PHOTOGRAMMETRIC ENGINEERING 
correlated with soil and topography (Figure 3) interpreters can often use 
forest sites based on topography to classify forest areas. 
Stand size.— This classification, based upon the size of the timber, re- 
quires some photo measurements. Texture and relative height of photo 
images may be used, but most foresters rely on the quantitative measure- 
ments of total height, crown diameter, and crown coverage, or, to some 
extent, tree counts to identify stand-size classes. These measurable factors 
have been found to correlate fairly well with such field criteria as height, 
diameter, number of trees, and volume. 
Condition.—The treatment a forest stand receives may càuse differences 
in species composition and in the quality and size of timber. One of the first 
classifications used by foresters on aerial photos was based on the condition 
of the forest. Forest fires, logging, and clearing all made characteristic pat- 
terns and changes in the normal photographic texture and tone of the forest 
(Figure 4). Furthermore, these differences in texture and tone were visible 
on photos flown many years after the change in forest condition occurred. 
Since species and size class are often correlated with these old burns or 
slashings, the forest can be stratified by condition. 
Rarely do foresters use only one means of classification in their forest in- 
ventories. Instead, most classification schemes recognize species, stand-size con- 
dition, and often site or age class. These schemes vary greatly according to the 
region, the character of the forest, and the type of inventory for which they were 
developed. In their attempt to adapt these involved schemes for use on aerial 
photos, foresters have produced many complicated photo-classification systems. 
In most of these schemes final interpretation must be completed in the field, 
since many classes are not recognizable on the available photos. 
As a result of this procedure the belief has spread that photo interpretation 
of forest conditions must be done in the field, and that interpreters can only be 
trained by work on field crews. Some field experience is necessary but if the 
classification scheme is simple and the classes used can be recognized on photos, 
an interpreter can be trained by means of carefully analyzed stereograms and 
previously recorded field data. A few hours spent comparing his interpretations 
with these standards may have as much training value as a number of days 
spent as a member of a field crew. 
Foresters have recognized in aerial photos a means of reducing the cost of their 
forest inventories. But they must learn to simplify their methods of forest 
classification if they are to make the most efficient use of the aerial photos. 
MEASUREMENT OF FOREST AREAS 
In forest inventories the area within each classification is of prime im- 
portance. Traditionally these areas have been sketched on a map, then meas- 
ured by planimetring or some similar method. 
Mapping procedure.—In this paper aerial mapping will not be discussed. 
However, most forest inventories do use a mapping procedure. Classifica- 
tions are identified under stereo and outlined within the net area of contact 
prints. Only the more obvious classes are identified in the office. Then the 
contact prints are taken to the field where the more obscure classes are 
identified and outlined. Annotated prints are then sent to a central office 
for compilation by standard photogrammetric methods. Details of both field 
mapping and compilation have been published in PHOTOGRAMMETRIC ENGI- 
NEERING and several other publications. Areas are usually measured on the 
completed maps.