Chapter 16
Contents
1. include lt stdio h gt main int an_integer 5 float a_float printf Integer value is d n an_integer a_float an_integer 2 printf Halved without type conversion the value o is f n a_float a_float float an_integer float 2 printf Halved with correct type conversion the value is f n a_float One of the best ways to learn about computer cartographic programming is to study the programs written by other cartographers The list of available sources is short but it is growing steadily Cartographers have been slow to publish their computer programs in a form suitable for use by others As a result many cartographers have duplicated the work of others without even having the ability to compare the two results The programs presented in this book are not complete programs in themselves but are building blocks around which more useful computer programs may be constructed In most cases writing a computer program using these functions will involve reformatting some existing digital cartographic data entering the data into a programming data struc ture appropriate to the task performing any necessary cartographic transformations and producing a map The cartographic transformations may involve implementing an algo rithm perhaps one published with a descriptive paper or journal article If the programming is done effectively each of these tasks can be performed by one or more progra2. and sometimes with the specific device driver installed Examples from Sun Soft s SunGKS are ws_type values of postscript hpgl and xgl_tool fora PostScript HPGL and a generic X windows window respectively Once opened the workstation should be activated or made ready for inter action using the function activate_workstation This function simply uses the workstation identifier given to open_workstation The digital canvas is now ready The programmer can establish the normalization transformation s read any data required and set any attributes and is then ready to draw If no segments need to be defined the primitives are simply included in the order used To draw and save primitives in a segment the function callopen_segment is used to create a new segment which includes all primitives and their attributes until the close_segment function is used Once defined segments can be redrawn re named copied between workstations or written into the metafile To redraw the entire map the function call redraw_all_segments could be used The way back out of the GKS system is the reverse of the way in After all segments have been closed the workstation can be cleared using clear_workstation de activated using the GKS function call deactivate_workstation and closed us ing close_workstation and finally GKS can be shut down using close_gks The GKS system also provides an immediate error exit from GKS so th
3. correct arguments Producing a skeleton pro gram in this way makes working on a large program much easier and also allows several people to work on a program at the same time The user interface is the last challenge facing the analytical and computer cartogra pher As we have seen the cartographer of today must have an understanding of the hard ware and software tools of computer cartography He or she must understand cartographic data cartographic data structures and the programming mechanisms by which they can be manipulated The analytical cartographer must understand cartograph ic transformations and be able to use the power of computer programming and the pro gramming environment to achieve a higher understanding of maps and mapping It is the user interface however that in the future will determine how computer map ping systems relate to the vast number of users who make up the rest of the world If car tography is to move away from being a discipline in which a practitioner disappeared for Sec 16 5 References 317 a time and then delivered a map if cartography is to become democratic enough to place the production of maps into the hands of the noncartographer then we must begin by writing mapping software that is usable This means effective user interfaces and mean ingful assistance with decision making and it means that much well written software re mains to be produced The opportunities presented by the wealth of d
4. 16 Writing Cartographic Software 16 1 THE PROGRAMMING ENVIRONMENT Writing a computer cartographic program rather than simply using an existing program establishes the degree of control over producing maps necessary for analytical cartogra phy All too often however the decision to write a program is taken too lightly The re sult is often confusion discouragement and worst of all a waste of time and resources With a little thought and planning however computer programs that produce maps and map based analyses can be made to work and to work well without the hours of grueling work now known as hacking This development has come about because of some significant improvements in the methods of computer programming Although programming has been around for many years only recently has software engineering the application of engineering and com puter principles to the act of programming turned the art of computer programming into more of a science Software engineering considers every aspect of a piece of soft ware from its purpose to its maintenance to the personnel who will produce it in addi tion to the language coding and debugging of the program The first stage of writing a computer cartographic program is the design During this stage we should ask what the software will accomplish what will be expected of the user how the software will be documented maintained and used and what its expected life ti
5. KS lists standard bindings for GKS for FORTRAN Ada and Pascal Because the C programming language has only recently been revised into an ANSI version there is as yet no standard C language bind ing to GKS This does not mean however that none exists merely that there are some differences in bindings between the products of different software vendors Fortunately the GKS standard is quite general when it comes to the major source of differences that is the naming of functions Function names are listed by English lan guage names rather than GKS function names Figuring out a function name for a differ ent binding or even transporting a piece of mapping software between computers or programming languages often consists therefore of translating the names of functions and assuring that arguments in the lists passed to the functions are consistent A conven tion used by some vendors for C functions is to use the full rather than abbreviated func tion names replacing blanks with underscores _ On some computers especially microcomputers there is a need to abbreviate due to DOS name limitations or limitations on the linking capabilities of C compilers In this discussion the simplest form of the name will be given A C language binding can be thought of as a set of predefined C libraries that contain all of the functions necessary to use the standard The functions break down into those that manage primitives and attributes those that ha
