POINT LOCATION - 14070

VALUES OF PLANE-TABLE METHOD - 14070_180

Engineering Aid 1 - Advanced Structural engineering guide book
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will locate the desired point, P. This intersection is used to orient the plane table. A check on a fourth location will prove the location. TRACING-CLOTH METHOD.— Another method you can use to plot the location of an unknown point from three known points is the tracing-cloth method of resection. Figure 9-6 illustrates this method. In the figure, points a, b, and c are the plotted positions of three corresponding known stations (A, B, and C). P is the point of unknown location over which the plane table is set. To plot the location of P you first place a piece of tracing paper (or clear plastic) over the map and select any convenient point on the paper as P’. Then you draw rays from P’ toward the three known stations. Next, you loosen the tracing paper and shift it until the three rays pass through the corresponding plotted points a, b, and c. The intersection of the rays marks the location of P, which can be pricked through the tracing paper to locate the point on the map. POINT LOCATION The horizontal location of points can be determined by triangulation using the plane table. Any two points plotted on the plane-table sheet can act as a base for triangulation. A ray drawn from each of these points to some unknown point will form a triangle, with the distance between the two known Figure 9-6.-Tracing-cloth method of resection. plotted points as the third side. The newly plotted position of the third point will be at the intersection of the rays. The rays to the unknown point maybe drawn while occupying the known stations. This is called intersection. The rays also may be drawn while occupying the unknown point, and this is known as resection. Resection The methods of resection were explained in the discussion of plane-table orientation. As you know, when using resection methods it is unnecessary to occupy known stations. While resection can be used with two known points, you should use mom than two points to determine the location of a point to a higher degree of precision. Intersection Intersection is accomplished by setting up and orienting the plane table at each of two or more known stations in turn. At each station, the alidade is pointed toward the unknown point, and a ray is drawn from the plotted position of the occupied station toward the point being plotted. As such rays are drawn from two or more stations, their point of intersection is the plotted position of the required station. Two points are the minimum requirement to establish a location. For more accuracy, however, you should occupy three or more points. Radiation In plane-table surveys when intersection is used, a series of radiating rays are drawn and marked. These rays all radiate from known stations. Points are located by drawing rays from one or more known stations. The intersection of the rays determines the plotted location of the desired points. When drawing rays, be sure to identify clearly the object that each ray is being drawn to. This is important since an object viewed from one direction may appear differently when viewed from another direction. This can lead to rays being drawn to the wrong object which will result in errors in plotting point locations. Progression Progression, or plane-table traverse, starts from a known position and uses a continuous series of 9-5

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