(400) 
The measure of plain distortion for any triangle results from the threefold 
value of the abscissa of the triangle centre of gravity. 
If we know the elements of tilt, these geometric-graphic methods allow to 
AY , A 
7 
  
v 
  
  
Fig. 5 
determine the values of distortion. Thus directions, distances, angles and plains 
on the ground can be deduced from vertical photographs in a quick and 
simple way. 
Finally I want to give the terms on which these geometric methods are based: 
y 
/ 
Aa — yr. f 
= h 5. 1 + (2x — x). t 
$i f 
F= E +, (1 Fa i) 
Hereby are: 
f focal length 
y nadir distance 
h flying height 
s, F distance relatively plain on the ground 
S, E » = » in the photograph 
yr,Xr picture co-ordinates of the point F' 
XM abscissa of the bisecting point of the distance s” in photograph. 
Measure of direction distortion: yr 
= ,, distance x ı 2xu —xr 
. /, 
E » plain 2: : 3 xs