2 PHOTOGRAMMETRIC ENGINEERING 
photo or a mosaic. Earlier procedures for estimating board- or cubic-foot volume 
mentioned a count of tree crowns on photos multiplied by the volume in an 
individual tree determined from field measurements. Some articles now sug- 
gested not only a count of trees but height measurements on photos using the 
shadow method. H. E. Seely, in several articles throughout this period, discussed 
classifying forest areas on both vertical and oblique photos using species, height, 
and age classes. 
During the 30's, forest inventories were started in Alaska, the Rocky Moun- 
tain Region, and in California by the U. S. Forest Service using available aerial 
photos. Extremely detailed procedures for field classification based upon form, 
relative size, and appearance of vegetation were worked out and improvements 
in mapping procedures and equipment were developed. The use of aerial photos 
in forest inventories spread rapidly in the United States after the Department 
of Agriculture began its standard 1:20,000 scale photo program in the late 30's. 
Articles at this time discussed tree height measurements by parallax, and 
studies made in Germany and other European countries considered the accuracy 
of photo versus field measurements. Forest inventories started by Japan in 
Manchuria and by other countries in the Far East used aerial photos. 
During the early 1940's great emphasis was placed on photo interpretation 
by the armed forces, and many foresters both in America and abroad were first 
introduced to the use of aerial photos at military intelligence schools. After the 
war these same foresters, skilled in interpreting vegetation and other terrain 
conditions, found in forest inventories the opportunity to capitalize on both 
their civil and military experience. 
And so from 1946 on the literature is filled with discussions of the technique 
of forest classification; the use of both infrared and panchromatic photos of 
varying scale; the technique of simple photogrammetric measurements of 
height, crown width, and crown coverage; the correlation of all of these with 
ground measurements, and the development of aerial volume tables. 
As statistical methods were accepted by foresters and sampling became a 
tool in forest inventories, some foresters recognized that complete mapping of the 
forest may not be a necessary step in securing timber volume. Instead, detailed 
photo interpretation of a number of sample plots and the field examination of a 
few of the same plots is now believed by many to be the most efficient form of 
forest inventory; particularly in areas of low relief. 
INTERPRETATION TECHNIQUES USED IN FOREST INVENTORIES 
The technique of these interpretations, whether the forest is to be sampled 
or completely mapped, falls logically under the headings of (1) classification into 
relatively homogeneous areas, (2) measurement of these areas, and (3) deter- 
mination of per-acre volume. 
CLASSIFICATION 
Forests are classified into like areas by qualitative interpretation helped out 
by an occasional photo measurement. The photos are viewed under lens stereo- 
scope and the classes identified by the tone, texture, shape, and relative size of 
forest areas and individual trees. Shadows are studied as well as tree images 
because they often indicate species, condition, or the size of the trees. 
The usual classifications fall into four general groups: (1) forest types, based 
upon species; (2) forest sites, based upon topographic and soil condition; (3) 
forest stand size, based upon diameter and height of the trees; (4) forest condi- 
tion, based upon past treatment of the forest.