—M———— M——M————À—————M——— LE 
108 PHOTO INTERPRETATION PICTURE, COLWELL 
know so little about the geometry of an oblique aerial photo such as this that they are 
likely to assume the distance, *BE", in this example to be 1720 feet; then measure the 
corresponding photo distance to establish the photo scale; and using this scale, erro- 
neously estimate the horizontal ground distances “AB” and “CD” by direct photo meas- 
urement and use of the same scale ratio as they erroneously had calculated previously 
for vertical distances! 
Lest the example just given be considered too extreme, an equally serious and far 
more common example will be given to further illustrate that many photo interpreters 
need better to understand how to make measurements on aerial photographs. 
b. Fig. 14 shows a portion of a vertical aerial photograph on which two timber 
stands, A and B, have been delineated. Both stands contain approximately the same 
  
Fig. 14. Portion of a vertical aerial photo on which two timber stands, A 
and B, have been delineated. Most photo interpreters would erroneously esti- 
mate the volume of timber in stand B to be much greater that that in Stand 
A. Compare with Fig. 15. (Photo by Airview Specialists Corp.) 
total number of trees, but Stand B appears to occupy more ground, its stand density 
seems higher, the average crown diameter of its trees appears to be larger and, when 
studied stereoscopically, the averag'^ height of its trees seems considerably taller. The 
four factors just mentioned happen vo be the ones most commonly used by a forest photo 
interpreter in estimating timber volumes. The cumulative effect of these four factors is 
such that the photo interpreter is likely to interpret the total board foot volume in 
Stand B to be several times greater than that in Stand A. Actually stand volumes within 
the two delineated areas are nearly identical. All of the above errors of interpretation 
and measurement come simply from ignoring the fact that Stand A, growing in a deep 
valley, was 10,000 feet below the camera lens at the instant this photo was taken, while 
Stand B, growing near the top of a high ridge, was only 7,000 feet below the camera 
lens. When reporting volumes of timber by species, errors such as these usually are not 
compensating, because the species composition and merchantable value of a high-elevation 
timber stand usually are much different from those of a low-elevation timber stand. 
In order to illustrate the magnitude of error that can be introduced in this way, a 
situation almost identical to that shown in Fig. 14 has been reconstructed in Fig. 15 in 
the form of a three-dimensional model. In this model each simulated tree was constructed 
from a small staminate pine cone, mounted on a pointed steel pin from a photogram- 
metrist’s “lazy-daisy” kit (also termed a “mechanical triangulator”). When the left 
photograph shown in Fig. 15 was taken, Stand B was situated only 7/10 as far below 
the camera lens as Stand A. For this reason it is subject to the same misinterpretations 
as were described for Fig. 14. 
When Stand B is lowered to the same level as Stand A, (as in the right photo of 
Fig. 15) it is seen to occupy essentially the same area, have the same stand density. 
contain trees of the same size, and have the same total volume as Stand A.