age level, the errors amount to (approximately) 1% or 2 dm 
for a 20 m high tree. 
2. 14. Effect of tilt on the determination of the base. 
If the camera axis was not vertical when the photograph was 
taken, it is difficult to determine the flying height by means of 
the scale, as the scale is not constant. Moreover, an incorrect 
base will be measured in the picture. 
An inclination at right angles to the flight direction (w) will 
not affect the determination of the base, but an inclination in 
the direction of the flight (59) will. For diverging camera axes 
; h m 
b! will be too great and, consequently, too small. Thus the 
) 
height differences will be too small. Converging camera axes will 
have the opposite effect. 
  
  
  
  
  
  
2) ; 
fe ; «c € (/9- ^p g) 
lv ca 0 ue E, 
C 55 
v le c 7 
| | 
N um S 
N X X / 
N | [@ 4, 
N i 
h | h | 
N | | / 
NONO | 7 
3 | 
À / 
y 
= 7 Nw Wu f 7 777 Z 77 
Figure 3. Figure 4. 
For diverging camera axis For converging camera axis 
h, Li, 
Is too small. ‚is too great. 
b b 
Errors in height differences caused by incorrect base due to 
tilt are shown in table 4 for various values of the divergency or 
convergency (5). The calculation is made with the assumption 
that the flying height is perfectly determined, and the mean 
value of the bases measured in the two photos is taken. 
The percental errors are constant all over the model. The q 
differences decide the magnitude of the errors. For the com- 
9