Figure 7-1.—Diagram of a two-base altimeter survey.
communication, a timepiece is needed for each
altimeter. These timepieces are synchronized, and the
altimeter readings are taken at prearranged intervals.
Figure 7-1 shows a diagram of the two-base method
when three altimeters are used. This figure shows the
known elevations of the lower (Sta. A) and upper (Sta.
B) base stations. Altimeter readings at each of the base
stations and at field station C are also shown. The
difference in elevation is computed by direct proportion,
using either the lower base or the upper base as
reference. For example, to find the differences in
elevation between Sta. A and Sta. C, we proceed as
follows:
Then this result is added to the elevation of Sta. A, as
shown in solution No. 1, figure 7-1. If we use the upper
base as a reference, you compute the difference in
elevation by using the same method; but to compute
from Sta. B, subtract the result, as shown in solution No.
2, figure 7-1.
For a more accurate result, altimeter surveys should
be made on days when there is not much variation in
barometric pressure. Windy days when detached clouds
are traveling rapidly should be avoided because
alternating sunlight and shade over the survey area can
cause fluctuations in the altimeter reading. Steady
barometric pressures generally occur on days with
gentle winds and an overcast sky. The recommended
time for observations is 2 to 4 hours after sunrise and 2
to 4 hours before sunset. Midday observation must be
avoided if possible. Remember, you must shade the
instrument at all times, and you must avoid jarring the
instrument suddenly during its transfer from one station
to another.
TRIGONOMETRIC LEVELING
When you know the vertical angle and either the
horizontal or slope distance between two points, you can
apply the fundamentals of trigonometry to calculate the
difference in elevation between the points. That is the
basic principle of trigonometric leveling. This method
of indirect leveling is particularly adaptable to rough,
uneven terrain where direct leveling methods are
impracticable or too time consuming. As in any survey,
the equipment that you will use in trigonometric leveling
depends on the precision required. For most
trigonometric-leveling surveys of ordinary precision,
angles are measured with a transit, or alidade, and
distances are measured either with a tape or by stadia,
which you will study in chapter 8. On reconnaissance
surveys the vertical angles may be measured with a
clinometer, and distances maybe obtained by pacing.
The method used in trigonometric leveling is
described in the following paragraphs:
7-2
