Surveying- Prismatic Compass


Comapass surveying is a branch of surveying in which directions of surveying lines are determined with a compass and the length of lines are measured with a tape or chain. In practice the compass si generally used to run a traverse.

In surveying,"Traverse" consists of a series of straight lines connected together to form a open or a closed polygon.

Methods of traversing: Depending on the type of instrument used for the measurement of angles the method of Traversing can be classified as under;

1.Chain Traverse

2.Compass Traverse.

3.Plane Table Traverse.

4.Stadia Traverse.

5.Theodolite Traverse.

In Compass Traverse the direction of the traverse lines are determined with a magnetic compass.



The prismatic compass is one of the magnetic compass in which there is a prism for taking observations. The prismatic compass is generally smaller in size than a surveyor compass.The prismatic compass is used to determine the whole circle bearing of the lines.It consists of

1.circular box, about 85 to 100 mm in diameter. The box is made up of brass or a non metallic material.At the centre of the box there is a hard steel Pivot which supports the magnetic needle. The needle used in a prismatic compass is board of form. The box is fitted with a glass disc at its top. When the compass is not in use,the box is covered with the brass disc.

The prism when carried on a mounting frame which can be raised or lowered for focussing of the prism. The image of the graduations is viewed through a small circular aperature in the prism mounting. Just above the aperature ther is an narrow slit or a small V-cut used as an eye vane. The object vane consists of a metal frame hinged to the box. It has a vertical hair. The object vane is usually provided with a hinged mirror so that the object which are either too low or high can be sighted by inclining the mirror. Dark coloured glasses are provided near the eye vane which can be interposed between the eye and the prism when sighting illuminous objects or the sun.

when the instrument is not in use, the object vane is folded on the glass cover. In this process, the lifting pin is pressed which lifts the needle off the pivot and holds it against the glass cover. Thus undue wear of the pivot is prevented. TOo dampen the oscillations of the needle and to bring it to rest quickly, a light spring brake is fitted inside the box. A brake pin provide belwo the base of the object vane when pressed comes into contact with the edge of the alluminium ring and stops its oscillations.The prismatic compass is mounted on the tripod while taking readings.


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