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Title
Transactions of the Symposium on Photo Interpretation

168
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962
lands that contain no roads or trails of any kind. The more mountainous the
terrain the more common is this situation. Even in our eastern forests located
near areas of highest population densities, occur areas requiring hours of foot
travel before work can start.
The density and character of the vegetation limits accessibility. There are
dense almost impenetrable new growth thickets of Douglas-fir in Oregon and
Washington, lodgepole pine in Montana and Ponderosa pine in Arizona and
New Mexico. In some places the down and lodged timber from old fires re
quires crawling under, while in others one can walk for hours on the down logs
without touching the ground. The chaparral of the Southwest often can be
penetrated only by crawling beneath the thorny shrub or by scrambling over
the top where it is very dense. The huge forested areas of Southeast Alaska are
accessible only by boat or plane and then a slow, laborious penetration of the
dense forest over centuries of down and rotting logs and through marshes and
swamps perched on very steep mountain slopes - a most difficult and time-
consuming operation. The open stand forests of the Central and Southern
Rocky Mountains are the most accessible.
Vegetation limits visibility. In the densest areas visibility is limited to a
relatively few feet. The accurate location and plotting of type boundaries is
an impossibility under extreme conditions. Type boundaries must be located
stereoscopically as discussed later.
Topography or terrain pose several accessibility limitations from the physical
point of view. These limitations can only be overcome by spotting field men on
mountain slopes or tops by helicopter and have them work downslope during
the course of the day.
The above limitations are ameliorated to the extent that roads and trails
are available for travel.
Soil survey involves essentially two operations: 1. Determining the kinds of
soils present and 2. Drawing boundaries on maps to show where the kinds of
soils occur. Soil surveyors have long made use of indicators or clues to locate soil
typelines. In general, these clues are 1. shape of the terrain or land forms, 2.
rock types, 3. drainage patterns, 4. vegetation types, etc. The validity of all
these indicators must be determined for each survey area because their relia
bility changes from place to place.
Soil surveys are never made in forested lands without stereo paired aerial
photos for plotting. Commonly two scales are desirable. The small scale photos
(1/60,000 or 1/70,000) are used to study stereoscopically the broad physical
characteristics of the area. This provides a working knowledge of the topo
graphy, geology, drainage, vegetation and present land use. The regular field
mapping is placed on larger photos having scales of 1/20,000 or 1/12,000. A
great deal of information is preplotted on the field photos such as roads, trails,
houses, power lines, etc. These help a great deal in determining one’s location in
the field.
Tentative office delineations of all soil type boundaries are made using those