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

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.

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 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.

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, or plane-table traverse, starts from a known position and uses a continuous series of direction and distances to establish positions. This method of point location is illustrated in figure 9-7. After you set up and orient the plane table at the first station, you draw the direction to the next point on the survey with a radiating ray. The distance between the occupied station and the new point is measured and plotted along the ray. The new plotted position is now considered a known position and can be occupied and used as the next station on the line. The plane table is setup and oriented over this station and another radiating ray is drawn to the next point. This process continues for the length of the traverse. Orientation plays an important role in plane-table traverse. Slight errors in direction at each setup can accumulate rapidly and become large in a short time. Long traverses should be avoided except in reconnaissance surveys.