4 
tions are known in the six orientation points. For cl = b we get after 
correction for the curvature of the earth as stated above 
2 c A h 
2do= bh (3) 
where 
h = flying altitude 
Ah — height error after correction for earth curvature 
c = camera constant 
2do = cenvergence of the model 
Photographs from different altitudes can be used to determine the 
variation with the height of the ^-convergence. In this way the influence 
of refraction in different altitudes perhaps may be studied and separat 
ed from the distortion caused by the lens. 
Another method to determine the ^-convergence is to use elevation 
control points 1, 2, 7 and 8 in Fig. 3. Radial distortion will result in 
equal differences in the two points 7 and 8 as in the two points 1 and 
2. Accordingly in a model without ^-deformation these four points 
should not have any height errors after correction for the curvature 
b2 
of the earth (+ for 7 and 8). If there are any the height error 
4K 
A h can be used to determine the convergence 
4 • A h 
2d a = — (4) 
Usually freedom from vertical parallaxes is considered as an indi 
cation that the relative orientation is completed. If the photographs 
do not have any distortions this is correct. 
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