a flag to designate that no default values should be utilized for parameters not specified in the projection definition Coordinate units units These are used to define distances when setting x and y coordinates 128 Chapter 4 For a full list of
400. ture ID value of 624 To find this error double click on this record in the Topology Checker error table QGIS will zoom to the location of the error shown in the following figure Here we can see two parcels overlapping The parcel on the right will be moved to the right to eliminate the overlap with the left parcel Overlapping parcel polygons 210 Chapter 6 Repairing an overlap between polygons Here are the step by step directions for repairing the overlaps found in the sample parcels shp data 1 Toggle on editing option on for the parcels layer and select the polygon to edit with the Select Feature by Rectangle tool E Each vertex will be displayed as red graphic Xs 2 The Node tool will be used to move the leftmost parcel and eliminate the overlap It allows individual vertices to be moved 1 Click on one of the parcel corners and the vertices will appear as red boxes 2 Click on the lower left vertex of the right hand side parcel to select it It will turn blue 3 Drag the selected vertex until it snaps to the boundary of the parcel it is overlapping A blue line will appear showing the location of virtual polygon boundary as you edit it as shown in the following figure Vertex of the right hand overlapping parcel being moved 211 Advanced Data Creation and Editing 3 Repeat the actions in the previous step for the vertex in the top left corner 4 Now the vertices have been snappe
401. ublished with PDF and ePub files available You can upgrade to the eBook version at www Packt Pub com and as a print book customer you are entitled to a discount on the eBook copy Get in touch with us at service packtpub com for more details At www PacktPub com you can also read a collection of free technical articles sign up for a range of free newsletters and receive exclusive discounts and offers on Packt books and eBooks pI PACKT https www2 packtpub com books subscription packtlib Do you need instant solutions to your IT questions PacktLib is Packt s online digital book library Here you can search access and read Packt s entire library of books Why subscribe e Fully searchable across every book published by Packt e Copy and paste print and bookmark content e On demand and accessible via a web browser Free access for Packt account holders If you have an account with Packt at www Packt Pub com you can use this to access PacktLib today and view nine entirely free books Simply use your login credentials for immediate access Table of Contents Preface Ix Chapter 1 A Refreshing Look at QGIS 1 QGIS download and installation 2 Installing QGIS on Windows 2 Installing QGIS on Mac OS X 2 Installing QGIS on Ubuntu Linux 2 Installing QGIS only 2 Installing QGIS and other FOSSGIS Packages 3 Tour of QGIS 4 QGIS Desktop 4 QGIS Browser 5 Loading data 6 Loading vector data 6 Loading raster data T
402. ues as QgsField in the header Each feature has its own unique ID that is useful to retrieve the feature directly For example to retrieve the feature with ID 3 we use the following code features myVector getFeatures QgsFeatureRequest 3 feature features next The preceding code could appear overcomplicated to retrieve a single feature in the next section we will show you the reason for this complexity by explaining the role of the QgsFeatureRequest class to retrieve features With the feature we can retrieve all the attributes and the geometry of the feature using the following code print feature attributes geom feature geometry print geom This will produce something similar to the following result 4 0 u Alaska 0 322511 lt qgis core QgsGeometry object at 0x99f0050 gt This shows an array of values we get from the attributes call and an instance of a QOgsGeometry result of the geometry method that is saved in the geom variable 348 Chapter 10 Exploring QgsGeometry The QgsGeomet ry class is a complex and powerful class that can be used for a lot of geometry operations and most of them are based on the capability of the underlying GEOS library GEOS means Geometry Engine Open Source It is an extensively used library to manage geometry entities You can find more information about this at http trac osgeo org geos It s difficult to describe the richness of this class an
