observed on the orthophotography depends on the same parameters. In fact 
from figure l it can be observed that the parallax of a point P is pro- 
portional to. the tangent of angle o of the inclination of the rays of light 
(ard accordingly it depends on the above said distance "bx' of projection- 
-« enters and difference in height Z); therefore for values higher than a 
and particularly higher than 45°, tg a moves to infinity and the paral- 
lax becomes so strong as to hinder the overlap of images. In fact whether 
we suppose to have a relief for similar values of a, the shifting of the 
image point of the top compared to its foot will be so strong that whether 
the images of the top are overlapped it is impossible to overlap those of 
the foot, and vice versa. 
  
  
  
  
Z 
c = camera focus 
of the negative film. 
a 
P 
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| Level of n,, n,, p — level 
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Figure 1 
Therefore in such a case only a central portion of orthophotomap can 
be stereoscopily observed; the amplitude of the bonder band, where there 
is no overlap of images, obviously depends, as already specified, on the 
values of tg a and namely on the distance between the projection center 
and the difference in height there existing. 
This inconvenience can be easily overcome by deriving the orthophoto- 
map from airphotographs taken just for this purpose. In case of extreme