Surveying and Levelling : STUDY OF COMPASS SURVEYING : Practical 6

 COMPASS TRAVERSING 

STUDY OF COMPASS SURVEYING

 

DESCRIPTION OF THE INSTRUMENTS

A. Prismatic Compass:- 

1. A magnetic needle is attached to the circular ring made up of aluminum. 

2. The needle is on the pivot to orient N and S ends.

3. The line of sight is defined by object vane and eye slit both attached to the compass 

bar.

4. The object vane consists of a vertical hair attached to a suitable frame while the eye 

slit consist of a vertical slit above the prism unit.

5. When the object is sighted, the sight vanes will rotate with respect to the NS end of 

the ring through an angle which the line makes with the magnetic meridian.

6. The reading increase in clockwise direction from 0˚ at south and to 90˚ at west end 

180˚ at north end and 270˚ at east end.

7. Break- pin is placed at the base of the object vane to clamp the oscillation of the 

needle while taking reading.

8. To sight the objects, which are too high or too low, a hinged mirror is placed.

9. Dark glasses are used to sight bright objects. 

Adjustments of prismatic compass:- 

(A) Centering:- 

a) It is the process of keeping the instrument exactly over the station.

b) It is done by dropping a pebble from the centre of the bottom of the instrument. 

(B) Leveling:- 

a) For which the tripod is provided with ball and socket arrangement to fix the compass on

level. 

b) Adjust the box in such a way that the graduated disc is swinging freely and appears to be

level. 

(C) Focusing the Prism:- 

a) The prism attachment is sided up or down till the readings are seen to be sharp and clear. 

B.Surveyors Compass:- 

a) The object vane is similar to that of prismatic compass.

b) The eye vane consists of a simple metal vane with the fine slit without the prism.

c) The graduation ring is directly attached to the box and not with needle.

d) The object is to be sighted first with the object and eye vanes and reading is taken against 

the north end of the needle by looking vertically through the top glass.

e) The card is graduated in quandrantal system having 0˚ at N and S ends & 90˚ at west and east 

ends.