x in the tangent plane which can be expressed 
  
  
L I? 
EE COH 
This displacement measured in the image corresponds to the distortion 
C ec I} P C 
dmg * 7:5 or ifl —g'L dr, — XR = 8 
Corresponding curves for different H and at c — 15 cm are shown in fig, $, 
The curvature of the earth results in a displacement of points on the ice surface 
in the direction towards the image centre, i.e. it corresponds to a negative dis- 
tortion. 
523.3 The sum of distorting effect of the refraction and of the earth 
curvature 
is In this connection the combined effect of refraction and curvature of the earth 
d is of particular interest. Adding the curves for dr, and dr, gives the group of 
curves shown in fig. 9. The curves are transformed in such way that they 
have a common 0-point at r = 9 cm. 
dry+ dre 
PA 
+20 + 
„I È 
  
-20 b 3 
4 
-30 + 
co 
5 = 
3 
ds 
i 
-40 L 6 < 
7 
50 E 
s 8 
Fig. 9. The dis 
3 tortion effect of 
-60 + 0 the sum of the 
km refraction and the 
earth curvature 
of the point from the nadir. This distance d corresponds to a radial displacement 
  
  
   
   
    
      
      
    
    
  
    
   
    
   
   
  
  
  
  
  
  
  
   
    
  
    
   
  
       
594  Distorti« 
In connection 
a check of the 1 
all pressure plate 
mm were found 
concavity being 
plate is conceive 
responding to tl 
l 
dry =", *dc. In | 
of an actual pre: 
525 Determü 
empirica 
Fig. 10 shows 
plotted both th 
ment of the ice 
can be derived 
flatness of the p: 
distortion curves 
the distortion cu 
agreement with 1 
In fig. 11 anot 
graphed by the 
scale 1 : 53,300). . 
good agreement 
526  Discussio 
The disturbanc 
several sources of 
result in an erro 
as an isolated occ 
most. practical pi 
case has such a s 
refraction. Since t 
an error with the 
the “distortion” i 
total amplitude o 
ie. had a total