angle. To compute the HI, the rod reading RB and the
DE are added to the elevation of B, or
HI = RB + DE + Elev. B.
2. DEPRESSION ANGLE FORESIGHT (fig, 7-3,
view B). The rod is below the instrument, and the
vertical angle is minus. The elevation at C equals the HI
minus the DE and minus the rod reading RC, or
Elev. C = HI – DE – RC.
3. ELEVATION ANGLE BACKSIGHT (fig. 7-3,
view C). The rod is above the instrument, and the
vertical angle is plus. The HI at F equals the elevation
at C plus the rod reading (RC) and minus the DE, or
HI = Elev. C + RC – DE.
4. ELEVATION ANGLE FORESIGHT (fig. 7-3,
view D). The rod is above the instrument and the angle
is plus. The elevation of G equals the HI plus the DE and
minus the rod reading (RG), or
Elev. G = HI + DE – RG.
As mentioned earlier in this section, the horizontal
or slope distances used for calculating the DE may be
obtained using various methods. For each method, there
are requirements and limitations that must be adhered
to. These requirements and limitations are discussed as
follows:
1. Measured distances obtained by horizontal
chaining should be corrected for standard error,
temperature, and sag before you compute the DE. These
corrections are discussed in chapter 12 of the EA3
TRAMAN. Under ordinary circumstances in the
Seabees, corrections for earth curvature and refraction
are not necessary. However, methods to perform these
corrections can be found in commercial publications,
such as Surveying Theory and Practice, by Davis,
Foote, Anderson, and Mikhail.
2. Measured distances obtained by slope chaining
also should be corrected as discussed above. In addition,
you must convert the slope distance to a horizontal
distance before computing the DE. As an aid in
computations, tables have been developed that provide
the following data:
a. Inclination corrections for 100-foot tape
b. Differences in elevation forgiven horizontal
distances and gradients from 0° to 45°
c. Differences in elevation for given slope
distances and gradients from 0° to 45°
d. Horizontal distances for given slope
distances and gradients from 0° to 45°
3. When using stadia, you should refer to the stadia
procedures and formulas described in chapter 8 of this
TRAMAN. With practice, stadia provides a rapid means
of determining the horizontal distances and elevations.
4. Electronic distance-measuring devices measure
the straight-line horizontal or slope distance between
instruments. When you use the same setup for slopes,
replace the electronic equipment with a theodolite and
either a target or a rod to measure the vertical angle. The
measured vertical angle can be used to convert the
measured slope distance to DE by multiplying by the
sine of the vertical angle.
LEVEL AND TRAVERSE
COMPUTATIONS
In this section we provide information on
procedures used in making level and traverse
computations. We also discuss methods of differential
leveling, including steps to follow in checking level
notes. Coverage includes information on adjusting
intermediate bench marks as well as a level net. In
addition, we describe several methods of plotting
horizontal control that may be used in determining the
bearing of the traverses. These methods include plotting
angles by protractor and scale, plotting angles from
tangents, and plotting by coordinates. We point out some
of the common types of mistakes that the EA may
encounter in making or checking computations, and we
provide some information about locating mistakes.
PRELIMINARIES TO COMPUTATIONS
Before computations are started, a close check on
the field data for completeness and accuracy is required.
This includes checking the field notes to ensure that they
accurately reflect what was actually measured; for
example, a deflection-angle note 79°01'R must be
checked to be sure that the angle actually measured
79°01' (by ascertaining that the sum of the angle and the
closing angle is 360° or within allowable differences)
and to ensure that the angle was actually turned to the
right.
A field measurement may itself require
transformation (called reduction) before it can be
applied as a value in computations; for example, field
notes may show plate readings for two-, four-, or
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