XYRectify tip sheet
Contents
1. N Parameters specifically K1 For most rectified measurements K1 is important but K2 and K3 as well as P1 and P2 are not absolutely necessary If the camera has been calibrated then it is best to click the Apply corrections box The pixel size mm should be correct relative to the c mm focal length as noted in Figure 2 NOTE it is always better to work with a camera that s been calibrated using iWitness to optimize accuracy for use in XYRectify Camera parameters Information Model Canon EGS DIGITAL REBEL T Sensor specifics Wide High Resolution pixels 2504 3456 Pixel size mm 0 0064 0 0064 Focal length and principal point c imm 26 0000 lt p ror ooon ye mm 0 0000 Distortions x1 Jo o000e 000 p1 o c000e 000 k2 0 0000e 000 Pz o 0000e 000 K3 0 0000e 000 Apply corrections Figure 3 4 At this stage the camera parameters are set up and the image will be imported Double click the image thumbnail to display it 5 Control Points XYRectify requires a minimum of 4 control points on a fairly planar surface and spread out in this example a roadway intersection The control points are XY coordinates that need to be accurate In this instance a total station was used to measure a quantity six XY coordinates but a total station is not the only means of obtaining these XY coordinates they could come from many different sources Create a Control Point file using MS N2. XYRectify tip sheet This tip sheet was prepared for users with a working knowledge of the iWitness close range photogrammetry software system The purpose of this tip sheet is to provide the basic understanding of using XYRectify and to present an accurate means of scaling the XYRectify rectified JPEG image using a CAD diagramming software For this procedure it is assumed the user has basic familiarity with the XYRectify software If not please refer to the XYRectify User Manual We will also use the CAD Zone Quick Scene diagramming software in combination with an originally shot oblique image that was later rectified using the XYRectify software Background Rectification is the image processing technique whereby an image taken obliquely to a plane is converted to an equivalent normal image of that Same plane An easy way to picture the rectification process is to consider the conversion of an oblique aerial image of the ground to a corresponding vertical image This process which requires the establishment of a minimum of four control points of Known XY coordinates in the object plane along with the marking of the four corresponding image points is carried out using XYRectify Following rectification XY coordinates can be measured within XYRectify or with the scale homogeneous rectified image and a CAD diagramming program 1 If it is at all possible it is best to pre calibrate the camera in Witness and use the focal lengt
3. cene will prompt you to select i e click on the image followed by carefully clicking on the bottom left and right corners of the image see figure 16 Note this is the tmage width as described earlier Eo imap Scale select imap Emily M LACA Figure 15 Figure 16 16 In Figure 16 notice how after clicking on the corner of the image see left arrow Quick Scene places a Miet color marker for the first Scale point The cursor on the right is illustrated as pre selecting the second scale distance Notice on the right how the crosshair is perfectly indexed to the corner of the image Click it to add the second Figure 17 Bitmap Scale Distance Between Points 257 B44 Enter Actual Distance 293 061 Adjust Bitmap Mow Close In Figure 17 type in the proper image width distance in our case it is 393 06 followed by clicking Adjust Bitmap Now The XYRectify planar rectified image is now accurately scaled and ready to be traced and or any of the planar surface entities can now be measured in the image within Quick Scene 2008 All Rights Reserved DeChant Consulting Services DCS Inc www iwitnessphoto com
4. h amp other camera parameters as the calibration settings for XYRectify If the camera is NOT in the XYRectify Global Camera Database see Figure 1 upon importing the image Figure 2 is presented X Rectify This camera is not in the database Camera details must be completed manually Camera parameters Image dimensions are 2504 pixels wide amp 3456 pixels high The following three fields must have valid entry data Enter camera make and model Camera name For default Enter Focal length in mm 26 000 Enter the following data if known otherwise leave as itis z JS Size of image pixel ron 2005 Camera Image Size of imaging sensor mm width 11 52 height 17 26 Principal point coordinates mm xp 0 0 YF 0 0 Radial lens distortion coefficients K1 0 0 Ke 0 0 KS 0 0 Decentring distortion coefficients Pl 0 000e 000 P2 0 000e 000 Figure 2 2 In this instance the focal length of 26mm is simply read in XYRectify from an image s EXIF header tag If the Pixel size is not correct the user should go to www dpreview com and select the Canon EOS 350D camera http www dpreview com reviews CanonEOS350D page2 asp and determine the pixel size In this case being Sensor width 22 2mm Max Image width 3456 pixels 0064mm In Figure 2 type 0064 in the Size of image pixel field 3 In Figure 3 note that the camera was not calibrated so there s no correction for the DISTORTIO
