- 3 — 
they arrive at the image plane. Because of their movement 
first towards and then away from a projection center, the 
effect of sources of image geometry changes is primarily of 
a radial nature. 
The image arriving at the image plane is then recorded 
on, in most cases, aerial film, which is not perfectly 
positioned into the image plane. This film is automatically 
moved forward for the exposure of the next frame. Because 
this process has to be performed in a short time, it is done 
using considerable force. The exposed picture is later 
proeesses and, in general, eopied for mensuration. The 
copying process contributes to the errors in the image 
geometry as does the processing of this secondary image. 
All sources mentioned in this paragraph and indieated in 
Fig. 1 with a thick frame contribute to the image deformation. 
Throughout the entire process of imagery recording and 
copying changes may also result from changes in the 
environmental conditions, primarily from changes of the 
strongly correlated temperature and relative humidity. 
Last, but equally important, are the differences 
between the geometry of the measured image and the measure- 
ments introduced by the operator in not setting absolutely 
error free on a point, and by the measuring instrument. The 
interpretation of the image by the operator is largely 
dependent upon the image quality. A discussion of the 
various sources of degradation of image quality is omitted 
here and, in its place, pointing errors are discussed.