Surveying Compass | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Surveying (Part 4)

Surveying Compass: The surveying compasses are of two to types:

(1) Prismatic compass’
(2) Surveyor’s compass

Comparison between a Surveyor’s Compass and Prismatic Compass :

1. Magnetic needle
2. Graduated ring
3. Sighting vanes
4. Reading system
5. Tripod

Surveyor’s Compass The needle is of edge bar type and also acts as an index.

(i) The graduated ring is attached to the box and not to the needle. This rotate along with the line of sight.
(ii) The graduations are in Q.B. system, having 0° at North and South and 90° at East and West. East and West are interchangeable.
(iii) The graduations are engraved erect.

(i) The object vane consists of a metal vane with a vertical hair.
(ii) The eye vane consists of a small vane with fine slit.

(i) The reading are taken directly by seeing through the top of the glass.
(ii) Sighting and reading can not be done simultaneously from one position of the observer.
The instrument cannot be used without a tripod

Prismatic Compass 

(i) The needle is broad needle type but does not act as an index.
(ii) The graduated ring is attached with the needle. This does not rotate along with the line of sight.
(iii) The graduations are in W.C.B. system having 0° at South or North.

(i) The object vane consists of a metal vane with a vertical hair.
(ii) The eye vane consists of a metal vane with a slit.

(i) The reading are taken with the help of prison provided at the eye slit.
(ii) Sighting and reading can be done simultaneously from one position of the observer.

Tripod may or may not be provided. The instrument may be used even by holding in hand:

Meridian : The fixed direction on the surface of the earth with reference to which, bearing : survey lines are expressed, called a meridian.