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A new definition of the normal case of aerial photogrammetry 
As stated above the use of the normal case of the terrestrial photo 
grammetry applied to aerial photogrammetry may have been a reason 
to the fact that the effect of the curvature of the earth has not been 
considered in a correct way. Therefore I think it will be useful in the 
treatment of the theories to define a special normal case of the aerial 
photogrammetry. 
The normal case of aerial photogrammetry is defined as a stereo 
model of two nadir photographs from the same altitude. 
The two photographs of this normal case have convergent nadir 
b 
directions. As is known the convergence angle is ^ or b c if the length 
of the base is b km. 
Particular attention is to be drawn to the following fact, illustrated 
in Fig. 2. 
If (p 1 and 0 2 are the absolute longitudinal tilts of the two photo 
graphs and these are put in an ordinary stereo plotter the 0 -tilts in 
relation to the plotter system will be 
b c 
T 
and 0 2 + ~ respectively 
A condition for the absolute orientation of the second model thus is 
that the 0 -tilt of the second photograph gets a correction — b c in 
relation to its value in the first model. 
Determination of the ^-convergence 
The single model 
If there is radial distortion in the pictures due to lens distortion or 
refraction, a ^-convergence will be introduced in the model. There are 
several possibilities to determine the ^-convergence if a sufficient 
number of elevation control points are known. It is favourable to use 
a model of an ice surface, where the heights of all points are the same. 
The height deviation in the middle of a model should in a ideal case 
points the height errors should be — in relation to a reference 
¿R. 
plane through the nadir points. If these deviations are not obtained, 
there must be a ^-convergence in the model. 
The convergence error can be determined for instance if the eleva-