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Figure 13-31.-Kinds of angles measured at the occupied station. | Up
Engineering Aid 3 - Beginning Structural engineering guide book | Next
Figure 14-1.-Common types of bench mark construction and application. |
CHAPTER 14
DIRECT LEVELING AND BASIC
ENGINEERING SURVEYS
Leveling is an operation that is used for
determining the elevations of points or the
differences in elevation between points on the
earth’s surface. This operation is extremely
vital for deriving necessary data required
for various engineering designs, mapping,
and construction. Data from a finished level
survey are used to (1) design roads, highways,
and airfields; (2) develop maps, showing the
general configuration of the ground; (3) calculate
volume of earthwork; and (4) lay out construc-
tion projects.
In this chapter, we discuss the basic principles
of DIRECT LEVELING and the types of
methods used; the duties and responsibilities of
the leveling crew; field procedures used in
differential leveling; precision in leveling; and
proper ways of handling leveling instruments and
equipment. INDIRECT LEVELING, such as
barometric and trigonometric leveling, adjustment
of level network, and end areas and volume of
earth’s computations, is not covered in this
book.
In this chapter, you will find a general
description of basic engineering surveys and
various construction-site safety hazards commonly
associated with the EA survey party. Other
types of engineering and construction surveys—
particularly those for curves and earthwork—will
be presented at the EA2 level.
BASIC TERMS USED IN
LEVELING OPERATIONS
Generally, the basic vertical control for
topographic survey and mapping is derived from
first- and second-order leveling. For many
construction projects and for filling gaps between
second-order bench marks (BMs), less precise
third-order leveling is acceptable.
In leveling, a level reference surface, or datum,
is established, and an elevation is assigned to
it. This datum may be assigned an assumed
elevation, but true elevation is required for the
establishment of a BM. A series of properly
established BMs is therefore the framework of any
vertical control.
Although further discussion will follow,
fundamentally, direct leveling describes the
method of measuring vertical distances (dif-
ferences in elevation) between the plane of known
or assumed elevation (datum) and the plane of
a point whose elevation you are seeking. Once
these distances are known, they may be added
to, or subtracted from, the known or assumed
elevation to get the elevation of the desired
point. These vertical distances are obtained
by use of a leveling rod and, usually, an engineer’s
level.
Some of the basic terms commonly used in
leveling operations are defined in the following
paragraphs.
BENCH MARK
ABM is a relatively permanent object, natural
or artificial, bearing a marked point whose
elevation is known. BMs are established over an
area to serve as (1) starting points for leveling
operations so the topographic parties can
determine other unknown elevation points and
(2) reference marks during later construction
work. BMs are classified as PERMANENT or
TEMPORARY. Generally, BM is used to indicate
a permanent bench mark and TBM, to signify a
temporary bench mark. TBMs are established to
use for a particular job and are retained for the
duration of that job. Throughout the United
States, a series of BMs have been established by
various government agencies. These identification
markers are set in stone, iron pipe, or concrete
14-1
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