photograph, and figure 11-3 shows the line map
made by the use of overlays.
are all used for measuring unknown angles and
distances and/or for laying off known angles and
distances.
BASIC SURVEYING INSTRUMENTS
MAGNETIC COMPASS
Most fieldwork done by an Engineering Aid
(especially at the third- and second-class levels)
is likely to consist of field measurements and/or
computations that involve plane surveying of
ordinary precision. This section describes the basic
instruments, tools, and other equipment used for
this type of surveying. Other instruments used for
more precise surveys will also be described briefly.
Surveying instruments come in various forms,
yet their basic functions are similar; that is, they
A magnetic compass is a device consisting
principally of a circular compass card, usually
graduated in degrees, and a magnetic needle,
mounted and free to rotate on a pivot located at
the center of the card. The needle, when free from
any local attraction (caused by metal), lines itself
up with the local magnetic meridian as a result
of the attraction of the earth’s magnetic North
Pole.
Figure 11-3.-Line map made by overlays from the aerial photograph in figure 11-2.
11-12
