Understanding Plane Table Surveying

Plane table surveying is a graphical method of surveying. It involves carrying out the fieldwork and the plotting simultaneously. This means that the survey map is prepared in the field while the survey is in progress.

What Instruments are Used in Plane Table Surveying?

The core equipment used in plane table surveying includes:

• Plane Table: This is the main instrument. It's essentially a drawing board mounted on a tripod. The survey paper is fixed on this board. The board needs to be level and oriented correctly at each station.
• Alidade: An alidade is a straight edge ruler with sights. It is used for sighting objects and drawing rays on the survey paper. The line of sight should pass through the object and the fiducial edge of the alidade.
• Plumbing Fork and Plumb Bob: Used for centering the table exactly over the ground station mark.
• Spirit Level: Used for leveling the plane table board horizontally.
• Trough Compass: A trough compass is used to orient the plane table by aligning the magnetic meridian with a line drawn on the paper. This is crucial for maintaining consistent orientation throughout the survey.

Method of Plane Table Surveying

In plane table surveying, the surveyor sets up the table at a station, levels and centers it, and then orients it using the compass or by sighting a known point. Objects are sighted with the alidade, and rays are drawn on the paper. Distances are measured (often using chain or tape, though stadia methods can also be used) and plotted to scale along the rays. This process is repeated from different stations, connecting the points directly on the map.

Therefore, plane table surveying fundamentally involves using a plane table along with instruments like an alidade, and importantly, a compass for orientation, among other accessories.