Variation be- 
ade by Abney 
ie for these 
adings and 
rpreter can 
ay. 
re more ac- 
ands rather 
e plot, the 
pod on the 
ventory as 
experienced 
er-all gross 
the ground 
the defects 
"vary con- 
calculation 
even those 
10ds for use 
     
  
  
  
   
   
  
  
  
   
  
   
PHOTO INTERPRETATION IN FOREST INVENTORIES 11 
An equally important problem is the requirement on most forest inven- 
tories that volumes be determined by species and diameter class. Since the 
interpreter cannot recognize all species, and because he cannot directly meas- 
ure diameter, he can only get these details from field plots. 'The most practi- 
cal procedure consists of adjusting his over-all volumes from gross to net by 
percentages from a few plots measured both on the photos and in the field. 
From the same plots he can secure percentages which can be used to dis- 
tribute his adjusted over-all volume by species and diameter classes. Studies 
indicate that the number of expensive field plots needed for such a procedure 
would be considerably less than are now taken on most forest inventories. 
TABLE 2 
COMPARISON PHOTO AND FIELD OVER-ALL VOLUME ESTIMATES 
FROM SURVEYS MADE IN THE CENTRAL STATES 
  
  
  
  
  
Photo Estimates Field Estimates 
. Mean . Mean : 
Size of Size of Size of 
per-Acre Cost per-Acre Cost 
Tract Volume Sample Volume Sample 
Acres Bd. ft. Plots Man Bd. ft. Plots Man 
days days 
40 6,537 +207 35 1-acre 1 6,241 +628 35 i-acre 4 
640  2,720::180 56 1-acre 1 2,6601 53 640 %-acre 20 
25,000. 1,230 + 86 370 1-acre 18 1,500 +300 86 +-acre 100 
100,000" - 5,251 = 301 37 1-acre 1 1 4,973::515 37 +-acre 8 
4,000,000 2,670 5,900 1-acre 110 2,620+ 90 1,030 #-acre 412 
  
  
Since no field plots were taken on the photo surveys gross photo volumes have been made 
comparable to field net volume by use of the cull deductions from the field surveys. 
EXTENT OF THE USE oF THESE TECHNIQUES 
At present the interpretative techniques used in forest inventories vary ac- 
cording to the experience and training of the interpreters and to the extent that 
their organizations are willing to rely on aerial photos. Most inventory pro- 
cedures were developed originally by foresters having little photo training 
Classifications, measurements, and other details were defined in terms with 
meaning to ground survey crews. The photo interpreter who attempts to adapt 
such survey procedures for aerial estimates soon finds many requirements be- 
lieved by most foresters to be indispensable, are completely impractical on 
aerial photos. At present most techniques in use are compromises. 
Classification of the forest on aerial photos with subsequent measurement of 
field plots is a general technique used by nearly all. The degree to which this 
classification is completed by stereo interpretation varies according to the 
classes recognized, the character of the forest, and the scale and quality of the 
aerial photos. 
Most inventories prepare type maps, particularly if the maps are to be used 
for other than strictly inventory purposes. This procedure is followed by our 
own national forests, the surveys now being made in Canada, the U. S. Forest 
Survey in the West, and by surveys in many other parts of the world. Most con- 
sulting foresters confine their use of photos to the preparation of better forest 
maps. 
But not all forest inventories insist on maps. Extensive areas covered by the 
U. S. Forest Survey east of the Mississippi River have been completed by photo