Plane table surveying

Plane table surveying

Principle

Parallelism i.e. the rays drawn from stations to object on paper are parallel to the lines from the stations to objects on the ground. 

Relative positions of the objects on the ground are represented by their plotted positions on the paper and lie on the respective rays. Table is placed at successive station. Graphical method, field work and plotting is done simultaneously (No field book). It is recommended for work where great accuracy is not required.

Accessories of plane table

1. Plane table 

Drawing board of well seasoned wood like teak is required. Top surface is leveled but lower surface has threaded circle plate for tripod.

2. Alidade

a. Plain alidade

It is metal or wooden ruler of length about 50 cm. One edge of the alidade is beveled and is called fiducial edge. It has vanes called object vane and sight vane.

b. Telescopic alidade:

It is used for clear view of the objects.

3. Spirit level:

It is a metal tube with spirit bubble used to level the plane table. Bubble is visible on top along graduated glass tube.

4. The compass:

There are two types of compass that can be used for plane tabling. They are trough campus and circular box compass.

a. Trough compass

It is a rectangular box with magnetic needle in it. The needle is marked 'D' on both ends.

b. Circular box compass

It consists of the pivoted magnetic needle. It has a square base plate and two bubble at 90 degree. It is used to make N – direction on the map.

 5. U-fork or plumbing for with plumb bob

It consists of U shaped metal strip which has equal arm length. Top arm is pointed to point on the plane table(centering) while buttom arm consists a hook to suspend the bob.