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Fig. 1. A high radio tower used for investigations of aerial cameras. The dimen- 
sions of the tower are demonstrated in the right figure and a view of a part of the 
tower in the left figure. 
2. Tests of aerial cameras from a high tower 
In 1956 a test field for aerial cameras was arranged under a high 
radio tower (about 125 meters). The tower (Fig. 1) is very stable and 
is from several points of view well suited for the tests. The principles 
of the tests can be described as follows. On the ground a concentric 
system of signalled control points is arranged, the geodetic coordinates 
of which are determined with high precision. The camera is then mount- 
ed in the tower with the center point of the lens accurately vertical 
above the center of the control point system. The test photographs are 
then taken after careful levelling of the focal plane frame. In varying 
different factors, for instance the temperature of the camera, the nega- 
4