Q 
  
© 
  
  
70 
60 
50 
40 
20 
  
  
FIGURE 1} 
' be expected by an observer on the ground (dotted lines) and in the air (solid 
     
    
   
  
  
   
   
  
    
  
    
  
   
  
    
We obtain from Fig, (10): 
ctn z = SB (88) 
Vo? + (0)? 
A denotes the correction to the zenith angle due to refraction. 
À = F(X, Yo Zo? 8» %) M) (89) 
M are meteorological parameters and 
[nf «ote qu Me 
N 
S 
Consequently, we obtain: 
2), = la + 0] V? ota (2 - à) d. Gi 
o 
The computation of x and y^ with formulas (12) is now carried out, using for 
each individual ray, the corresponding (X), (Y), and (2), coordinates. As the 
(X)(Y)(Z) values converge during the iteration cycles to the final answer, so 
will the corresponding A correction converge to the correct refraction value. 
Fig. ll shows the gener&l character of the refraction values as they must 
lines). 
Refraction in seconds of arc is presented in its functional relation to 
the elevation or depression angle of the line of sight and to the height above 
the reference ellipsoid of target point or observer, respectively. 
The tables No. l &nd No. 2 show the same information in somewhat more 
detail. The columns headed by A/1? show the changes of refraction for 3e change 
"of a specific elevation or depression angle, respectively. 
It appears pr&ctical to assume that for aerial precision measurements the 
210mm - 60° lens cone under 209 tilt presents the most stringent requirements 
with regard to refraction. Therefore 35° for a minimum depression angle appears