6. ORMATION OPTIONS next menu write array to a file filter array data isopach invert array resample array subsample array slope aspect analytical hill shading linear trend surface Convert feet to meters Unravel an array Double an array Quit aacadtdraunkK SerME B Select option number t Figure 16 3 A menu driven software package Terrapin See the companion disk Sec 16 4 Cartographic Software and the User Interface 315 Edit Data Classify Map Legend Tent BERIT Style DefineNeatline EraseNeatline DbiNeatline Erap pime fikirga Figure 16 4 A window and menu based software package MacChoro program the so called pop up menu The windows environment has been popularized by microcomputers especially Apple s Macintosh and Microsoft s Windows Probably the highest level of sophistication is achieved when mapping software uses all the capabilities of the Graphical User Interface running on top of a computer s oper ating system In this case the program can inherit the properties of the GUI such as window colors button and menu formats This allows all of the capabilities of the GUI to be used including windows icons menus buttons dials and pointers Examples of this approach are Sun s SUNOS Solaris X windows Motif Microsoft s Windows NT and IBM s OS 2 Because the window functions are callable from computer programs the programmer has the ability to incorporate programs that use a
7. anuals and running through learning exercises With the editor the programmer can enter into a file the C language computer instruc tions that produce a map known as the source code A beautifier is a utility program that reads the source code for a computer program and rewrites it with correct indentation and consistent spacing and with uniform typing specifications If you have never used a beautifier it is difficult to appreciate that correct order within a program can be a very effective debugging aid and can even help check for logical consistency It is not necessary to make programs terse and cramped Any savings generated by such code can be eliminated by the encoding of a single error that becomes hidden by the sloppy program layout Many beautifiers are also syntax checkers A syntax checker is a utility program that checks source code files line by line and detects errors in language syntax that is inconsistencies in the rules and requirements of the programming language A good syntax checker will find errors point the programmer to the exact place where they were found and give at least a hint as to which rule has been violated For example a common C language syntax error is to forget to terminate a state ment with a semicolon A suggestion by the checker such as Syntax error in line 306 Writing Cartographic Software Chap 16 23 is vastly inferior to Missing terminating for line 23 It should be noted that using the
8. at the programmer can escape from errors elegantly without exiting the graphics levels using emergency_close_gks The sequence is illustrated in Function 16 1 Few differences exist between bindings with one exception Many of the C language bindings use single variable parameters as function arguments This method follows closely the FORTRAN standard A line for example could be sent to a line drawing polyline function using polyline number_of_points amp x 0 amp y 0 This provides the function with the number of points in the vectors x and y and the start ing addresses of the points Alternatively at least one major GKS binding uses a structure of type POINT and instead passes the address of the entire structure to the GKS function Each GKS C binding implementation has its own details Usually there is a large header file often called gks h which contains all the values of the function arguments in a definition header file so that for example color number 6 on a specific device is predefined as define RI eal J oy 310 Writing Cartographic Software Chap 16 Function 16 1 Program gksbasic Demonstrate GKS open close kee 12 93 Function 16 1 xy include lt gks ansicgks h gt include cart_obj h main Gchar conn NULL Gchar wstype postscript Gws ws 1 Open up GKS if gopengks stdout GMEMORY exit Open the workstation
9. compiler to find syntax errors is a waste of time and can lead to a whole new kind of compound logic syntax error The next stage is to compile the syntactically correct program With prior syntax error checking the compiler is often satisfied fairly easily This is absolutely no guarantee however that the program is correct or even workable Using C the final pass of the com piler called linking has great importance because C shares program modules between programs Linking to system libraries such as the math and windows libraries is done at this stage as also is linking to the libraries that constitute GKS and the GKS C language binding Linker error messages are usually to communicate the fact that a named or required library was not found or that its name was misspelled After a successful compilation and linking C usually produces two new versions of the program The first from the compil er is the object module that is the compiled machine language version of the program or module After successful linking another version the executable program is pro duced Under MS DOS typically a program called prog c_ will produce an object module prog obj and an executable program prog exe To run the program the lat ter is used Often after all these stages have been completed the program either produces incor rect results or crashes Programs that crash can be started under the control of adebug ger Debuggers allow the pro
