features, as well as the larger topographic fares, such
as stream channels and swamps, can be observed
directly from aerial photographs. The photos also can be
used to prepare a base map for portrayal of the field data
by tracing planimetric details from an uncontrolled
mosaic with spot elevations added from field surveys.
The geologist may satisfactorily use contact prints of
aerial photographs in place of the base map except
where large-scale plans for engineering purposes are to
be the base. In such a case the distortion within an aerial
photograph does not permit plotting of geological data
commensurate with the accuracy of the final plan.
Map Base for Detailed Geological Surveys
Detailed geological surveys generally cover a
specific map area geographic region, or specified site
from scales of 1:62,500 to 1:600 or larger. In general,
the very large scales are used for specific engineering or
mineral development problems.
SITE PLANS AND PROPILES. Geological
data affecting foundation designs at construction sites
are plotted on plans drawn to scales of 1 inch = 50, 100,
200, or 400 feet. Contour intervals may range from 1 to
10 feet, depending upon the roughness of the terrain
Plane table mapping is suited to plotting the topographic
features, ranges, and reference points used to locate drill
holes, rock outcrops, and other geologic data. When
plotting contours on a 1- or 2-foot interval, you should
try to locate points that are actually on the contours or
to determine elevations at the intersection of closely
spaced grid lines staked out on the site. In addition to a
plan, the geologist may require that profiles be drawn
along selected lines or that the boring logs of test holes
be plotted to suitable scales.
USING A TOPOGRAPHIC MAP AS A BASE
MAP. The base map for a detailed geological survey
is a complete topographic map or plan with relief
expressed by contours. Simple colors and symbolization
of basic details are used so that they will not conflict
with the overlay of geological information that is shown
by colors and symbols. Published topographic maps are
used where suitable. The geological survey is expedited
if the map base is from a quarter to double the scale of
the map on which the information is to be presented.
Enlargements of the base map, rather than other maps
of a larger scale, are generally used to satisfy these
requirements. This permits the direct reduction of
geological data to the scale of the final map with a
minimum amount of drafting.
When no topographic map is available or if the
existing maps are not suitable, a base map or plan must
be prepared from detailed topographic surveys. Culture
and relief (contours) should be shown in the greatest
detail possible. The survey for the base should conform
to third-order accuracy where large geographic areas are
concerned. Maps made from aerial photographs by
precise instrument methods can be used in place of field
surveys. Altitude or elevation of the intersection of
boreholes and the surface should be accurate to the
nearest one-half foot.
Sometimes there is a requirement for pedological
mapping for the purpose of locating the limits of sand
or gravel deposits suitable for concrete aggregates, road
materials, or for other construction operations. In such
a case, the pedological survey conducted under the
direction of the soils engineer and the surveyors
mission would be one of support to the soils engineers
The engineers objective in a pedological survey is
to prepare data in plan and profile symbolizing soils and
outcropping on maps, overlays, and sketches for
subsequent engineering uses. The following approaches
may be used in conjunction with a soils survey
1. Aerial photography may be used when an
extensive area is to be surveyed. Usually no survey
measurements are required in this case.
2. Maps of an area that extend several square miles
are required when an initial study or technical
reconnaissance is needed for an engineering project.
Low-order survey measurements usually suffice for the
preparation of a reconnaissance sketch upon which the
soils engineer can plot the pertinent data.
3. A sketch of an airfield, for example, is frequently
required by the soils analyst before construction
planning can be initiated. In this case, the surveyor
applies low-order measurements to prepare a sketch
(1 inch = 100, 200, or 400 feet) upon which the soils
engineer plots the results of soil tests and findings.
Photo coverage of the area under consideration aids
in the establishment of control for the pedological
survey. The use of vertical aerial photographs in the
planning phase of outlining ground control will speed
the survey regardless of the size of the area to be