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THREE-POINT  METHOD
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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