investiga-
hoto inter-
ig airfield
le a means
the general
rrain con-
ipling and
\s data are
the photo
Its beyond
, and lines
he ground.
overlooked
problems.
ne terrain
associated
conditions
)ts permits
air photos
avoid the
ralues and
wn airfield,
s on these
of the site
ent costly
| in some
ntified or
ic, hydro-
he dimen-
in. Photo-
landforms
of micro-
ditions on
k and soil
(647)
grading required may be possible by photogeologic interpretation in some
situations, but in others, particularly in some areas of residual soil, an estimate
from air photos alone would be highly tentative.
Approaches are determined by the position and height of obstructions
(topography, trees, or others) relative to the possible runway alignments.
Measurements can be made photogrammetrically or roughly estimated by
measuring shadows or by comparison with known or inferred heights of houses,
characteristic trees, or other objects. Obstructions must be appraised not only
in regard to possible blocking of aircraft approaches to runways but also to
their possible interference with radar control that is, the casting of *radar
shadows."
Subgrades and foundations are determined by the soils and bedrock con-
ditions. A large literature exists on methods of photogeologic and soils inter-
pretation and considerable success in this field has been achieved. Geologic and
topographic setting, geography and climate, ecologic relationships, and the
texture, color, and tone of the photographic print all enter into the interpreta-
tions. In some situations a study of burned areas or other exposed ground can
provide an estimate of erosion problems to be expected after clearing.
Surface drainage features are readily noted on air photos. In site selection,
flooding possibilities are an important consideration, and topographic position
may be a clue. For example, a site on a low flood plain among meander scars
may be suspect, whereas one on a high terrace may reasonably be expected to
be above flood levels. Photographs taken during times of flood are an effective
means of rapid delineation of flood-free areas. Many subsurface drainage con-
ditions, also, can be interpreted by air photos. Areas of poor drainage commonly
show up as prominent darker patches. As noted before, patterns of subsurface
drainage and wet soils sometimes can be seen more readily on air photos than
on the ground.
Clearing chiefly involves vegetation. The size of areas requiring clearing
can be computed quickly from air photos. Botanical identification and knowl-
edge of plant habits can provide estimates of the difficulty of clearing and
grubbing. The white spruce of Alaska, for example, has a shallow root system
allowing it to grow where permafrost occurs at a shallow depth. Identification
of this situation would lead to the estimate that clearing and grubbing could be
easily and rapidly accomplished by bulldozer.
Natural construction materials must be located close to the construction
site. Sand bars and rock exposures, sources of such materials, generally show
up prominently on air photos. Photogeologic interpretation can turn up many
other hidden sources of construction material; in regions covered by glacial
ground moraine, as in the north-central United States, some of the best gravel
deposits are located in eskers, which are low ridges of distinctive shape easily
identified on air photos. By tracing rock beds between outcrops, deposits might
be located that would make more convenient quarry sites than the more obvious
exposures. Ecologic interpretation and other photogeologic techniques may even
identify the type of rock available.
Water is necessary not only for construction but for the permanent supply
of the airfield installations as well. An estimate may be possible as to the ade-
quacy of water supply available in nearby surface-water bodies. Appearance of
the drainage pattern, banks, channels, and other drainage aspects can be a clue
