METHOD OF USING A COMPASS

The following temporary adjustments are required to take a reading from a compass,:

(a) Centring: Compass must be fixed to the stand and set over the station. Stand should be moved inward-outward or in a circumferential direction to centre the compass legs of the tripod. Plumb may be used or a pebble dropped to check centring from the centre of the compass.

(b) Levelling: perfect levelling is not necessary in compass survey, but it should be enough to allow free suspension of magnetic needle. For inspection levelling a bubble level is provided in many compasses. After centring bubble might be ensure in the middle of the circle provided for it in the level. Circumferential movements may be provided to the legs of tripod if it is not within that circle, so that without disturbing centring the levelling is achieved.

(c) Focussing the prism: To focus the prism on graduated circle, in prismatic compass, its attachment is slides up or down until the readings are clearly visible. In surveyors compass there is no such requirement.

For observing bearing of a line the following steps are required, say, AB:

(a) Level the compass.

(b) Centre the compass over A.

(c) Take the reading and note it in the field book.

(d) Rotate the box until ranging rod at B is sighted through the line of sight.

(f) Bring the needle to rest using knob.

(g) Focus the prism, if prismatic compass is used Care should be taken to see that the line of sight is not disturbed between the line of sighting the object and the time of reading the bearing.