403. ugins that control events generated by QGIS This chapter will give you an overview on the following topics e Learning Python e Loading rasters by code e Loading vectors by code from files or database e Describing vector structure and how to browse and edit features e Using Processing Toolbox algorithms by code and executing your custom algorithm e Calling external algorithms e Interacting with canvas events to draw or pick values from a raster or vector Where to learn Python basics This chapter is not intended to give you an introduction to Python programming There are a lot of free online resources and MOOC http en wikipedia org wiki Massive open online course courses on the web 337 PyQGIS Scripting The main resources can be obtained directly from the Python homepage at https www python org about gettingstarted where there is a big collection of guides and free books and tutorials Tabs or spaces make your choice During programming in Python it s important to give special attention to edit code with correct indentation Avoid mixing spaces and tabs because it can generate errors that can be difficult to understand especially for someone who is a beginner at Python programming Loading layers Loading layers in QGIS involves different steps which are as follows 1 Load the layer This step creates a variable with the layer information and related data 2 Register the layer in QGIS so that it
404. ulation as an attribute Instead this data is contained in an Excel spreadsheet It is possible to join additional tabular data to an existing attribute table There are two requirements which are as follows e The two tables need to share fields with attributes to match for joining e There needs to be a cardinality of one to one or many to one between the attribute table and the standalone table To create a join load both the GIS layer and the standalone table into QGIS Desktop QGIS will accept a variety of standalone table file formats including Excel spreadsheets dbf files and comma delimited text files You can load this tabular data using the Add Vector Layer menu by navigating to Layer Add Layer and setting the file type filter to All files as shown in the following screenshot 12 _NAAANNlMMM M _ Chapter 1 Open an OGR Supported Vector Layer ere Ji PACKT Data County Organize New folder Fl Name Date modified Type Yp cq Libraries CI D Counties dbf 10 21 2014 5 18 PM DEF File Documents mae Counties prj 10 7 2014 12 02 PM PRI File a oe A Counties qgs 10 21 2014 5 18PM QGIS Proj C Counties sbn 10 7 2014 12 02 PM SBN File Counties sbx 10 7 2014 12 02 PM SBX File C Counties shp 10 7 2014 12 02 PM_ SHP File Counties shx 10 7 2014 12 02 PM_ SHX File ZE County_attributes xls 10 21 2014 5 17 PM Microsoft Pictures F Videos ef Homegroup Computer a
405. untu To install the ubuntugis package remove the http qgis org debian lines from the etc apt sources 1list file and run the following commands sudo apt get install python software properties sudo add apt repository ppa ubuntugis ubuntugis unstable sudo apt get update sudo apt get install qgis python qgis qgis plugin grass QGIS is also available for Android We have not provided detailed installation instructions because it is in alpha testing at the moment However there are plans to have a normalized installation process in a future release You can find more information about this at http hub qgis org projects android qgis The download page is available at http qgis org downloads android A related app has recently been announced and it is named OField for OGIS For a short time it was named QGIS Mobile It is described as a field data capture and management app that is compatible with QGIS At the time of writing this it was in invite only alpha testing It is eventually expected to be available in the Android Play Store You can find more information on this app athttp www opengis ch tech blog 3 A Refreshing Look at QGIS Tour of QGIS QGIS is composed of two programs QGIS Desktop and QGIS Browser Desktop is used for managing displaying analyzing and styling data Browser is used to manage and preview data This section will give you a brief tour of the graphical user interface components of b
406. uration and value while not changing the hue click on the triangle Color swatches The color swatches panel shown in the following screenshot provides an interface to manage color palettes and select colors from the palettes Recent colors 4fab19 dedede cabcde To switch between color palettes select the desired color palette from the drop down box at the top The following three default color palettes are listed in the drop down box e Recent colors This contains the most recent colors selected in the color picker This palette cannot be modified 55 Styling Raster and Vector Data Standard colors This contains the colors that are always available as quick selections in QGIS Project colors This contains the colors that are stored within the QGIS project file The three default color palettes can be quickly accessed by clicking on the down arrow next to a color display as shown in the following screenshot E E E E E The current color can be added to a color palette by clicking on the plus sign button The color s selected in the palette can be removed by clicking on the minus sign button To apply a label to a color in a palette double click on the space to the right of the color swatch enter the label and press Enter on the keyboard The label will be displayed as a tooltip when hovering the mouse over the color in the palette quick access menu that was shown in the preceding s
