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REPORT OF COMMISSION VII 29
makes extensivé use of eight-inch-per-mile enlargements for soil surveys and
farm planning purposes. In California it recently mounted some mirror stereo-
scopes on extension arms, so that enlargements on which original field survey
data had been mapped could be viewed in three-dimensions. In addition to
aiding use of the photos in farm planning work, the stereoscopic view permits
correcting slope boundary lines and other details not discernible on the single
print. The coarseness of the enlargement severely limits the extent of interpreta-
tion possible, but the contact prints can always be inspected under a lens
stereoscope for study of detail. :
Photo-interpreters generally prefer to put their annotations directly on
the contact prints while studied under the stereoscope. While glossy prints
may be somewhat sharper and lead to more precise interpretation, they do not
take ordinary pencil or ink well. Grease pencil or pigment ink may be used
but are difficult to work with; colored pencils having a strong, easily sharpened
lead, are much better. Hence semimatte prints, which take colored pencil well,
and permit ready erasure, are most desirable. Most natural resource inventories
entail more or less field use of the photos, so the more durable double-weight
paper has a distinct advantage over single-weight.
2. FOREST INVENTORIES
Probably more persons are directly and indirectly doing aerial photo-inter-
pretation in connection with forest inventories than in any other field of re-
source inventorying. In government and private employ in the United States
alone, there are approximately 100 foresters whose main occupation centers
around the interpretation and use for forest inventory purposes. Many others
deal with photos occasionally or indirectly in matters related to forest invento-
ries. Estimates are not available on the numbers of forester-interpreters in other
countries, but Germany, France, England, Canada, the Scandinavian countries,
and others, have for many years been using photos in forest inventories. As
recently as 1950 the Italian Government started experimenting with photos in
the field of forestry. Relatively little has been done by the Latin-American or
the Asiatic or African countries.
'The basic objective of a forest inventory is to determine the volume of timber
on a given tract of land. Other objectives are to determine the location and area
of stands, their species composition, site quality, density, and size class or age
class. Other information may be desired, such as accessibility, determination
of virgin or cutover condition, amounts of insect and disease damage.
Forest inventories may be designed to cover hundreds of square miles, or
only a few acres. Each survey may have its own distinctive set of objectives,
and the techniques to be followed. It is not easy to describe the modus operandi
of such variable procedures, nor is it feasible to restrict such a description to
those which represent primarily advancements since 1948 when the last Inter-
national Congress was held in the Hague. The following comments are based
in part on published material, and also on replies to questionnaires sent to per-
sons having knowledge of the use of aerial photographs in natural resource
inventories.!
The main distinctions between different techniques of using photos to aid
forest inventories are in the extent to which needed information is obtained
from the photos or secured by field work. The most elementary function is a
photo classification which separates all lands into two categories: forest land
and nonforest land. The current federal forest survey in the southeastern part of
1 A list of persons who replied to questionnaires is given later.
