Compass is an instrument which can be used to measure the direction of a survey line with respect to magnetic north-south. The magnetic north-south direction which is the reference direction is called meridian (reference direction) and the angle between the line and the meridian is called bearing. Use of compass for measuring direction of a line simplifies the surveying to a great extent. Construction of different types of compasses, the system of noting bearings of the lines, some problems associated with measurement with compass are explained and then field work involved in compass survey is presented.
Disadvantage of chain surveying is that, in it only distances are measured and hence area is to be covered with a network of triangles. If the length as well as angle of a line can be measured with respect to a known direction then it is possible to plot a line, independent of length of other lines. Hence, in such cases there is no compulsion of going for a network of triangles only.
HOW TO USE A COMPASS
To take a reading from a compass, the following temporary adjustments are required:
[1] CENTERING
The compass should be fixed to the stand and set over the station. To centre the compass legs of the tripod stand should be moved inward-outward or in a circumferential direction. To check centring plumb may be used or a pebble dropped from the centre of the compass.
[2] LEVELLING
In compass survey perfect levelling is not necessary, but it should be sufficient to permit free suspension of magnetic needle. For checking levelling a bubble level is provided in many compasses. After centring bubble should be ensured in the middle of the circle provided for it in the level. If it is not within that circle, circumferential movements may be provided to the legs of tripod so that without disturbing centring the levelling is achieved.
[3] FOCUSING THE PRISM
In prismatic compass, to focus the prism on graduated circle, its attachment is slide up or down till the readings are clearly visible. There is no such requirement in surveyors compass.
The following steps are required for observing bearing of a line, say, AB:
· Centre the compass over A.
· Level the compass.
· Focus the prism, if prismatic compass is used.
· Rotate the box till ranging rod at B is sighted through the line of sight.
· Bring the needle to rest using knob.
· Take the reading and note it in the field book.
