PHOTOGRAMMETRIC ENGINEERING 
ORE 
FrG. 6. Stereogram illustrating use of parallax wedge to measure tree heights. Variation be- 
tween height measurements made on 1:20,000 photos and height measurements made by Abney 
level on the field averages less than 6 feet when parallax wedge technique is used. 
simple conversion tables. He then reads average per-acre volume for these 
measurements from his aerial volume table and records both readings and 
volume for each plot. Using this procedure an experienced interpreter can 
classify and estimate volume on at least 50 one-acre plots per day. 
For individual trees, total height and other measurements are more ac- 
curate on large scale photos. But the forester must consider stands rather 
than individuals. Because of stand variability within the acre plot, the 
average measurements used in this technique are about as good on the 
1:15,000 or 1:20,000 scale photos normally flown for forest inventory as 
they would be on the larger, more costly scales. Furthermore, experienced 
interpreters using reliable aerial volume tables can estimate over-all gross 
volumes which will not vary significantly from those measured on the ground 
(Table 2). 
Gross volumes are used since the interpreter cannot recognize the defects 
on which cull deductions are based and these deductions may vary con- 
siderably between stands. However, many foresters consider the calculation 
of cull percents to be a weak point in most forest inventories, even those 
made on the ground. Research may give us equally accurate methods for use 
on photos, 
  
  
pU VW IN 
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