find out whether there are great differences in elevation in the 
model or not. 
If the camera axis has not been perfectly vertical the scale 
corresponding to a certain level will not be the same all over 
the picture. To obtain the best evaluation of the flying altitude 
the distance used for the scale determination should be sym- 
metrical in relation to the center of the picture. For the scale 
determination of the photos used for the research work, a photo 
map of good quality was used for the Hällnäs field, and for the 
V. Skedvi field a control point densening performed at the Geo- 
graphical Survey Office by means of Multiplex was utilized. 
For the Ostad field was used a base determined in field. 
A forestry map available was not concidered sufficiently ac- 
curate. Therefore, to get a more reliable scale, a number of 
points were plotted in the Autograph Wild A6. 
The determination of h was based upon 3—— distances in 
each photo. The greatest deviation from the average value of h 
(calculated from these distances) did not normally amount to 
20 metres. In one case (Ostad. Flying altitude 2500 m) it 
amounted to 50 metres, probably due to tilt. The errors in the 
determination of h generally amount to 1 %, approximately. 
Errors in the determination of h will have a systematic effect 
on the result and, consequently, they do not influence the 
comparison between different scales, because corrections for 
systematic errors are applied to the results. If the object is to 
find out about systematic errors in tree height determinations, 
it is of course important to make a perfect determination of the 
flying altitude. 
2. 12. The effect of differences in level. 
An incorrect value of h can affect the measured differences 
in elevation in various ways dependent on conditions. The height 
of a tree growing on a level which is isolated and has not in- 
fluenced the determination of the flying altitude and the photo- 
graphy base, will be incorrectly determined unless h and b* are 
brought to correspond to the level of the tree. For levels higher 
than the reference level the tree heights will be overestimated 
(positive errors). The errors amount to the values shown in 
Table 2.