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.
499