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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
differ lithologically. The measurements are frequently made on photogeol- 
ogical maps, constructed by means of radial triangulation and sketchmaster. 
The basic data, i.e. the length and orientation of streams, have some very 
serious deformations on the photogeologic maps thus constructed. The main 
errors are the following: 
(i) Error due to tilt, 
(n) Error due to central projection, 
(hi) Error due to deformation caused by radial triangulation and sketch- 
master restitution process, and 
(iv) Error due to inaccurate sketching. 
Two of these errors will not be discussed - the first one because it becomes 
negligible with high-quality verticals, and the last because of its unpredicta 
bility. From now on we shall consider strictly vertical photographs.