ps, 
(399) 
photograph co-ordinates coincides with the principal point H’ and that the x- 
axis coincides with the principal line. 
Distortion of direction is then measured by the oridinate yr’ of the foot 
F which belongs to the perpendicular from the principal point to the given 
A) 
direction. This measure has to be multiplied by the constant term 7 (nadir 
distance y and local length f) in order to get the real distortion of direction. 
Considering all the distortions which belong to all directions going 
through one point P' the measures of distortions are the ordinates of a circle. 
This circle, the so-called 
*direction circle", has a ^y 
diameter equal to the 
distance H'P' and its 
central point is iden- Fas 
tical with the bisecting 
point of the distance 
H'P. The mentioned 
ordinates are then re- wr 
sulting from intersec- 
tion of the given direc- 
tions with the circle. 
By extension of H'P' H' x 
beyond P' to the point 
which has a distance 
from P’ equal to the Fig. 2 
half of HP’ we find 
that this point then can be interpreted as the central point of a circle with the 
diameter HP’. This circle may be named “distance circle” as it determines the 
distortions of distances. If all distances which have the common bisecting point 
P’ are considered, the 
4 y! abscissas of the circle 
are the measures of dis- 
tance distortion which 
have to be multiplied 
/ 
l 
  
  
N 
V 
  
  
p by ; in order to get the 
real distortion of distan- 
ce. The abscissas of the 
H X circle points are result- 
ing from the intersec- 
tion of the distances 
with the circle. 
The deduced distortions of angle result as follows: The sides of the angle 
are intersected by the direction circle. Perpendiculars shall fall from these points 
to the y'-axis. The distance of both foot points is the measure of angle distor- 
"4 
  
Fig. 3 
> 
J 
tion which has to be multiplied by + in order to get the real distortion of 
f 
angle. 
31