sometimes 1:20 000. In measurement with stereoscopes and 
stereometers the mean square errors vary around + 2 m. The 
best result (1.2 m) has been obtained in measurements with the 
Stereocomparator. 
In the investigations made here the two operators measured 
the height of the trees and stands marked in the pictures, inde- 
pendently. The measurements were performed, as stated, by 
means of mirror stereoscopes (magnification 4x) and stereo- 
meters on enlarged pictures, scale 1:15 000. With three test 
fields involved and three different scales for each field, the work 
was rather extensive, implying measurements of 600 trees and 
350 stands per operator. 
As, for several reasons, systematic errors can be expected in 
the measurements, these errors were first calculated as the 
difference between the mean value of true heights and the mean 
value of the heights measured on the photos. After corrections 
for systematic errors the mean square errors were calculated. 
The results obtained are shown in Table 8. 
2. 21. Systematic errors in measuring tree heights with stereo- 
meter. 
Table 9 
Systematic errors (in metres) in measuring tree and stand heights. 
  
  
Opera- Single trees | Mean Stands Mean 
tor Scale val : RÉ I : 
or H Sk oO | "We H Sk oO | AU 
" 1:16 000 1.1 21 17 — F 04 1.6 0.7 
1 : 26 000 -95 1.8 24 2.2 02 +0.3 1.3 0.5 
1 : 33 000 0.0 1.9 1.8 1.5 01] +04) +08 +04 
2 1:16 000 +H09/—17 0.7] - | + 4.0 02| +15 
1:26000 | +22 +04 3.5 1,1| +06) + L6 0.5! +05 
1:33000 | +1.0/ +19 25) -04! + 1.30 +521 +051 + 2 
In measuring single trees the heights are usually under- 
estimated. This fact has been stated before by, among others, 
Spurr in 1948, and the former Committee of Forest Photogram- 
metry in 1951 and is considered due to the fact that a small 
tree top is not fully visible in the picture because of limited 
resolving power. 
18