Aquaculture Engineering (2+1)

Compass surveying - Adjustment of prismatic compass

Unit 1- Surveying

Compass surveying

Adjustment of prismatic compass
There are two types of adjustments

• Temporary adjustment
• Permanent adjustment

Temporary adjustment
Temporary adjustment consists of

• Centring
• Levelling
• Focussing the prism

Centring: Centring is the process of keeping the instrument exactly over the station. The centring is invariably done by adjusting the tripod legs. A plumb-bob may be used to judge the centring and it is not available, it may be judged by dropping a pebble from centre of the bottom of the instrument.

Levelling: If the instrument is a hand instrument it must be held in hand in such a way that the graduated disc is swinging freely and appears to be level as judged from the top edge of the case. Generally a tripod is provided with ball and socket arrangement with the help of which the top of the box can be leveled.

Focussing the prism: The prism attachment is slided up or down for focusing till the readings are seen to be sharp and clear.

Permanent adjustment:
Permanent adjustment are those adjustment which are done only when fundamental relation between the parts are disturbed.