Different methods of Plane Table Survey - Plane Table Survey .

Methods of plane tabling can be divided into four distinctive heads:-

1. Radiation

2. Intersection

3. Traversing

The first two methods are generally employed for locating the details while other method is used for locating the plane table stations.

Radiation:-

In this method, a ray is drawn from the instrument station towards the point, the distance is measured between the instrument station and the points, and the point is located by plotting to some scale the distance so measured. Evidently, the method is more suitable when the distances ae small (within a tape length) and one single instrument can control the points to be detailed. The method has a wider scope if the distances are obtained tacheometrically with the help of telescopic alidade.

The following steps are necessary to locate the points from an instrument station T :

1. set the table at T, level it and transfer the point on the sheet by means of plumbing fork, thus getting point t representing T. Clamp the table.

2. Keep the alidade touching t and sight to A. Draw the ray along the fiducial edge of the alidade. Similarly, sight different points B,C,D,E etc., and draw the corresponding rays. A pin may be inserted at t, and the alidade may be kept touching the pin while sighting the points.

3. Measure TA,TB,TC,TD,TE etc., in the field and plot their distance to some scale along the corresponding rays, thus getting a, b, c, d, e, etc., join these if needed.

 Intersection:-

The method of intersection is mainly used for mapping details. If this is to be used for locating a point which will be used as subsequent plane table station, the point should be got by way of intersection of at least three or more rays. Triangles should be well conditioned and the angle of intersection of the rays should not be less than 45 degree in such cases. Graphic triangulation can also proceed without preliminary measurement of the base, as the length of the base line influences only the scale of plotting.

Following procedure to locate the points by the method of intersection:

1. Set the table at A, level it and transfer the point A on to the sheet by way of plumbing fork. clamp the table.

2. With the help of trough compass, mark the north direction on the sheet.

3. Pivoting the alidade about a, sight it to B. Measure AB and plot it along the ray to get b. The base line ab is thus drawn.

4. Pivoting the alidade about a, sight the details C, D, E etc., and draw corresponding ray's. 

5. Shift the table at B and set it there. Orient the table roughly by compass and finally by backsighting A.

6. Pivoting the alidade about b, sight the details C, D, E etc. and draw the corresponding ray's along the edge of the alidade to intersect with the previously drawn ray's in c, d, e etc. The positions of the points are thus mapped by way of intersection.

Traversing:-

In the case of radiation the observation are taken to those points which are to be detailed or mapped while in the case of traversing the observations are made to those points which will subsequently be used as instrument stations. The method is widely used to lay down survey lines between the instrument stations of a closed or unclosed traverse. 

Procedure:

1. Set the table at A. Use plumbing fork for transferring A on to the sheet.  Draw the direction of magnetic meridian with the help of trough compass. 

2. With the alidade pivoted about a, sight it to B and draw three ray. Measure AB and scale off ab to some scale. Similarly, draw a ray towards E, measure AE and plot e.

3. Shift the table to B and set it. Orient the table accurately by backsighting A. Clamp the table. 

4. Pivoting the alidade about b, sight to C. Measure BC and plot it on the drawn ray to the same scale. Similarly, the table can be set at other stations and its traverse is completed. 

It is to be noted here that the orientation is to be done by backsighting. 

If there are n stations in closed traverse, the table will have to be set on at least (n-1) stations to know the error of closure though the traverse may be closed even by setting it on (n-2) stations. 

At any station a portion of the traverse may be checked if two or more of the preceding stations are visible and are not in the same straight line with the station occupied.