Table 6 
Errors caused by the approximation of the formula at different flying 
altitudes and height differences. 
  
Height Flying altitude 
difference 2500 m | 4000 m : 5 000. m 
  
15 m 0.9 dm 0.6 dm 0.4 dm 
20 m 16 » 10 » 08 » | 
30m | 36 » 29.» ha 1.809 
For tree heights of 15—20 m the error is approximately 1 dm. 
2. 17. Errors caused by movements of the trees due to wind. 
A source of error not sufficiently investigated before is the 
effect of the movements of the trees in windy weather. In strong 
wind the displacement of the tree crown from its normal position 
is considerable. The component of the movement parallel to the 
photography base causes errors in the height determinations 
while movements at right angles to the base produce vertical 
parallaxes which disturb the adjustment of the floating dot to 
the tree top. 
If the displacement of the tree top in the direction of the base 
from one picture to the other is denoted a (in metres) the cor- 
; s e ;:,,1000.a-c, h. 1000.a.c 
responding height difference is M nd bi : 
1 ) ) 
c — 15 cm and b! — 90 mm the error in height is = 1.7-a metres. 
For 
  
The sign of the error depends on the direction of movement. 
No vertical parallaxes disturbing the stereoscopical viewing of 
the tree tops will mean relatively calm photography weather. 
The sources of error mentioned above will be most significant 
for measurements by means of mirror stereoscope and stereo- 
meter. Such factors as distorsion and film and paper shrinkage 
will be of no importance in tree height measurements. 
2. 19. Accuracy in adjusting the floating dot. 
For judging the effect of errors in measurements dealt with 
above, it is important to know how precisely a floating dot can 
be adjusted to a terrain detail. This question depends on the 
15