407. ustomization with PyDev already installed 326 Chapter 9 Installing Aptana Aptana Studio 3 can be downloaded from the project homepage at http aptana com Just unzip the folder and execute the executable file ApbtanaStudio3 inside the unzipped folder The installation version of Aptana Studio 3 should be at least version 3 6 1 because some previous versions have bugs that don t facilitate code writing If a version greater than 3 6 is not available then it s necessary to upgrade to the beta version To upgrade follow the instructions mentioned at http preview appcelerator com aptana studio3 standalone update beta Setting up PYTHONPATH To allow QGIS to be connected with the PyDev daemon it s necessary that the PyDev daemon path be added to the PYTHONPATH environment variable To find the path to add look for the pydevd py file it will be in the AptanaStudio installation path If you find more than one version get the path that has the highest version number For example in my Linux installation there are the following paths users ginetto Aptana Studio 3 6 plugins org python pydev_3 0 0 1388187472 pysrc pydevd py users ginetto Aptana Studio 3 6 plugins org python pydev_3 8 0 201409251235 pysrc pydevd pyc In this case we should use the following path users ginetto Aptana Studio 3 6 plugins org python pydev_ 3 8 0 201409251235 pysre 327 Creating QGIS Plugins with PyQGIS and Problem Solving
408. utes select Left Zip and Right Zip from the drop down menu Since the Streets shp sample data does not have zip code attributes these options will the left blank none The Bldg Setback map units option can be used to offset geocoded address points from the street centerline This should represent how far buildings are from the middle of the street in map units In this case the map units are in feet Enter a map unit value of 20 Name the output shapefile by clicking on Browse button Geocoding operations rarely have 100 percent success A Street names in the street shapefile must match the street Q names in the CSV file exactly Any discrepancy between the name of a street in the address table and the street attribute table will lower the geocoding success rate 181 Advanced Data Creation and Editing 16 The tool will save a list of the unmatched records Complete the Not Found Output List field by clicking on Browse button name the comma delimited file Any records that are not matched will be written to this file This allows you to easily see and troubleshoot any unmapped records 17 Click on OK 18 The output will be a point shapefile and a CSV file listing the addresses that were not matched In this example the output shapefile will have 199 mapped address points There will be four unmatched records described in the Not Found CSV list The following screenshot shows the completed Geocode from Street Layer
409. uting Sciences at Texas A amp M University Corpus Christi He has a PhD in geography from the University of Georgia and holds a master of science in computer science and a bachelor of science in geographic information science degree from Texas A amp M University Corpus Christi Richard actively does research in cartography systems integration and the use of geospatial technology for disaster response Richard is an advocate of FOSS4G and building FOSS4G curriculum He is one of the coauthors of the FOSS4G Academy http foss4geo org Richard has collaborated with other writers in his field but Mastering QGIS is his first book I would like to thank my wife and daughter for putting up with my late night and weekend writing sessions I would also like to thank my coauthor Kurt Menke for being patient with my edits I would especially like to thank the editorial team you have made my first book writing experience an excellent one Outside those directly involved or affected by the writing of this book I d like to thank my academic and life mentors Dr Stacey Lyle Dr Thomas Hodler Dr Gary Jeffress and Dr Robin Murphy for providing their support and good wishes as I begin my career In addition to teaching me you have inspired me to have the confidence to teach and write To those of you reading this I hope I do my mentors justice by providing a clear and useful text to assist you in mastering QGIS www allitebooks com Dr Luigi P
410. verviews are stored in the external aux file The file will have the same base name as the original input file e Resampling method The resampling method used to downsample the input raster e Levels This provides the options for the levels for which the overviews should be built The values represent the amount of reduction in resolution for each level For instance level 8 represents a resolution of one eighth of the original raster Multiple levels can be selected e Custom levels A custom set of levels can be specified Levels must be specified as positive integers that are separated by spaces e Profile If External or Internal overview formats are selected a compression profile can be selected The profile sets parameters that are required to reach the selected compression level Once a profile is selected it appears underneath the text box The Build overviews tool is essentially a frontend for the gdaladdo GDAL command So at the bottom of the tool window the GDAL command is displayed and modified when options are selected To manually change the GDAL command press the Edit button to make the GDAL command editable The gdaladdo GDAL command builds or rebuilds a pyramids for raster data For more detailed information Q about this command and its parameters visit the documentation page at http gdal org gdaladdo html 160 Chapter 5 Converting between raster and vector data models QGIS provides t
