Aquaculture Engineering

Plane table surveying - Instruments - II

Unit 1- Surveying

Plane table surveying
Instruments used for plane table surveying

Spirit level
A small spirit level may be used for ascertaining if the table is properly level. The level may be either of the tabular variety or of the circular type essentially with a flat base so that it can be laid on the table and is truly level when the bubble is central. The table is levelled by placing the level on the board in two positions at right angle and getting the bubble central in both positions.

Compass
The compass is used for orienting the plane table to magnetic north. The compass used with a plane table is a trough compass in which the longer sides of the trough are parallel and flat so that either side can be used as a ruler or laid down to coincide with a straight line drawn on the paper.

Drawing paper
The drawing paper used for plane tabling must be of superior quality so that it may have minimum effect of changes in the humidity of the atmosphere. The changes in the humidity of the atmosphere produce expansion and contraction in different directions and thus alter the scale and distort the map. To overcome this difficulty, sometimes two sheets are mounted with their grains at right angles and with a sheet of muslin between them. Single sheet must be seasoned previous of the use by exposing it alternatively to a damp and a dry atmosphere.