10 
2 
Let us assume that photographs 1 and 2 were exactly vertical, that 
is, with the tip and tilt of each equal to zero, = 0,^^ = 0, ^ = 0, 
^2 = 0). While the camera axes of these photos will be normal (vertical) to the 
geoid in their respective locations, they will not be parallel. The convergence 
of these axes is equal to the angle f which is proportional to the distance 
between the pictures and the radius of the earth. To achieve a parallax-free 
model, the relations of the camera axes have to be reproduced in the plotting 
instrument, that is, they must have the same convergence angle . Therefore, 
the values of > derived from the horizon photos, need to be corrected for 
this convergence before they can be introduced into the plotting instrument. 
When photo 1 is introduced into the left plate holder of the instrument, the 
T U? 
value of /2 has to be added to the lderived from the horizon camera. 
T* 
When photo 2 is introduced into the right plate holder, the value of °/2 has 
to be subtracted from the determined from the horizon camera for picture 2. 
Thus: 
? 2 
(instrument) 
(instrument) 
(horizon) + 
(horizon) 
V n