411. very large rasters Estimate faster may be preferred to save time With the Load min max values set click on the Load button to calculate the Min and Max values With the Min and Max values set we can turn our attention to the Contrast enhancement parameter The Contrast enhancement parameter sets how to stretch the color gradient across the cell values of the gray band The following four methods are available for Contrast enhancement e No enhancement No enhancement is applied The color gradient is stretched across all values in the entire gray band While this may be desired sometimes it may tend to make the raster look overly gray 68 Chapter 3 e Stretch to MinMax This method stretches the color gradient across the eray band between the Min and Max values It generally produces a higher contrast a darker rendering than No enhancement All cell values below the Min value are assigned the lowest gradient color and all cell values above the Max value are assigned the highest gradient color Stretch and clip to MinMax This method stretches the color gradient across the gray band between the Min and Max values It produces the same rendering as the Stretch to MinMax method except that all cell values below the Min value and all values above the Max value are assigned no color and they are transparent e Clip to MinMax This method stretches the color gradient across all values in the gray band which is the same result a
412. widely used transformation types They are also commonly referred to as first affine second and third order transformations The higher the transformation order the more complex the distortion that can be corrected and the more computer power it requires Polynomial 1 requires a minimum of three GCPs It is suitable for situations where the input raster needs to be stretched scaled and rotated Polynomial 2 or Polynomial 3 should be used if the input raster needs to be bent or curved Polynomial 2 requires six GCPs and Polynomial 3 requires 10 GCPs Thin Plate Spline This transforms the raster in a way that allows for local deformations in the data This may give similar results as a higher order polynomial transformation and is also suitable for scanned imagery It requires only one GCP Projective This is useful for oblique imagery and some scanned maps A minimum of four GCPs should be used This is often a good choice when Georeferencing satellite imagery such as Landsat and DigitalGlobe There is no one best Transformation type You may need Q to try several and then determine which generated the most accurate transformation for your particular dataset The following screenshot shows setting the Transformation type setting within the Transformation settings window ie Transformation settings Transformation type Polynomial 2 Linear Resampling method Helmert Polynomial Compression Polynomial 2 Polynomial 3 Thin Pla
413. xisting CRS and modify it Click on the Copy existing CRS button which will open the Coordinate Reference System Selector window Enter 102003 in the Filter text box to find the USA Contiguous Albers _ Equal _Area_Conic projection Select the found projection and then click on OK This will copy the Proj 4 string back to the Parameters text box in the Custom Coordinate Reference System Definition window In the Parameters text box modify the Proj 4 string by changing it to proj aea lat 1 42 5 lat_ 2 45 lat_0 43 75 lon 0 71 x _0 0 y_0 0 datum NAD83 units m no_defs The modified string should look like the one shown in the following figure HP Add new CRS Remove Name New England Albers Equal Area Conic Parameters proj aea Hat_1 42 5 Hat_2 45 Hat_0 43 75 Hon_O 71 x_0 0 y_0 0 datum NAD83 units m no_defs Copy existing CRS Click on OK to close the window and store your new custom CRS With the creation of the custom CRS we can apply it as our project CRS to perform an on the fly CRS transformation by navigating to Project Project Properties CRS The new custom CRS can be found at the bottom of the CRS list under User Defined Coordinate Systems as shown in the following screenshot 130 Chapter 4 New England Albers Equal Area Conic The following figure shows the New England states with the custom CRS that we created defined Quite a difference A 2 Advanced field calculations