10. computer cartography most software that produced maps ran in batch mode The means of interaction with the program was by supplying a set of param eters in a file or even on punched cards that were read and acted upon by the software Information could include the names of files the number of data points in a file the pre cise format of an input record or the numbers of options listed in the documentation Al though batch processing was capable of generating maps errors were common since the map output came at the end of a long sequence of operations which happened quite slow ly often overnight Even generating a map could take several days but debugging the software for mapping could take weeks or months As interactive computer systems became commonplace many computer mapping systems moved to interactive processing The earliest user interface in such an environ ment was the command driven program An example is MicroCAM Figure 16 02 A command driven program reads one command from the user at a time and acts upon it either immediately or when a general action command is given Commands usually in volve a keyword such as PLOT or READ and are followed by a list of values that supply parameters to the program For example to read a grid data file the command READ Input_file 200 300 I could be used to read a grid of integers from file Input_file with 200 rows and 300 columns Note that these commands can be placed togethe
11. consistent interface that is easy for all users because it embodies concepts which are common between computers Further more many vendors of these operating system GUIs support GKS so that GKS calls to locator devices make calls to the mouse and the workstation becomes a window such as an Xterm An example is xv an X windows based program to display and manipulate images Figure 16 5 At a higher level of sophistication several graphics languages and 316 Writing Cartographic Software Chap 16 hf Jimi ete Figure 16 5 A full GUI software package xv XV is copyright 1993 by John Bradley Used with permission toolboxes now support GUl independent programming so that a generic program is compiled with a binder to a particular GUI such as MacIntosh Windows or Motif For computer cartographic programs it is important that the means of interaction with the user be kept in mind at all times Modular and flexible programs make it easy to use one command system during software development and another when the program is fin ished A better approach however is to write the user interface first The entire user in teraction system can then be debugged tested and refined to make the program more friendly before the program even works For this purpose many large programs are first written with function stubs one or two line functions that simply report that they were called in the right place and with the
12. e in teraction with the user is necessary must be determined and the prompts the user is to receive should be planned Decisions are made about the valid responses to prompts and system defaults that will be taken without the intervention of the user are planned Final ly the programming language is chosen and planning begins for the use of the program ming environment The programming environment is the entire set of tools available to the programmer to produce working and correct programs An ideal environment should include an editor a debugger a syntax checker a beautifier a compiler and a linker The editor is the pri mary tool by which the programmer enters control statements data and documentation into the computer Most people have their favorite editor and a decision to stay with one s favorite is sound because many editors have now been converted for use in a large number of programming environments As a minimum the editor should be highly inter active and should allow the partial and complete reading and writing of files file concat enation the joining files together searching for text strings simple movement through the file global searching and substitution making changes to the whole file The editor also should be easy to learn and remember Although allegiance to one editor is one way of remembering commands the additional features offered by an alternative editor may well be worth a few hours spent reading m
13. gopenws ws conn wstype Activate the workstation gactivatews ws Generate a map if Deactivate the workstation gdeactivatews ws Close the workstation gclosews ws Close down GKS gclosegks Also different implementations split the libraries up differently Sometimes the language binding is separate from the main GKS library This must be taken into account when compiling and linking C programs New GKS implementations language bindings and device drivers become available frequently Trade journals the so called glossies and graphics conventions are good Sec 16 3 Writing Your Own Mapping Program 311 sources of information about these changes In addition several GKS implementations are now appearing as shareware Although these versions are unlikely to support the more exotic computers and graphics devices the more common ones may be found In addition several companies now make add on software packages built over the GKS In a few cases these systems support advanced mapping functions Other GKS im plementations are very low level Ob for example do not support many interactive func tions and allow output only to the metafile These versions come with metafile translators which convert the metafiles into the device specific graphics calls of a great number of different graphics devices Among the most popular are translations to Post Script for output on la
14. gram to be run stopped and the contents of variables listed to determine which variables had what values when or to see which modules were the source of the error The sequence editor to beautifier to syntax checker to compiler to linker to debugger may be repeated many times before a program even runs let alone works and gives correct results Figure 16 1 Graphics introduces yet another level of debugging Graphic debugging is assisted by the GKS C language binding and by the error control built into the GKS standards which can echo function names and the values passed to them on an ongoing basis The actual graphics created are also useful debugging tools Blank screens often mean an error in the transformational geometry involved while errors with primitives and attributes can gen erate graphics better suited to the walls of the Museum of Modern Art than to cartogra phy Once a program has been written and debugged it is time to return to the documen tation and program purpose to check and verify the program First the program should be tested with a small test data set for which the results are known Few such data sets exist in cartography so checking with hand calculators is often in order Attention should be given to special case exceptions to computations such as taking the square root of neg ative values or testing for the intersection of vertical parallel lines Once small data sets check out a real world cartographic data set
