were taken, type of film, location of the object in the stereo 
model, perspective, direction of illumination, shadows. Photo 
interpretation requires application of commonsense, field ex- 
perience, training in photo studies, etc. 
In selecting the samples in the test fields the aim was to get 
all forest types and stages of development represented. There- 
fore, the samples vary very much, which makes them difficult 
to interprete. 
3. 11. Stand mean height. 
As a rule the stands are fairly homogeneous and the height 
of the canopy is easy to determine. Two or several different 
height classes in the stand or a very dense canopy obscuring the 
ground make the height determination difficult. Estimation of 
stand heights without measurements requires known heights for 
comparison. Four comparison areas in each test field were used. 
3. 12. Density. 
By density is here meant volume density, obtained from the 
intensity table of T. Jonsson. What is directly estimated in 
the photos is crown density, i.e. the portion of a given area 
covered by tree crowns. Between volume density and crown 
densiy there is a correlation, which, however, is not fully known. 
Usually it is possible to estimate volume density by means of 
sample areas. 
The possibilities of looking into a stand are more and more 
reduced the more remote from the center of the picture the stand 
is, and, therefore, stands around the edges of the picture seem 
to be denser than center stands. 
3. 13. Timber supply. 
Usually the timber supply of different tree species is estimat- 
ed by means of tables, basing on height and density. This method 
can be used even for estimations in air photos, where by means 
of sample areas used for comparison it may be possible to 
estimate timber supply directly. Often a combination of these 
two methods is used. 
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