132 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
0.06279/ 
r = 
tan a 
(ii) For a given r, the error will be within permitted limits for a line with 
pitch 
0.06279/ 
tan a <1 
r 
Similar, shortened equations could easily be worked out for every error 
limit adopted. 
Thus, in the statistical analysis of drainage patterns, the error could be 
approximately rectified as follows: 
- By a thorough examination of the stereogram, the drainage is divided into 
classes according to pitch angles of unit stream sections (a-classes). 
- The pitches of drainage in the a-class are measured e.g. by the projection 
sphere method, Wallace (1950). 
- With the thus estimated a-classes there are //'-classes to be found, according 
to //-values which transfer the datum into neighbouring classes. 
When measuring a large quantity of unit stream sections, plastic overlays are 
constructed for / = const., and varying r, a and //'-classes. By laying these 
sheets on the photograph, it can be immediately read off whether the datum 
stays in the same r-class, or has to be shifted into one of the adjacent r-classes.