number.of samples (40.—50) were chosen in various types of 
stands. Besides some other data collected, the average height 
of the stands was measured, and the samples were marked on 
the photo. These tree and stand heights were determined by 
means of mirror stereoscope (magnification 4 >) and stereo- 
meter. The measurements were made by trained stereo ope- 
rators. 
In order to isolate the effect of photo scale all pictures were 
enlarged to the scale 1: 15 000 (4.2 inches to a mile). 'This means 
that all the pictures have been involved in the enlargement pro- 
cess and, moreover, they present approximately the same ap- 
pearence to the interpreter. To attain the highest picture quality 
the picture surfaces were high glossed. Efforts were made to 
obtain a fairly constant image contrast. It was difficult, how- 
ever, to avoid differences in this respect. 
1. 3. Aerial photos used for interpretation. 
The use of aerial photographs for photo study and photo in- 
terpretation in planning work ete. has become more and more 
important. In these respects air photos have become specially 
useful in forestry. To establish the effect of the scale on the 
use of the photos for these purposes is very difficult, because 
one has here to do with estimations of more or less subjective 
character. What is most desirable when using the photos is 
to be able to dinstinguish between different stands and to decide 
what type of timber one has to do with. Therefore, the investi- 
gations were carried out in the following way: For the samples 
chosen out in the three test fields data of average heights, 
density, growing stock (timber supply), tree species and age 
were collected in the field. The samples were then estimated 
concerning these factors by three different persons studying 
the photos. The persons are educated in forestry and two of 
them are specially trained photo interpretors. None of the persons 
had seen the test fields or were familiar with them. The photo 
estimations were based upon four samples within each test field 
used for comparison. For these samples the interpretors were 
informed about the data collected in the field and guided by 
these data he estimated the rest of the samples. Without samples 
for comparison or local knowledge it is very difficult to estimate 
4