Aquaculture Engineering (2+1)

Unit 1- Surveying

Plane table surveying

Working operations
Three operations in plane table surveying

• Fixing : Fixing the table to the tripod
• Setting : i) Levelling the table ii) Centring iii) Orientation
• Sighting the points
  

Fixing
Fix the plane table to the tripod properly

Setting
Levelling
for small scale work, levelling is done by estimation. For work of accuracy an 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.

Centring
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 centring the plane table.

Orientation
Orientation is the process of putting the plane-table into some fixed direction so that a line representing a certain direction on the plane is parallel to the direction on the ground. This is the essential condition to be fulfilled when more than one instrument station is to be used. If orientation is not done, the table will not be parallel to itself at different positions resulting in an overall distortion of the map. The process of centring and orientation dependent on each other. For exact work the centirng and orientation is repeat until the work is accurate.

There are two main methods of orientation of the plane table

• Orientation by means of trough compass.
• Orientation by means of back sighting.