Introduction. The Ortho-rectification Process

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1 Introduction An ortho-image is an image in an orthogonal perspective, when the projective rays are parallel and the viewpoint is at the infinite. More details about this subject can be found in the technical literature about ortho-rectification and ortho-image generation. The e-foto software uses the method of differential rectification for generating an ortho-image. Such a method requires a DSM model, from which it is possible to generate its corresponding ortho-image. The differential rectification algorithm starts from the ground-space domain back into the imagespace, what is so named a bottom-up projection approach. This bottom-up procedure starts from each center of a grid cell interpolated from a DSM, which ground coordinates are known, and projects it back into the image-space, by means of the collinearity equations. Therefore both interior and exterior orientation parameters of each image, along with the DSM, are a prerequirement for the generation of an ortho-image or an orthomosaic. The Ortho-rectification Process After making the Interior Orientation, the Exterior Orientation (by spatial resection or by phototriangulation), the automatic extraction of a DSM and the interpolation of a grid file, we can now run the Ortho-rectification. If the process was done in different steps, just upload the previously saved files. Then, on the main screen of the e-foto, in the menu Execute, select Ortho-rectification or press the CTRL+R simultaneously. Page 1 of 8

2 To run the Ortho-rectification, we must locate the previously saved DSM file (extraction and interpolation), with the extension *.dsm. The ortho-image resolution will be automatically filled on the Ortho-image resolution field, as saved during the process of the DSM execution. In the Ortho Image File Format you can choose in which format you want to generate your ortho-image (E-FOTO ortho-image or GeoTIFF). To start the process, click on the button (Run Ortho-rectification). This can be seen in Figure 1. Figure 1 The ortho-rectification screen After running the Ortho-rectification it will open a window displaying the superimposed images, as shown in figure 2. Page 2 of 8

3 Figure 2 Result of ortho-rectification displaying the resulting orthomosaic After the orthorectification you can evaluate the quality of the ortho-image using the Ortho-Image Quality Tool, to do that you can click on the button. In Figure 3 we can see the window of the Ortho Image Quality Control Tool. Page 3 of 8

4 Figure 3 The Ortho-Image Quality Control Tool Now, we will load a file with a certain amount of reference points to be measured and compared with the ortho-image. In Figure 4 we can see 12 points plotted in the ortho-image to compare with their original location. Page 4 of 8

5 Figure 4 Points loaded in the ortho-image We can test the quality of a specific point compared to the sketch. In Figure 5 below, with the Zoom tool we approach the point here indexed as "5" in the table. Now we need to seek its location in the sketch to check if is well referenced in the ortho-image. Page 5 of 8

6 Figure 5 Zoom in to Reference point 5 to checking the ortho-image positional quality Now we will compare the location in the image with the actual location of the point on the ground. To do that we should zoom in to the point and click in the button and them in the correct position of the point in the Ortho-rectified image. In the table which is depicted in the bottom of the ortho quality checking will appear the X and Y coordinates in the image and its corresponding coordinates in the ground reference coordinate system. (Figure 6). Page 6 of 8

7 Figure 6 Ground Coordinates of Quality checking point 5 taken from the ortho-image and directly measured in the terrain When you finish the measurements of all the points you want, you can delete the remaining points by selecting the point to be eliminated and then clicking in. To proceed with the calculation of the errors, click on the button. In Figure 7 you can see the measured point, the errors and the deviations. Page 7 of 8

8 Figure 7 Errors in Ortho-image quality checking, their statistics, highlighting the planimetric error calculated for point 5 Any contribution for correcting and improving this tutorial is very welcome. Please send your comments and/or suggestions to the e-foto team at <END OF TUTORIAL> Page 8 of 8