graphy and tilted camera axis affect the systematic error. Errors 
due to flying height, base and topography will not amount to 
more than a few per cent of the tree height (approximately 0.5 
metres) while the systematic errors amount altogether to 10 % 
or 1.5—2 metres. 
There seems to be a marked difference in systematic errors 
between single trees and stands. The heights of the stands are 
slightly underestimated and in some cases considerably over- 
estimated. Presumably, it is easier to adjust the floating dot 
to the canopy than to a single tree top. 
The systematic errors also seem to depend on diversities in 
viewing and interpretation skill of the operators. 
2. 22. Mean square errors in tree height measurements with 
stereometer. 
Table 11 
Mean square errors (in metres) in tree height and stand height measure- 
ments with stereometer. 
  
  
Single trees Al to- Stands All to- 
Scale - 2 sther — - ell d 
H Sk Ó |sethne H Sk O6 | ge! 
1:16000 | — |+287|+198| +216) - + 1.80) + 281) + 2.44 
1:26000 | + 2.05 | + 2.54 | + 2.06 | + 2.28 + 2.62 | + 188 +285 + 2.55 | 
1:33000 | + 2.16 | + 2.56 | + 2.22 | + 2.34 + 2.84 | + 2.08 +244 + 252 
1:16000 | — +248, + 285 + 272] — + 1,74 + 2.94, +255 
1:26000 | + 1.84 + 1.16 + 3.29 + 1.53 + 1.86| + 1.53 | + 2.90 + 2.25 
1:33000 | + 1.75] + 2.77] + 3.02 | + 2.74 | + 2.10 | + 257 | + 2.74 | + 2.54 
The mean square errors lie around 2.5 m and the variation 
is surprisingly small. There are no marked differences between 
different scales nor between the three test fields or between 
single trees and stands. 
ividently differences which may exist between different scales 
are too small in comparison with other sources of error to in- 
fluence the result. 
According to investigations previously performed, the accuracy 
in adjusting the floating dot would cause a mean square error 
of = 0.6 m. Many trees, however, are very fuzzy in the picture. 
In many cases a low contrast between the tree crown and the 
20