Expressed in a coordinate system as indicated in fig. 1 and considering our choice of the reference plane (1) is: 
(2) 
, x 2 +b. x+y 2 -y 2 
dZ = q_ 
2R 
Expressed in the coordinate system of the right-hand photograph one gets: 
(3) 
dZ = - 
(Z-ZJ 2 , 2 2. 
(x +b. x+y -y Q ) 
2R. f 
in which b is the air base at the scale of the right-hand photograph. The x-parallax in the right-hand photograph, ne 
cessary for compensation of this elevation error is: 
(4) 
dx = 
B.f 
dZ 
(Z-Z ) 
' o' 
(3) Substituted in (4) gives: 
(5) dx = 
B 
2R.f 
(x 2 +b. x+y 2 -y 2 ) 
Corr 
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4