The primary traverse ABCD runs around the
perimeter of the tract. Were this tract sufficiently small
and level, then details within the whole tract could be
located from only the primary control points; that is,
from stations A, A1, B, B1, C, C1, D, and D1. In this case,
however, the size (or perhaps the character) of the terrain
made it necessary to establish additional control points
within the perimeter of the tract, such as D2, A2, and B2.
These stations were established by running traverse
lines (called crossties) across the area from one primary
traverse station to another. It should be noted that, since
each secondary traverse closes on a primary control
point, errors cannot accumulate any farther than the
distance between the primary stations.
Field notes for the survey sketched in figure 8-1
must contain (1) notes showing the horizontal locations
of the stations and (2) level notes for determining the
elevations of the stations.
Vertical Control
In topographic surveying, bench marks serve as
starting and closing points for the leveling operations
when you are locating details. Although for some
surveys the datum may be assumed, it is preferable that
all elevations be tied to bench marks which are referred
to the sea-level datum. In many areas, particularly in the
United States, series of permanent and precisely
established bench marks are available. As a surveyor,
you must make every feasible effort to tie in your
surveys to these bench marks to ensure proper location
and identification. Often, the established horizontal
control marks are used as the bench marks because the
level routes generally follow the traverse lines.
Vertical control is usually carried out by direct
leveling; however, trigonometric leveling may be used
for a limited area or in rough terrain. When you establish
the primary vertical control to use in a topographic
survey for an intermediate-scale map, four degrees of
precision are used as follows:
1. 0.05 foot ~ distance in miles. This order is used
as the standard for surveys in flat regions when the
contour interval is 1 foot or less. It is also used on
surveys that require the determination of the gradient of
streams or to establish the grades for proposed drainage
and irrigation systems.
2. 0.1 foot i distance in miles. This order is used
in a survey when the contour interval of the map is 2
feet.
3. 0.3 foot ~ distance in miles. This order is used
for a contour interval of 5 feet.
4. 0.5 foot i distance in miles. This order is used
for a contour interval of 10 feet and may be done by
stadia leveling, a method that is very advantageous in
hilly terrain. Stadia will be discussed later in this
chapter.
You use the third or fourth orders of precision for a
large-scale map that generally has a contour interval of
1 or 2 feet. For an extensive survey of a large area, use
the third order; for surveys of a smaller area, use the
fourth order.
Once the topographic control has been established,
your next major step in a topographic survey is to locate
the details horizontally and vertically in the vicinity of
each control point or station. These details consist of
(1) all natural or artificial features that will appear on
the map and (2) enough ground points and spot
elevations to make the drawing of contour lines
possible.
The methods and the instruments used in
topographic surveys depend upon the purpose of the
survey, the degree of precision needed, the nature of the
terrain to be covered, the map scale, and the contour
interval. For a high degree of accuracy, you should
locate azimuths with a theodolite or transit. Measure
horizontal distances with the chain or the electronic
distance measurement (EDM) device. Determine
elevations with a level.
The following sections discuss two methods that are
commonly used to locate topographic details. A third
method (topography by plane table) is discussed in the
next chapter of this TRAMAN.
LOCATING DETAILS BY
TRANSIT AND TAPE
In the EA3 TRAMAN you studied the procedures
used to tie in and locate points, using a transit and tape.
These same procedures are used for tying in and locating
topographic details. Determine the vertical location (or
elevation) of the detail points, using direct or
trigonometric leveling procedures. Horizontally locate
the details either by directions or distances or a
combination of both. Use a method, or a combination of
methods, that requires the least time in a particular
situation. Directly measure the dimensions of structures,
such as buildings, with tapes. When details are
numerous, assign each one a number in the sketch and
key the detail to a legend of some kind to avoid
overcrowding. For directions, use azimuths instead of
deflection angles to minimize confusion. Locate details
as follows:
8-2
