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ment
REPORT OF COMMISSION VII 31
war by various private foresters in the East, and by the Northeastern and
Central States Forest Experiment Stations. It is being tested in the Northwest
by the Pacific Northwest Forest and Range Experiment Station and by several
private companies.
Not all workers use the same techniques. However, many use some variation of
technique *a" above. In regions of rough topography, and particularly where some
kind of stand or forest type map is desired as one product of an inventory, the classifica-
tions are mapped on photos and transferred to base maps. This provides the base for
forest area determinations. In regions of gentle topography, however, and particularly
where a forest map is not a requirement of the survey, the classifications are merely esti-
mated from pinpoint samples, usually systematically spaced on the photo coverage. This
method is a less expensive one than the method involving mapping of classifications.
However, in regions of rough topography appreciable errors may occur in area estima-
tion by this method unless the technique is complicated by modifications which reduce
bias from photo image displacements to a minimum.
Comparative stereograms and photo-interpretation keys have proved helpful not
only on inventories where ocular estimates aided by stereograms are part of the pro-
cedure, but on other surveys as well. On any survey the interpreters can do a con-
sistently better job where aided by photo-interpretation illustrations keyed to familiar
ground detail.
Ground work has proved essential in developing photo techniques and training
new interpreters. Ideally some field work should be done by interpreters periodically as
new field conditions are encountered on a survey. The best interpretation can be done,
other factors being equal, by interpreters who are familiar with details of the area being
inventoried.
+
The above methods are by no means restricted to usage in the United States.
With variations, they have been and are being used in other countries. For
example, inventory work in Italy (conducted since that reported at the 1948
International Congress) has corresponded to method b(2). Dr. Duilio Cosma
reports that his work involved a preliminary photo classification according to
species and vegetation condition. Within the subdivisions, one-hectare plots
were analyzed and measured on the ground, to determine number of trees, tree
height, and crown cover. Establishment of correlations between ground measure-
ments and corresponding photo measurements provided the means for volume
determinations from photo measurements alone.
As another example, a modification of method "a" was used to make a
reconnaissance survey of Liberian forest resources (Mayer, 1951). This survey
was made by one technical forester; he was also an experienced photogram-
metrist. He compiled an uncontrolled aerial mosaic of the available 1:40,000
scale photographs, and prepared rough photo-maps of the major forest areas.
They were used in conjunction with the contact prints to establish the location
and areas of various forest conditions. By observation on the ground of species,
volumes, and other conditions, an inventory sample was made. Photo-interpreta-
tion enabled correlations between ground and photo conditions necessary to
the photo analysis of the extensive areas not traversed. The methods followed
in this survey of Liberia might well serve as patterns for reconnaissance surveys
of other undeveloped and inaccessible forest areas of the world.
The main consideration in this paper is the operation of interpretation,
which is more or less a dominant feature of any kind of a forest inventory.
Photography, photogrammetry in the stricter sense, surveying, and cartog-
raphy, are all related but independent functions. Forest interpreters have as
their main tools simple lens stereoscopes and colored pencils. They deal funda-
mentally with vegetation, so their first problem is the recognition of kinds of
