or annotations. As an amplification of the map data,
more complete descriptions of outcrops are entered in
notebooks with the entries keyed to the field map.
Surveyors support the geologist by preparing basic
topographic maps on which they plot the results of
geological investigations and then make such tie
measurements to geological features as the geologist
may require.
The geologist uses simple survey methods in
plotting geological features on a field map. Where an
outcrop can be located with reference to a cultural or
relief feature, it is generally plotted on a map by spot
recognition. In other cases, the relationship of a
geological feature to a recognizable topographic feature
is established by using a magnetic compass to determine
direction and by pacing or taping to measure distance.
Slope or small differences in elevation are measured by
using a clinometer or hand level, while an altimeter is
used where there are large differences in elevation.
When the geological survey is keyed to a large-scale
plan, the geologist generally uses a plane table and plots
data with accuracy commensurate with the accuracy of
the base plan.
Base Map Surveys
The survey for the base map should normally take
place before the geological survey, because the
geologist uses the map in the field to plot his data and
to determine his position by identification of
topographic details. If aerial photographs are available,
the base map need not be made before the geological
survey since the geologist can use the aerial photograph
as a plotting base and later transfer the data to a base
map. However, if possible, the base map should be
prepared in advance, even in this case, as the number of
aerial photographs needed to cover an area is generally
too large to be handled in the field.
Plane table topography is the method best suited to
relatively open country. In the absence of detailed
instructions, the following specifications are generally
satisfactory:
1. BASE DIRECTION. To determine a base
direction, take from a known base a side in a
triangulation net or a course of a basic control traverse.
2. LOCAL HORIZONTAL CONTROL. Use
plane table traverses run in closed circuits or between
known control stations of a higher order of accuracy or
locate plane table stations by graphical triangulation.
3. LOCAL VERTICAL CONTROL. Where the
terrain is relatively level, carry elevation along traverses
by vertical angle or stadia-arc measurements, adjusting
elevations on closure at a basic control station. For
rugged terrain mapped at one of the larger contour
intervals, barometric or trigonometric leveling is
suitable.
4. SIGHTS. Use telescopic alidade.
5. DISTANCE MEASUREMENTS. Use, in
general, stadid or graphical triangulation to locate points
and stations. Certain measurements can be made most
conveniently by pacing or rough taping.
6. CONTOURING. Locate and determine the
elevations of controlling points on summits, in valleys
and saddles, and at points of marked change of slope.
Interpolate and sketch contours in the field, using these
elevations for control.
7. ACCURACY. Distance measurements by stadia
should be accurate to 1 part in 500. Side-shot points
located by pacing or other rough measurements should
be accurate to within 25 feet. Take sights for traverse
lines or graphical triangulation with care to obtain the
maximum accuracy inherent in the telescopic alidade.
The error in the elevation of any point, as read from the
finished map, should not exceed one half of the contour
interval.
Topography may be located more conveniently in
heavily timbered country by stadia measurements from
transit-stadia traverse than by the use of the plane table,
although the time required for plotting will be increased.
The specifications listed above are generally applicable.
Read horizontal angles on traverses to 1 minute and
horizontal angles for side shots that will be plotted by
protractor to the nearest quarter of a degree. Read
vertical angles for elevation determination to 1 minute
or use the stadia arc. Keep complete and carefully
prepared stadia notes and sketches to assure correct
plotting.
When the geologist indicates that a map of a lower
order of accuracy will fulfill his needs, plane table or
compass traverses are suitable.
Use of Aerial Photographs
If aerial photographs are available, the geologist
generally uses them instead of a map. The most
satisfactory results are obtained from large-scale
photographs, 1:15,000 or larger. Some topographic
features, such as some ravines, rocky knobs, or
sinkholes, are too small to be shown on maps. These
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