Plane Table Surveying is a graphical method of survey in which the field observations and plotting are done simultaneously. It is simple and cheaper than theodolite survey. It is most suitable for small scale maps. The plan is drawn by the surveyor in the field, while the area to be surveyed is before his eyes. Therefore, there is no possibility of omitting the necessary measurements.
Equipments and Accessories for Plane Tabling
The following instruments are used in plane table surveying.
- Plane Table
- The plane table is mounted on a tripod
- The tripod is generally of open frame type, combined rigidity with lightness. The tripod may be made to fold for convenience of transportation.
- Tripod is provided with three foot screws at its top for levelling of the plane table.
- The alidade is useful for establishing a line of sight.
- Two Types of alidade are used.
- Simple alidade
- Telescopic alidade
- Trough Compass
- The trough compass is required for drawing the line showing magnetic meridian on the paper. It is used to orient the table to the magnetic meridian.
- When the freely suspended needle shows 0° at each end, a line is drawn on the drawing paper which represents the magnetic north.
- Spirit level
- A Spirit Level is used for ascertaining If the table is properly level.
- The Table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both positions.
- U-Fork with Plumb bob
- Water proof cover
- An umbrella is used to protect the drawing paper from rain.
- Drawing paper
- Drawing paper is used for plotting the ground details.
- Drawing accessories
Principle of Plane Table Survey
The principle of plane tabling is parallelism means,
Principle: “All the rays drawn through various details should pass through the survey station.”
The Position of plane table at each station must be identical, i.e. at each survey station the table must be oriented in the direction of magnetic north.
Method of Setting up The Plane Table
Three processes are involved in setting up the plane table over the station.
Levelling and Centering
- The Table should be set up at convenient height for working on the board, say about 1m. The legs of Tripod should be spread well apart and firmly into the ground.
- The table should be so placed over the station on the ground that the point plotted on the sheet corresponding to the station occupied should be exactly over the station on the ground. The operation is known as centering the plane table. It is done by U-fork and plumb bob.
- For levelling the table ordinary spirit level may be used. The table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both directions.
The Process by which the positions occupied by the board at various survey stations are kept parallel is known as the orientation. Thus, when a plane table is properly oriented, the lines on the board are parallel to the lines on ground which they represent.
There are two methods of orientation
- By magnetic needle
- In this method, the magnetic north is drawn on paper at a particular station. At the next station, the trough compass is placed along the line of magnetic north and the table is turned in such a way that the ends of magnetic needle are opposite to zeros of the scale. The board is then fixed in position by clamps. This method is inaccurate in the since that the results are likely to be affected by the local attraction.
- By back sighting
- A= First survey station
- B= Second survey station
- Suppose a line is drawn from station A on paper as ab, representing line AB on ground
- The table is turned till the line of sight bisects the ranging rod at A. The board is then clamped in this position.
- This method is better than the previous one and it gives perfect orientation.