will locate the desired point, P. This intersection isused to orient the plane table. A check on a fourthlocation will prove the location.TRACING-CLOTH METHOD.— Anothermethod you can use to plot the location of an unknownpoint from three known points is the tracing-clothmethod of resection. Figure 9-6 illustrates thismethod.In the figure, points a, b, and c are the plottedpositions of three corresponding known stations (A, B,and C). P is the point of unknown location over whichthe plane table is set. To plot the location of P youfirst place a piece of tracing paper (or clear plastic)over the map and select any convenient point on thepaper as P’. Then you draw rays from P’ toward thethree known stations. Next, you loosen the tracingpaper and shift it until the three rays pass through thecorresponding plotted points a, b, and c. Theintersection of the rays marks the location of P, whichcan be pricked through the tracing paper to locate thepoint on the map.POINT LOCATIONThe horizontal location of points can bedetermined by triangulation using the plane table. Anytwo points plotted on the plane-table sheet can act asa base for triangulation. A ray drawn from each ofthese points to some unknown point will form atriangle, with the distance between the two knownFigure 9-6.-Tracing-cloth method of resection.plotted points as the third side. The newly plottedposition of the third point will be at the intersection ofthe rays. The rays to the unknown point maybe drawnwhile occupying the known stations. This is calledintersection. The rays also may be drawn whileoccupying the unknown point, and this is known asresection.ResectionThe methods of resection were explained in thediscussion of plane-table orientation. As you know,when using resection methods it is unnecessary tooccupy known stations. While resection can be usedwith two known points, you should use mom than twopoints to determine the location of a point to a higherdegree of precision.IntersectionIntersection is accomplished by setting up andorienting the plane table at each of two or more knownstations in turn. At each station, the alidade is pointedtoward the unknown point, and a ray is drawn fromthe plotted position of the occupied station toward thepoint being plotted. As such rays are drawn from twoor more stations, their point of intersection is theplotted position of the required station. Two points arethe minimum requirement to establish a location. Formore accuracy, however, you should occupy three ormore points.RadiationIn plane-table surveys when intersection is used,a series of radiating rays are drawn and marked. Theserays all radiate from known stations. Points are locatedby drawing rays from one or more known stations. Theintersection of the rays determines the plotted locationof the desired points. When drawing rays, be sure toidentify clearly the object that each ray is being drawnto. This is important since an object viewed from onedirection may appear differently when viewed fromanother direction. This can lead to rays being drawnto the wrong object which will result in errors inplotting point locations.ProgressionProgression, or plane-table traverse, starts froma known position and uses a continuous series of9-5

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