The experiments made by the committee (Chapt. 4) have yielded illumi- 
nating results in many basic questions, and also in respect to certain prac- 
tical applications. In some instances, however, they have proved too limited. 
The experiments relate to vertical photographs chiefly taken during the 
summer. The majority cf these were taken with wide-angle cameras on a 
scale of 1: 20 000 with normal overlap. But to a lesser extent photographs 
have also been taken with normal-angle cameras of various types, and 
also on large scales up to 1:2 000 (vide p. 196). Some of the most impor- 
tant experimental results must now be mentioned. 
The quality of the site can only be uncertainly estimated for isolated 
stands; on the other hand, the average for a larger number of stands may 
be of practical value. To provide a basis for site quality estimates it is 
generally required that pictures of stands that have already been studied 
from the ground shall be used. 
The number of trees per unit of area can be counted in favorable in- 
stances. Error can be limited to a few per cent in the older spruce stands. 
Pine and birch both present greater difficulties in dense stands. In young 
growths the number of trees can usually not be counted. 
Conifers can usually be clearly distinguished from deciduous trees by 
means of infrared photographs. For other forestry purposes, however, these 
seem to be of less value than panchromatic photographs. It is still not 
possible to distinguish pine and spruce with certainty, but experiments 
with various filters and different sorts of film are now being made. 
Height of trees and stands has been measured in various stereoinstru- 
ments. For the most part reflecting stereoscopes with micrometers have 
been used. Usually the height has been underestimated by between 1 and 
2 meters. The reason for this is thought to be that the slender tops of the 
tree crowns cannot be seen in the stereo-photographs. It is furthermore 
often hard to see the ground between the trees because of shadow, ground 
vegetation and boughs. Height measurements calculated from the length 
of shadow give a similar result. Photographs of higher quality are needed. 
The crown diameter has proved to be a useful ancillary factor when 
estimating the breast-height diameter of a tree. It is, however, often hard 
to see and measure the crowns, particularly when they are small, densely 
packed, or in the shadow. The quality of the photographs, as well as the 
scale, are of great importance. 
Stand density appears clearly in the picture and can be used for many 
purposes. 
The. volume of large isolatéd trees is approximately estimated from 
preliminary studies of crown diameter, tree-height, and sometimes den- 
sity. Sample plot volumes have been estimated by comparing them with 
photographs of similar experimental areas, where estimates have pre- 
viously been made on the ground (cf. site quality). This method has 
yielded rather uncertain results for isolated trees and stands, but for large 
forest areas appears to be practical and useful in cases, where there is 
relatively little need for certainty in the results. It should be possible to 
develop these methods and they may have great importance in the future. 
    
  
  
  
  
  
  
  
  
   
  
  
   
  
  
  
  
  
  
  
  
  
   
  
  
  
  
  
  
  
   
   
   
  
  
  
  
  
  
  
  
  
  
  
  
  
   
  
    
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