414. xt file containing all the information about the plugin This file is read by Plugin Manger to manage the plugin inside QGIS For example in this file there are the plugin classification tags or the minimum QGIS version in which the plugin can be run Makefile is a set of instructions used by the make command to compile resources and to manage some shortcuts to compile documentation or to clean previously compiled files and so on Usually it s not necessary to edit it The icon png file is the plugin icon As explained previously it would be modified with a definitive plugin icon The resources qrc file is the file that instructs Qt about how to manage the icon Usually it s not necessary to edit it other than adding more icons or changing the filename of the icon png file More information about the resource file can be found in the Ot documentation at http qt project org doc qt 4 8 resources html The resource_rc py file is the compiled version of the resource qrc file and it s generated after compilation with the make command The test_plugin dialog_base ui file is a file in XML format describing the layout of the user interface of the plugin It s strictly necessary only if the plugin needs its own GUI The GUI structure can be edited manually but usually it s better to use the Qt framework to edit it The framework is called Qt Designer or Qt Creator and it can be downloaded from http qt project org downloads The GUI d
415. y removing and adding features inside an editing session Running processing toolbox algorithms QGIS s versatility is due mainly to two reasons The first is the ability to customize it by adding functions thanks to its plugin structure The second is the power of the processing toolbox that can connect different backend algorithms such as GRASS GIS SAGA GDAL OGR Orfeo Toolbox OSM Overpass and many more with dedicated providers In this way for example we can access all GRASS processing algorithms by using QGIS as the project and presentation manager Another important ability of the processing toolbox is that it can be used to join together all the backend algorithms allowing you to connect the best algorithms For example we can connect GRASS as a producer for another algorithm that is better developed in another backend such as SAGA Here QGIS processing becomes the place where you can add your specific algorithm in a more complex and integrated workflow 359 PyQGIS Scripting This section is focused on how to code the execution of algorithms that are already available in the processing toolbox The main points to learn are as follows e Looking for a processing toolbox algorithm e Discovering parameters accepted by the algorithm e Running the algorithm In the following sections we will be using the processing commands after we have imported the processing module with the following code import processing If the pr
416. yTable the geom sql myWhere After the URI string is created we can use it to create a new vector layer with the following code myVector QgsVectorLayer uri uri myVector postgres The third string parameter specifies the data provider which in the case of vector data that is hosted on a SpatiaLite database would have the value as spatialite As usual to visualize the vector we have to use the following code if myVector isValid QgsMapLayerRegistry instance addMapLayer myVector 344 Chapter 10 The preceding code visualizes the vector only if it has been correctly loaded Failures can happen for reasons such as errors in parameter settings a restriction of the vector provider or a limitation by the database server For example a PostgreSQL database can be configured that would allow access to a vector table only for a specific group of users Vector structure The QgsVectorLayer class is more complex that its raster equivalent To describe it we will first approach basic layer parameters and then we will explore how the vector is organized We will explore some classes that are involved in the vector structure that represent rows and headers The basic vector methods We will explore the vector class working on a real vector we will load alaska shp in the myVector variable This variable is an instance of the QgsVectorLayer class This means that all methods of the vector are documented at http
417. yle Manager s list of color ramps Brewer For more information and an interactive color selector gt The ColorBrewer color ramps are based on the work of Cynthia visit the ColorBrewer website at http colorbrewer2 org 63 Styling Raster and Vector Data Adding a cpt city color ramp If you do not want to add a color ramp from scratch a large collection of existing color ramps from the cpt city archive of color gradients is available for use in QGIS To add a cpt city color ramp click on the Add item button and then choose ept city This opens the cpt city color ramp window shown in the following screenshot 4 cpt city color ramp All by author GRASS color ramps 33 aspect aspectcolr 94 Diverging Greens Greys QGIS amp cb celsius corine grass Precipitation l Reds 7 E i E Temperature curvature differences elevation etopo2 Topography Topography bathymetry Top of the cpt palettes evi gdd Top of the svg palettes Selection and preview Information Palette elevation Path grass elevation License GPL 2009 http www gnu org copyleft gpl html Save as standard gradient Color ramps can be selected by theme or by author by choosing the appropriate tab at the top of the window In either case the color ramps are presented in an expandable tree on the left with a listing of color ramps in each tree element on the right When a color

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