€ 
Consequently, a given tree crown will be more underestimated 
if a smaller scale is used, which is also stated by Spurr. Such 
a difference is not proved by the investigations made here, where 
pictures enlarged to 1: 15 000 have been used. This may be due 
to the different enlargements of the negatives, which may mean 
that the resolving power of the 1: 16 000 negatives is not fully 
utilized. 
With an optical magnification of 4 X in the instrument the 
linear magnification is 4.3 X for the largest and 8.8 »X for the 
smallest photography scale. 
According to Spurr, the type of crown considerably affects 
the systematic error. Trees with thin crowns are more under- 
estimated than trees with wide crowns. The trees here measured 
are chiefly pines with normal crowns. In the Háüllnàs field: 
pines only. In the V. Skedvi field: pines, some spruce and birch. 
In the Ostad field: pines, some spruce and broad-leaved trees 
of different species. Pines and spruce have different crowns but 
the material was not extensive enough to allow distinction be- 
tween them. 
For the Ostad field, however, conifers and broad-leaved trees 
have been distinguished. Table 10. 
Table 10 
Systematic errors (in metres) for conifers and broad-leaved trees 
in the Ostad field. 
  
  
Conifers Broad-leaved trees 
Operator Scale . Sr 
Number Number 
1 1:16 000 40 — 2.6 65 — 1.9 
1:26 000 38 — 3.6 69 — 1.8 
1 : 33 000 43 — 2.9 62 — 1.0 
2. 1:16 000 49 — 3.1 12 0.7 
1 : 26 000 48 5.0 73 - 2.3 
1 : 33 000 49 3.4 11 1.8 
Conifers having cone-shaped crowns are more underestimated 
than the broad-leaved trees, which is in accordance with in- 
vestigations performed earlier. 
The systematic errors are also influenced by the accuracy in 
the determination of flying altitude and base. Moreover, topo- 
19