should check and clear parallax before the first
sighting and should not readjust it until all
sightings from the setup are complete.
4. Rod improperly plumbed. This error is
caused by a rodman who does not pay attention
to his work. The instrumentman can call attention
to plumbing if it is at a right angle to his line of
sight, but he cannot see it in the direction of line
of sight. The use of a rod level or waving the rod
will avoid this error.
5. Unstable object used for a TP. The rodman
causes this error by selecting a poor point of
support, such as loose rocks or soft ground. As
the rod is turned between sights, the weight of the
rod can shift a loose rock or sink into soft ground.
The elevation of the TP as used for the next BS
can change appreciably from the value that had
been computed from the previous FS. This error
can be avoided by using the turning pin or pedestal
when the ground does not present solid points.
6. Rod length erroneous. This error results in
either too long or too short rod readings at each
point. In a survey predominately over slopes, this
error will accumulate. The rod length should be
checked with a steel tape at intervals to locate this
7. Unbalanced BS and FS distances. The
unbalanced distances do not cause the error. It
is caused by the effect on the line of sight from
residual adjustment and leveling errors and the
effect of curvature and refraction errors. Readings
you take at a long distance will have a greater
error than those at a short distance. This
unbalance may not be critical on one setup but
can be compounded into a considerable error if
the unbalance continues over several setups. By
balancing the sight distances at each instrument
setup, if possible, and the sums of the BS and FS
distances at every opportunity, you will keep these
errors to a minimum.
8. Earths curvature. This produces an error
only on unbalanced sights in leveling. When the
BS distances are constantly greater than FS
distances, or vice versa, a greater systematic
error results, especially when the sights are long.
To eliminate this error, you must maintain a
balanced sight distance in every BS and FS
reading, not just their sum total between BMs (the
error varies directly as the square of the distance
from the instrument to the rod).
9. Atmospheric refraction. This error also
varies as the square of the distance but opposite
in sign ( + or ) to that caused by the earths
curvature. The effect of atmospheric refraction
is only one-seventh of that caused by the earths
curvature. In first- and second-order leveling, the
effect of refraction is minimized by taking the BS
and FS readings in quick succession and avoiding
readings near the ground. (They should be taken
at least 2 ft from the ground.)
10. Variation in temperature. If a portion of
the telescope is shaded and some parts are exposed
to the suns rays, it produces some warping effect
on the instrument that may affect its line of sight.
This effect is negligible in ordinary leveling; but
in leveling of higher precision, this effect may
produce appreciable error. This is one of the
reasons why surveyors use an umbrella to shield
the instrument when doing more refined work.
BASIC ENGINEERING SURVEYS
AND CONSTRUCTION SITE SAFETY
An engineering survey forms the first of a
chain of activities that will ultimately lead to a
completed structure of some kind, such as a
building, a bridge, or a highway. An engineering
survey is usually subdivided into a DESIGN-
DATA SURVEY and a CONSTRUCTION
This section discusses the basic engineering
surveys commonly performed by an EA survey
party in support of military construction activities.
In addition, various types of occupational hazards
relating to specific surveying operation are also
presented in this section together with the
precautions or applicable abatement procedures
that must be carried out to deter injury to the
survey crew and/or damage to surveying equip-
ment or material.
Surveys for roads and streets involve both field
work and office work. The extent of each type
of work depends on the magnitude and complexity
of the job. Some phases of the work may be done
either in the field or in the office, and the
decision as to the exact procedures to be followed
will be influenced by the number of personnel
available and by the experience and capabilities
of the individuals involved.
This type of survey is conducted for the
purpose of obtaining information that is essential
for planning an engineering project or develop-
ment and estimating its cost. A typical design-data