Orthorectification (orthorectification) of an image is a mathematically rigorous transformation of the original image into an orthogonal projection and elimination of distortions caused by the relief, shooting conditions and the type of camera.
As a result we have opportunity to create orthophoto images, orthophoto maps, orthophotoplans and other orthorectified (orthorectified) images and products.

What is Orthotransformation? This is a process of geometric image correction, which removes perspective distortion, reversals, distortions caused by lens distortion and others. In this case, the image is reduced to a planned projection, that is, one in which each point of the terrain is observed strictly vertically, in nadir. To perform this transformation, you must remove the distortion caused by the relief. Therefore, for transformation you need a terrain model, you need to know the height of the terrain for each point of the image.
Why is it necessary to perform orthocorrection of space images, because spacecraft are shooting from a very high altitude (hundreds of kilometers) and distortion is minimal? The fact is that the spacecraft cannot shoot in nadir all the time, otherwise it would have to wait a very long time when it passes over the given point, because the capture of the observation equipment does not exceed 20 km, and the duration of the complete coverage of the turn-to-turn interval by the capture bands is more than 100 days. To eliminate this drawback, the spacecraft are "turned" and most of the frames are obtained promising. It should be noted that shooting angles can be up to 45 degrees, and at this height, this leads to significant distortion.
Orthorectification process.
The generation of an orthophoto requires an accurate height model of the surface (DSM). Relief displacement is corrected by taking each pixel of the DSM and finding the equivalent position in the image. The intensity or color for this position is determined by resampling and the resulting value is assigned to the pixel in the orthoimage.