That is a fancy way of saying that the only things you can reliably measure on a photograph are angles from the centre and even that is only true if the photograph is taken perfectly parallel to the ground. Of course that happy state never exists and every photograph is rotated, tipped and tilted to a greater or lesser extent and the effect of that has to be removed. To make a map one is simply measuring visible things on an aerial photograph. Using digital techniques one can reliably make those measurements with an accuracy of about 0.02 mm. Miscellaneous errors - It will be appreciated that with these sort of accuracies there are a host of other nasties that creep in.
Lens distortion is a bad one but it is constant and once it is known it can be eliminated.
Film instability due to temperature change and stretches from bad handling is not a constant and causes problems. There are other lesser contributors such as atmospheric refraction and earth curvature but the effects of these are normally minimal. However the key to the overwhelming advantage of digital orthophotography lies in the ability of the computer to take in a pair of stereo images and automatically derive the critical ground heights of that terrain. Although the mathematics to solve this problem has been known for more than fifty years, the technology to actually do it has only become available in the last four or five years. This 'stereo coverage' provides the information required to produce an orthophoto.