3-6 Orthoimage Generation

Once DEM is given, orthoimage can be generated automatically from rasterized image data of aerial photograph based on central projection.

There are two procedures as follows.

Orthoimage from Stereo Matched DEM
In case of stereo matching, the left image is fixed as regularly spaced grid, while the right image will be irregularly spaced grid because of the terrain relief. The product, that is DEM, is accordingly based on irregularly spaced grid on the ground. Therefore resampling and interpolation should be executed to convert it to regularly spaced grid on the ground, as shown in Figure 3.14.

Orthoimage from Regularly Spaced DEM

Some areas will have regularly spaced DEM, possibly produced from the existing topographic maps. In such cases, regularly spaced DEM or three dimensional coordinates (X, Y, Z) can be projected on the image plane as shown in Figure 3.15 The projected grid will be irregularly spaced grid, which must be interpolated to form regularly spaced image data file.

Any image based on other geometry such as mechanical scanner, pushbroom scanner, radar etc. can be also converted to orthoimage using the same principle.