drawing board or table is oriented. Orientation
consists of rotating the leveled table around its vertical
axis until the plotted information is in exactly the same
relationship as the data on the ground. There are
several methods of orienting the plane table. Some of
these methods are discussed below.
Backlighting
The usual method of orienting the plane table
is by backlighting. Using this method, you orient the
board by backlighting along an established line for
which the direction has previously been plotted. Fig-
ure 9-3 illustrates this method.
In figure 9-3, points a and b are the previously
plotted locations of points A and B on the ground.
First, you set up and level the table at point B. Then
you place the straightedge of the alidade along line ba
and rotate the table until the alidade is sighted on
point A. Once the alidade is sighted on A, the table is
clamped and the orientation is checked by sighting
on another visible and previously plotted point. The
direction to any other visible point can be plotted as a
ray from the plotted position of the occupied station.
Orientation by Compass
For rough mapping at a small scale, you can use a
magnetic compass to orient the plane table. If the
compass is fixed to the table, you orient by rotating
Figure 9-3.-Orientation by backlighting.
the table about its vertical axis until the established
bearing (usually magnetic north) is observed. If
the compass is attached to the alidade, you first place
the straightedge along a previously drawn line
that represents a north-south line. The table is then
oriented by rotating it until the compass needle points
north.
As you should recall from your study of the EA3
TRAMAN, you know that the earth’s magnetic field
and local attraction will greatly affect the pointing of
the compass needle. For these reasons, you should
avoid using the compass to orient the plane table when
orientation by backlighting can be accomplished.
Resection
Orienting a plane table by backlighting or by
compass requires occupying a station whose position
has been plotted. Resection, however, enables you to
orient the plane table without setting up at a previously
plotted station. This technique uses two or more
visible points whose positions are plotted on the plane
table. From these plotted points, rays are drawn back
toward the occupied but unplotted point.
TWO-POINT METHOD.— The two-point
method of resection is used to orient the plane table
and establish the position of a station when two
previously plotted points cannot be occupied. A
description of the two-point method is as follows:
In figure 9-4, A and B are visible, but inaccessible,
control points. Points a and b are the plotted positions
of A and B. The location of unplotted point C is
approximately estimated and marked c. D is a selected
Figure 9-4.—Two-point method of resection.
9-3
