Table 2 
Errors in % of 4h, Zh being measured on levels above the reference 
level as shown in the table. 
  
  
| Level above Flying altitude | 
reference ; à 4 | 
level 2 500 4000 | 5000 | 
25 20% |’ 193% | 10% | 
50 41% 25% [720% X 
100 8.5 % 52% | 41% 
200 18.1 % 105%. 85% 
If the level above the reference level is known for a special 
case corrections according to the table are justified. 
2. 13. Effect of differences in flying altitude. 
With the two nadir points on different levels or the lenses at 
different elevations (dz), it will be necessary to vary the factor 
  
I according to the position in the stereo model of the part 
) 
measured to obtain a correct height determination of that part. 
In practical work use is made of the mean value of the bases 
measured on the photos and the average of the flying heights 
calculated from data in the photos. 
In reality there exist a number of possible positions for the 
nadir point in relation to the reference plane. Here it is assumed 
that the nadir points are on different levels and that the flying 
altitude corresponding to these levels is determined according to 
Figure 2. If in this case the mean values mentioned above are 
. . . h i o : 
used for the calculation of bi the height differences obtained 
) 
are too high at one side of the model and too low at the other 
side. 
Provided that correct values of the flying height can be cal- 
culated for the two photos the following maximum errors in 
height differences (in 95) are obtained.