15. igital cartographic data now becoming available are extraordinary How effective this revolution in cartography will be in solv ing the problems of our world however depends on a new generation of analytical and computer cartographers 16 5 REFERENCES Enderle G K Kansey and G Pfaff 1984 Computer Graphics Programming GKS the Graphics Standard Symbolic Computation New York Springer Verlag Foley J D and A VanDam 1982 Fundamentals of Interactive Computer Graphics Reading MA Addison Wesley Hearn D and M P Baker 1986 Computer Graphics Englewood Cliffs NJ Prentice Hall Johnson N 1987 Advanced Graphics in C Programming and Techniques Berkeley CA Osborne McGraw Hill Myler H R and A R Weeks 1993 Computer Imaging Recipes in C Englewood Cliffs NJ Prentice Hall Kernighan B W and D M Richie 1988 The C Programming Language 2d ed Pren tice Hall Software Series Englewood Cliffs NJ Prentice Hall Lamb D A 1988 Software Engineering Planning for Change Englewood Cliffs NJ Prentice Hall Press W H B P Flannery S A Teukolsky and W T Vetterling 1988 Numerical Recipes in C The Art of Scientific Computing New York Cambridge Univer sity Press
16. m modules Remember that these modules should become your own per sonal library of cartographic functions and that for best effect they should be shared freely with your friends and any other interested persons The best way to find weaknesses and errors and to optimize the code is to release pro grams to other users Users will expect information such as a manual and installation de tails Also remember however that as a user of computer cartographic software you are under an assumed obligation to report problems errors and weaknesses to the author of any piece of software for without this feedback no improvement will take place It is also entirely appropriate to report more favorable items of news Sec 16 4 Cartographic Software and the User Interface 313 16 4 CARTOGRAPHIC SOFTWARE AND THE USER INTERFACE The success or failure of a piece of cartographic software is determined not by innova tion sophistication or accuracy but instead by usability Many excellently written and designed computer programs have been passed over entirely because they lack the ele ments that make the capabilities of the program accessible to the user The usability of most software and this includes computer cartographic software is therefore strongly de termined by the user interface of the program The user interface is the entire set of means by which the user communicates with software to achieve a specific mapping purpose In the early days of
17. me is Professional software engineers accomplish this step with group meetings and consider a large number of possible alternatives before making selections Once the software s purpose documentation and user interactions are determined the next round of decisions should involve a schedule the choice of a language a strategy for debugging a plan for checking and verification and a plan for the software s use and update The schedule is important Programming can lead to large amounts of time being 304 Sec 16 1 The Programming Environment 305 wasted on incorrect solutions and often the timetable is the only force directing the pro grammer to give up and try another way The first draft of a user manual and reference guide or the writing of on line help facilities is a good way to help in planning organiz ing and making design decisions relating to the software Typically during the early phase of the design the software is seen as being able to accomplish too much so a real istic pruning is often necessary In the next phase of design the software itself is started but no programs are written The components are broken down into modules and decisions are made about how inter action between modules is accomplished Modules should be chosen so that they are log ical divisions of the program which can be programmed tested and maintained in isolation from the program as a whole At this stage the places in the program wher
18. ndle interaction with the user those that control the workstation those that perform transformations those that manage seg ments and the metafile those that manage errors and those that allow the programmer to query GKS to establish any of the above In all this makes several hundred functions because for every primitive attribute and interaction function there is a corresponding query function For example the func tion set_fill_area_color_index has a parallel GKS function inquire_fi area_color_index There is a formal structure for the use of GKS functions The sequence applies for all GKS levels and implementations The first GKS operation that a program must perform is to open GKS This performs an initializa Sec 16 2 The GKS C Language Binding 309 tion of the software on this system for this application The function used is open_gks This function sometimes uses as a parameter the name of a file that GKS will use to write GKS error messages The file should be opened in write mode by the user s program and the value passed should be a pointer to type FILE The next stage is to open the workstation This is accomplished using the function open_workstation Arguments should include an integer identifier for the work station to be used in successive workstation calls and a unique number or text string that identifies the type of workstation to GKS This number is provided with the GKS lan guage binding
