Backlighting

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