- 5 - 
image is possible in almost every photogrammetric instrument, 
and, secondly, systematic deformation patterns can more easily 
be recognized after that transformation. This can be seen 
by comparing Fig. 2a showing the deformation of a processed 
grid after a similarity transformation without scale change, 
with Fig. 2b showing the deformation of the same grid after 
a proper similarity transformation. The vectors indicate 
corrections necessary to derive the original point location 
from the actually measured location. The decision in favour 
of presentation of corrections rather than errors was made 
in view of the fact that the shrinkage of film is in general 
larger in areas near the image frame than in the central part 
of the image. Error vectors would point towards the center, 
while correction vectors point away from it, thus making the 
graph easier to read. 
PHOTOGRAMMETRIC REFRACTION 
During the last four years photogrammetric refraction 
has been discussed repeatedly and either exclusively ([ 1h91, 
[54],[66]) or together with other sources of image geometry 
changes ([351,[64]). A short abstract of Saastamoinen's 
work [49] is presented in the following. 
On their path towards a photogrammetric camera carried 
by an airplane, the imagery rays pass through alr of decreasing 
density and are refracted away from the vertical. In the 
a small angle is obtained between the incoming