Methods of Plane Table Surveying

Methods Of Plane Table: 

Methods of Plane tabling can be divided into four distinct heads:

1. Radiation
2. Intersection
3. Traversing
4. Resection

The first two methods are generally employed for locating the details while the other two methods are 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 that point , and the point is located by plotting to some scale the distance so measured. Evidently, the method is more suitable when the distances are small (with 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.

Intersection (Graphical Intersection): Intersection is resorted to when the distance between the point instrument station is either too large or cannot be measured accurately due to some field conditions. The location of an object is determined by sighting at the object from two plane table stations (previously) plotted and drawing the rays. The intersection of these rays will give the position of the object.

Traversing: Plane table traverse involves the same principles as a transit traverse. the method is widely used to lay down survey lines between the instrument stations of a closed or unclosed traverse. It is to be noted here that the orientation is to be done by back-sighting. If there are n stations in a closed traverse, the table will have to be set on at least (n-1) stations to know the error of closure through the traverse may be close even by setting it on (n-2) stations. At any section 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.

Resection: Resection is the process of determining the plotted position of the station occupied by the plane table, by means of sight taken towards known points, locations of which have been plotted.

The Resection methods are:

• Resection after orientation by compass.
• Resection after orientation by backsighting.
• Resection after orientation by three-point problem.
• Resection after orientation by two-point problem.