drawing board or table is oriented. Orientationconsists of rotating the leveled table around its verticalaxis until the plotted information is in exactly the samerelationship as the data on the ground. There areseveral methods of orienting the plane table. Some ofthese methods are discussed below.BacklightingThe usual method of orienting the plane tableis by backlighting. Using this method, you orient theboard by backlighting along an established line forwhich the direction has previously been plotted. Fig-ure 9-3 illustrates this method.In figure 9-3, points a and b are the previouslyplotted locations of points A and B on the ground.First, you set up and level the table at point B. Thenyou place the straightedge of the alidade along line baand rotate the table until the alidade is sighted onpoint A. Once the alidade is sighted on A, the table isclamped and the orientation is checked by sightingon another visible and previously plotted point. Thedirection to any other visible point can be plotted as aray from the plotted position of the occupied station.Orientation by CompassFor rough mapping at a small scale, you can use amagnetic compass to orient the plane table. If thecompass is fixed to the table, you orient by rotatingFigure 9-3.-Orientation by backlighting.the table about its vertical axis until the establishedbearing (usually magnetic north) is observed. Ifthe compass is attached to the alidade, you first placethe straightedge along a previously drawn linethat represents a north-south line. The table is thenoriented by rotating it until the compass needle pointsnorth.As you should recall from your study of the EA3TRAMAN, you know that the earth’s magnetic fieldand local attraction will greatly affect the pointing ofthe compass needle. For these reasons, you shouldavoid using the compass to orient the plane table whenorientation by backlighting can be accomplished.ResectionOrienting a plane table by backlighting or bycompass requires occupying a station whose positionhas been plotted. Resection, however, enables you toorient the plane table without setting up at a previouslyplotted station. This technique uses two or morevisible points whose positions are plotted on the planetable. From these plotted points, rays are drawn backtoward the occupied but unplotted point.TWO-POINT METHOD.— The two-pointmethod of resection is used to orient the plane tableand establish the position of a station when twopreviously plotted points cannot be occupied. Adescription 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 positionsof A and B. The location of unplotted point C isapproximately estimated and marked c.D is a selectedFigure 9-4.—Two-point method of resection.9-3
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