3. 14. Composition of the stand. 
The proportion between different species in a stand is ex- 
pressed in tenth of the whole timber supply. 
In pictures taken with panchromatic film it is very difficult 
to differentiate species. When the pictures are taken shortly 
after leafing time the broad-leaved forest is lighter in the photo 
than the conifers. No attempts to distinguish between different 
broad-leaved species have been made. Pines and spruces can 
hardly be differentiated unless they appear separately in pure 
stands. 
3. 15. Age. 
In judging the age of a stand, first the height and then the 
pattern aid the interpreter. Pattern depends on the number of 
stems and the development of the crowns, both factors connected 
with age. Field experience or sample areas are necessary even 
in this case. 
3. 16. Preparation of the interpretation work. 
The interpretation work was done by stereoscopical viewing 
of the pictures by means of a lens stereoscope (focal length 8.5 m, 
\ 
magnification 3 X). The result of the interpretation work con- 
cerning height and timber supply was prepared in the same way 
as the result of the measurements. Mean square errors were 
'aleulated after corrections of systematic errors. Concerning 
other stand factors than height and timber supply an account is 
given of the numbers of incorrect estimations distributed into 
different error classes. 
Each interpreter had to estimate about 120 samples for each 
scale, taking five different factors into consideration. 
3. 2. Results of the interpretation work. 
3. 21. Estimation of stand mean heights (see Table 14). 
To facilitate comparison, systematic errors and mean square 
errors are given separately in Tables 15 and 16.