19. r into a file and submitted to gether in batch mode implying that command mode contains a batch mode The next level of sophistication in terms of the user interface is to provide a menu in which commands can be selected by number or letter Figure 16 3 shows a typical menu for a computer mapping program Using a menu selections can be chosen by entering a number instead of the name of the option in the same way that you can order from a Chi nese menu by asking simply for number 6 instead of La Zi Ji Ding Many menu systems also allow the name of the command to be typed thus supporting command lines also Increasingly operating systems are taking advantage of the ability of computer pro grams to use windows Windows are areas of the screen where interaction can take place or where graphics can be generated A windowing system allows the windows to be moved resized or selected to be visible often with the use of a mouse Figure 16 4 Selections can be made by moving the mouse to a header and pulling down the menu or the next menu level and by holding the mouse button down and moving the mouse Menus can also appear on the screen at the appropriate time during the execution of the 314 Writing Cartographic Software Chap 16 Aicrotanet 3 4 Ite lnter College nL ARES 5 Sal Tht E Howth at Ciwina ert Faint Faipaste Fidsexit Figure 16 2 A command line driven software package MicroCam Terrain_Mapping TRANSF
20. ser jet printers or to protocols of desktop publishing packages al lowing the output from computer programs to be pasted directly into a paper article set of documentation or even a textbook Other packages for example CorelDraw and Is landDraw allow metafiles in CGM format to be opened for use and editing 16 3 WRITING YOUR OWN MAPPING PROGRAM Assuming that you have read much of this book have read about the C programming lan guage understand cartographic data structures and have gone through the software de sign process described above how do you actually go about writing a computer program to generate a map During the design process you have asked yourself what kind of map you wish to produce You have blocked out a set of modules which can do any necessary input structure the data perform any necessary transformations and then generate the map The next and most important step is to write the program First you should carefully research the various C compilers available to you The newsstand microcomputer journals contain large amounts of information to assist in de ciding which compiler to buy If you have access to a larger computer see if the C lan guage is available and how it is supported If the computer runs the UNIX operating system then many of the tools available as part of the programming environment dis cussed above are available UNIX can also run on many microcomputers and is the most common workstation opera
21. should be used Special cases should be deliberately introduced into the data to check circumstances that may not have been foreseen during design Real data has a habit of introducing these spe cial cases as well as other limitations such as size restrictions into the program and test ing the program s design limitations Failure of the program at any of these stages may demand going back to a module to Sec 16 1 The Programming Environment 307 make corrections It may even be necessary to redesign a whole module which is possible kn a a gt fee sera 4 SD comme A DO O o A gt E peua Figure 16 1 The programming sequence only with structured programs If at this stage it is found that the entire program struc ture is poor this is a good time to give up as no number of program patches can repair a poor overall structure or logic As a final stage many programs are optimized This involves analyzing often using other computer programs each module to see how much time is spent in each function and at each stage in each function The most time consuming tasks are reworked first each optimization making the program run faster or use less memory Some C compilers and operating environments such as UNIX have built in optimizers and program run time analyzers the O compiler option and the profile program In other cases op timization is a painstaking process with each improvement taking seconds off the r
22. ting environment Do not start writing programs until you are familiar with the language When you do make lots of deliberate mistakes and see how the various parts of the programming envi ronment react to them If C is your first language then as mentioned in Chapter 15 you should seek out instruction from a computer scientist If you are coming to C from Pascal or a similar language you should have very few problems but you should find an appro priate C language tutorial and work carefully through it Pascal programmers should not be tempted to rewrite C to look like Pascal although it is possible Although many of the concepts from languages like FORTRAN will help in the conversion it is better to learn the structured approach from scratch A feature that eventually becomes the source of many errors in C programs is the dis tinction between the types of variables The C language supports integers unsigned short regular and long characters floats and doubles and although variables must be assigned types exclusively type conversions are not automatic For example to make a floating point real equivalent of the integer 5 it is not enough simply to assign the value The type should be explicit in the statement also This is illustrated in the Function 16 2 312 Writing Cartographic Software Chap 16 Function 16 2 Remember to explicitly perform type conversions in C otherwise beware
23. un times for the program Having given a description of the programming environment it goes without saying that it is critical to cartographic software Much time will be wasted if the programmer 308 Writing Cartographic Software Chap 16 avoids using the tools available tools that are now parts of the typical C programming environment A little time invested in learning the sequence used in Figure 16 1 can save hours of wasted time and can result in much more effective and better constructed com puter cartographic software The programmer should think first and foremost about the map being produced then about the specifics of the programming language in use Above all the design should be organized and planned before a single line of computer code is written Writing computer programs and designing them are two entirely different tasks and the worst place to think about design is inside three levels of open control statements 16 2 THE GKS C LANGUAGE BINDING In some earlier chapters particularly in the discussion of the symbolization transforma tion the GKS standard was presented as a means by which cartographers can write com puter programs that produce maps We saw that the standard can be implemented at different levels with different device support and functionality for different levels We also saw that the link between programming languages and graphics under the GKS stan dard is the language binding The ISO specification of G
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