. their locations are transferred to photos. The plots are then judged to be forest
30 REPORT OF COMMISSION VII
the United States goes only that far with photos, primarily because available
photography is not new enough to show current conditions of forest stands in
a region where changes occur rapidly. Sample plots are located on maps, and
or nonforest by examination of the tone and texture characteristics shown on a
single print. (Stereoscopic viewing is not used.) The percentage of plots falling
in each category is used to determine acreage of the two broad classes. Certain
of the plots classified as forest land are then examined and measured in the
field, to obtain statistical samples of desired data on volumes, species, growth,
etc.
Other U. S. inventory methods have been rather neatly summarized by |
Mr. R. C. Wilson? as follows:
Specific photo-interpretation techniques used in making forest inventories are
varied, but mostly fall in the following categories:
a. Classification and stratification of stand-size classes and forest types and. subsequent |
field inventory. |
Here interpretation is mainly based on qualitative stereosopic evaluation of |
photo factors of shape, size, tone, texture and shadows, with checks occasionally
by quantitative evaluation of the measurable photo factors. The classifications |
are often mapped out and delineated directly on photos and eventually these |
boundaries are transferred to maps. The classification then is used to control field |
inventory plots. In some surveys the classifications are merely noted at pin-point |
locations on photos to provide a photo sample for estimating the area of the |
various classification categories. These are used to control a system of field
inventory plots. This method has been termed the "triple" or "double" sampling
method of survey, since a fraction of the pinpoints classified on the photos are |
inspected or cruised in the field to provide correction factors to apply to the
photo classification and to provide more detailed data than may be obtained by
photo-interpretation alone.
b. Estimating volume directly from aerial photos
(1) Ocular estimates with stereograms. This method requires ocular estimates
of volume by stereoscopic study on a large number of photo plots through com-
parisons with a series of stereograms for which field volumes are kown. It proved |
accurate on several projects where the factors of photography (time of day, scale, |
etc.) were constant. It was successfully used by Spurr in 1945 for a survey at
Amherst, Maine, also by. the Central States Forest Experiment Station, and in a
limited way on a private timber survey in Washington in 1949.
(2) Tree aerial volume tables. These are similar to conventional volume
tables based on total tree height and d.b.h. except measurement of crown diam-
eter and correlation with stem diameter is substituted for d.b.h. measurement.
Harvard Forest and the U. S. Forest Service cooperated in 1945 in developing a
table used for white pine in the northeast. Other tables have been made and
applied in other parts of the East and South for other species. More tests must
be made before full applications and limitations of this method can be deter-
mined for a variety of U. S. species and types. It may be mainly applied in open
stands and over gentle topography.
(3) Stand-aerial volume tables. This, the most promising method of photo
volume estimation, has seldom been used independently of ground inventories.
For even-aged stands variables of density, total height, or average crown diam-
eter, if measured carefully on photos, often give good estimates of gross volume.
Most successful application has been coordinated with a few field plots which
precisely define size classes and defect and breakage. Field plots are used to
adjust gross volume estimates from photos. The method has been used since the
? Forest Economist, U. S. Forest Service, Pacific Northwest Forest and Range Experiment
Station, Portland, Oregon. Quoted from reply to questionnaire. Unpublished.
