Locating right-of-way boundaries, as well as staking out fence lines, if necessary.
PART G - CONTROL SURVEYS
1-12. Control Types. Control surveys establish reference points and reference lines for detail surveys.
Control may be either horizontal or vertical.
a. Horizontal Control. Horizontal control is a basic framework of points in which the
horizontal position and interrelationship of have been accurately determined.
(1) Horizontal Control by Traversing. A surveying traverse is a sequence of lengths and
directions of lines between points on the earth, obtained by or from field measurements and used in
determining the positions of the points. A surveying traverse may determine the relative positions of the
points that it connects in a series.
(a) Closed Traverse. A closed traverse is one that ends at the point at which it began (see
Figure 1-4, page 1-16).
(b) Open Traverse. An open, or open-end, traverse is one that ends at a point other than
the one at which it began (see Figure 1-5, page 1-16).
(2) Horizontal Control by Triangulation. Triangulation is a method of surveying in which
the stations are points on the ground that are located in a series of triangles. The angles of the
triangulation net are measured by using instruments, and the lengths of the sides are derived by
computation from selected sides that are termed baselines-the lengths of which have been obtained from
precise direct measurements on the ground.
b. Vertical Control. Vertical control (also called elevation control) is a series of bench marks or
other points of known relative vertical position that are established throughout a project. In a
topographic survey, for example, a circuit of bench marks is established over an area at convenient
intervals (usually every half mile along a coordinate system on government property) to serve as starting
and closing points for leveling operations. They also serve as reference marks for grades and finished
floor elevations for structures in subsequent construction work. Since these bench marks will be needed
from time to time to establish other elevations, it is important that the work be accurately done so that
elevations referred to by one bench mark will check with those referred to by any other bench mark in
the circuit. The bench marks must be established in a definite point of more or less permanent character
so that they will not be disturbed.