for rot, poor form, or other cull.
PHOTOGRAMMETRIC ENGINEERING
DETERMINATION OF PER-ACRE VOLUME
Few forest inventories rely on volume estimates made directly from aerial
photos. Although this has been the dream of some foresters and photogram-
metrists for nearly 30 years and has been the object of considerable study, most
inventory procedures use per-acre volumes computed from field plots.
Field volumes.—Vield plots of known area are selected from mapped
areas or from plots classified on photos. Field parties visit each plot and
make precise measurements of the trees. From these measurements, volumes
are computed and the per-acre average for each classification is determined.
The number of plots needed for a given statistical accuracy can be estimated
in advance of the field work and their distribution can be determined from
the previous photo classification. It is this control of the field work which
many foresters consider the most important contribution of photo inter-
pretation to forest inventories.
Although this selection of field samples by photo study is undoubtedly a
great advance over previous ground inventory methods, the emphasis, other
than in classification and mapping, is still on field procedures. The average
volumes are determined by field measurements, and the number of field
plots cannot be reduced below that needed to get reliable field volumes.
Furthermore, in remote and inaccessible areas where it would cost too much
to take field plots the method will not work. True aerial estimating requires
a technique of quantitative interpretation which will give average volumes
directly from measurements made on the aerial photos alone.
Photo volumes.—Per-acre volumes have been estimated directly from
aerial photos by (1) comparing the plots to be estimated with representative
stereograms whose per-acre volumes are known and (2) by measuring tree
or stand images and using an aerial volume table.
The comparison with representative stereograms showing plots of known
volume makes use of the ability of the interpreter to recognize and compare
minute likes or differences, even though he may not be able to measure them.
Although used successfully on a few estimates, this procedure is weak in
that the volume measured on the specific plots for which stereograms are
made is seldom an average for the class, and the infinite variation in forest
conditions may require a large number of stereograms. Furthermore, differ-
ences in specifications (focal length of the camera, flying height, or photo
base) between the photos studied and those used in preparing the stereo-
grams will cause like stands to appear radically different to the interpreter.
Aerial volume tables both for the individual tree and for the entire stand
have been developed and show great promise (Table 1). In general these
tables correlate average gross? board or cubic volumes obtained from field
measurements with measurable photo factors of total height, crown diam-
eter, and crown coverage or number of trees. Occasionally photo measure-
ments have been correlated with tree measurements made on the ground
and the final table worked out from forest yield tables.
Tree tables correlate individual tree gross volume with photo measure-
ments of total height and crown diameter. Since the desired per-acre volume
is a product of the tabular volume and a tree count, they are most usable
in scattered stands of mature trees and on large scale photos. In dense stands
or on small scale photos where tree counts are slow and very inaccurate,
tree tables are impractical.
*? Gross volume—the total board- or cubic-foot volume of the tree or stand without deductions
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