5. ion Transformation performed WES RMS 0 472 Number of measured CPs 6 minimum oF 4 Rectified Image reate Image W Full image Bounded by control points 10 Figure 9 The Save As dialog will now be presented Give the resulting Projective Transformation Image a new name we ll call it crash Save it to your working folder The newly created rectified image we named crash jpg is now planar rectified and an image thumbnail of it is presented above the Control thumbnail in Figure 10 Note the rectified image crash jpg is also saved to your working folder Figure 10 2 crash jpg Control Points 9 Output File from XYRectify The saved JPEG is accompanied by a tfw or World file which defines the XY reference coordinate system for the rectified image The TFW ASCII text file is named the same name as your image in this example case crash twf located in your working folder The image can now be imported into your favorite CAD package The TFW File created during the XYRectify Process After marking the control points in XYRectify a TFW world file is automatically created and saved in the working folder There are 6 lines of numbers that represent the TFW file The TFW six line numbers look similar to this 0 3190419410 0 0000000000 0 0000000000 0 3190419410 115 3208527345 204 8477400810 Explanation of the TFW line numbers LINE 1 The first line va
6. k in the Label field with the red plus and it becomes a yellow plus Figure 6 Mark the corresponding location of the control point in the image and it becomes a green plus See Figure 7 r j Label x Y vx w A E re 22 900 122 550 E center_se 26 800 19 980 E rrer e 31 250 79 260 EAR 4 090 30 030 Eea 57 660 32 220 z v m Projective Transformation Transformation performed NO RMS Number of measured CPs aa minimum of 4 Rectified Image Create Ima e V Full image Bounded by control points 10 Close Do this same action for the remaining minimum of 4 control points Note in Figure 7 that the Rectified Image in the lower left of the dialog box displays two options Full Image and Bounded by control points The default is Full Image The operator can choose to rectify either the entire original image full image or a portion of the original image bounded by control points equal to the area encompassed by the control points plus a border area of approximately 10 In this example we are going to leave the tick mark on the default Full Image Figure 8 illustrates all 6 Control Points Marked from the Control Points Dialog Box 8 Click the Create Image in Figure 9 Control Points E centeR_bE 26 800 EJ verer_pe 31 250 EJ LEFT cL 4 090 E kiste 57 660 E Last PT 266 600 Projective Transformat
7. lue represents the scale of any pixel in X LINE 2 The second value represents the rotation in X LINE 3 The third value represents the rotation in Y LINE 4 The fourth value represents the scale of any pixel in Y negative because Y is flipped LINE 5 The fifth value represents the X value of the top left pixel LINE 6 The sixth value represents the Y value of the top left pixel Using this tip sheet all that is necessary for the reader to understand is that the first line of the TFW file represents the size of the pixel in the Measured Units of the 4 Control Points measured within XYRectify Therefore if one knows the size of the image format one can simply calculate the UNIT size of the image format in width X or height Y To acquire the XYRectify JPEG image format in pixels simply right clicking on the JPEG image Summary tab See Figure 11 below Figure 11 Properties General Summary Property Value Image 4 width 1232 pixels lt _ Height 1798 pixels Horizontal Resolutio an vertical Resolution 1 dpi Bit Depth 24 Ly Frame Count 1 Description 4 Title 4 Subject 4 Keywords Ly Comments Origin a Author lt lt Simple In our example the pixel size reported in the TFW file is 0 3190419410 The numbers of pixels in the image width is 1232 Figure 11 Therefore 3190419410 x 1232 pixels is 393 06 If the Units of Measuremen
8. otepad Type in the name of the control point and then its X and Y coordinates separated by TABS See Figure 4 Save the file in your working folder control Notepad p m In our example Lwr_BE means Lower Bulls Eye as defined by the white X and the white circle in the rectified image File Edit Format View Help Left_CL is the end of the double centerline on the left side note the image is actually illustrated upside down so that is why it appears on the right side in the image marking process noted in Figure 4 Figure 7 6 Click on the File Import Control and open the newly created control points file XYRectify Untitled wie Window Help Hew Ctrl M Open Chrl 0 Close Save Chrl 5 Save As Import Images o 1 sample 2 sampled 3 result applied corrections 4 cps 4 result applied corrections 4 cps be 7 A Control Points thumbnail is presented in the thumbnails located to the left side of the screen see Figure 10 Double click it and the control points dialog will be presented as in Figure 6 Control Points Boe 22 900 122 550 CENTER_BE 26 000 19 980 LIPPER_BE 31 250 79 260 LEFT L 4 090 30 030 E Richt 57 660 32 220 Projective Transformation Transformation performed MG RMS Number of measured CPs 0 minimum oF Rectified Image reate Image i Full image atte Bounded by control points 10 Close Clic
9. t from the Control Points was Feet then the calculation means the XYRectify image is 393 06 feet in the overall width of the image s format NOTE Pixels are dimensionally square generated as part of the XYRectify process For accurate CAD scaling all that is required at this point is to calculate either the width of the pixels relative to the image format s width or conversely the pixels height relative to the image format height Using the Calculation for CAD Scaling As previously noted we are using Quick Scene by CAD Zone as the CAD diagramming program for this tip sheet The Crash Zone as well as other CAD diagramming programs offer image scaling functionality Figure 12 In Quick Scene use the File Pull down Menu and select Import Images Figure 12 fo Quick Scene 4 1 Untitled1 Evaluating 0 Remaining FACS Edit Draw View Drawing 4ids Window Help 3 c TE 6 FM Export Images i Prine PR Printer Setup PT Exit Fes Af o0 D 393 06 Er OBeax qm Back Figure 13 illustrates the XYRectify rectified image a Quick Scene 4 1 Untitled Evaluating 0 Remaining Fic Edit Draw View i Drawing Aids Window Help TE a n DSE RAS o AARLAAGD Draw Edit Figure 13 REBAB Er E 90 A a as 6 Symbols 7 Learning Center e Quick Scene Click on the Bitmap scale icon see Figure 14 Draw Edit Figure 14 Figure 15 Quick S
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