/ from aerial 
photogram- 
study, most 
ots. 
'om mapped 
ich. plot and 
nts, volumes 
determined. 
be estimated 
rmined from 
work which 
photo inter- 
idoubtedly a 
phasis, other 
The average 
aber of field 
ld volumes. 
st too much 
ting requires 
age volumes 
irectly from 
presentative 
asuring tree 
ots of known 
ind compare 
easure them. 
e Is weak in 
eograms are 
ion in forest 
more, differ- 
ht, or photo 
the stereo- 
interpreter. 
entire stand 
eneral these 
d from field 
rown diam- 
to measure- 
the ground 
to measure- 
acre volume 
most usable 
dense stands 
inaccurate, 
out deductions 
    
   
  
    
   
   
  
  
PHOTO INTERPRETATION IN FOREST INVENTORIES 9 
Stand volume tables correlating per-acre gross volume from field meas- 
urements with photo measurements of average total height, crown diameter, 
and crown coverage can be used on small and medium scale photos. The 
three measurements per plot take little time, and since the tabular volumes 
are already averages of several plots, these tables are the most direct ap- 
proach to the required per-acre volume. 
TABLE 1 
AERIAL VOLUME TABLES 
  
Visible Total Height (Feet) Average Crown Cover (Per Cent) 
Crown ————— 
Diameter 20 30 40 50 60 70 Height 15 35 55 75 95 
advocated for forest inventory. 
  
| Stand Volume Tablet 
| 
r a XI AT. ok 
[ree Volume Table (10-14 Ft. Crown Diameter) 
  
  
  
  
  
| Stand —————— 
  
  
  
  
  
Feet Cubic Feet Feet Cubic feet per acre 
10 2.2 83.2 4.6, 7,4 41.0. — 20 250 400 450 500 650 
11 2.8 38.8 5,5 8.3. 12.2 — 30 300 450 500 550 750 
12 34 45 6.7 95 13.6 21.2 40 350 500 600 650 850 
13 — 658.8. 8.0 10.7 15.1 22.9 50 400 550 700 750 1,000 
14 — 6.7 93 12.1 16.6 247 60 550 800 950 1,050 1,300 
| 70 900 1,250 1,400 1,500 1,700 
  
* Tree volume tables give average volume for individual trees of given measurements. Volume 
must be converted to per acre figure by count of trees on the acre. 
t Stand volume tables give average per acre volume for stands of given measurements and 
crown cover. Desired per acre figure is read directly from table. 
'The use of either type of table requires some elementary photogram- 
metry. In some areas? tree or stand heights are determined by shadow meas- 
urements. The most generally usable system, however, seems to be by paral- 
lax measurements. Although simple parallax bars to be used with lens stereo- 
scopes are available at low cost, the most practical measuring device seems 
to be the parallax wedge (Figure 6). The cost is but a fraction of that for 
other portable height finders, there are no moving parts to get out of ad- 
justment, and the accuracy obtained by trained operators on simple parallax 
difference measurements seems to be identical with that obtained by far 
more expensive instruments. 
Crown diameters are measured with a wedge or a dot type crown diam- 
eter scale, consisting of a series of small dots of graduated diameter. These 
simple devices are usually printed on film. Micrometer attachments for 
lens stereoscopes have been designed for such horizontal measurements, but 
they are more complicated, more expensive, and less usable than the crown 
scales. 
The proportion of the acre covered by tree crowns is measured by com- 
parison with a crown coverage scale. This device consists of a graduated 
series of squares with randomly selected black dots covering 5 to 95 per cent 
of their surface. All these comparative measurements are made under stereo- 
scope in order to reduce the effect of shadows. 
As he studies the plot under his stereoscope, the interpreter makes what 
he considers average readings of the three factors for that plot. He converts 
these readings to measurements in ground units by means of a slide rule or 
3 Canada, for example, where long shadows are common and oblique photos are frequently