a) Temporary Adjustment
The adjustments which are required to be made at every set up of the instrument are known or stationary adjustment. It includes:
The process of centering the instruments i.e. making the pivot exactly vertically over the ground station mark is called centering. The compass is fixed on the top of a tripod. By adjusting the legs of the tripod, centering is achieved. A plumb bob may be hung from the center of the circular box, to check the cantering of the compass. If the compass is centered perfectly, the pebble will fall exactly over the ground station mark.
If the bob is not provides, centering can be done by dropping the stone freely from the center of the bottom of the cylinder box.
The process of holding the compass in such a way that its graduated rings swings freely, is called levelling. The levelling is done by eye judgement. Generally the compass is provided with a ball and socket arrangement attached to the tripod for achieving quick levelling of the instrument.
3) Focusing the prism
The process of moving up and down the prism for obtaining the figures and graduations sharp and clear, is called focusing the prism.
b) Permanent Adjustment
The adjustments which are made only if the fundamental relations between the various parts of a compass, are disturbed due to careless handling or otherwise are called permanent adjustments.
The following fundamental relationship between different parts of a compass, are established by making permanent adjustments:
1) when plate bubbles. if provided are at the centers of their run, the vertical axis of the compass should be truly vertical.
2) When the instrument is perfectly levelled, sight vanes should be vertical.
3) The ends of the needle and the center of the picot should lie in the same vertical plane.
4) The center of the pivot should coincide with the geometrical center of the graduated ring.
Its main adjustments includes:
1) Adjustment of the plate levels
To make the vertical axis truly vertical when plate bubbles are at he center for their run.
2) Adjustment of sight vanes
To make sight vane and object vane vertical when the compass is level.
3) Adjustment of magnetic needdle
To straighten the magnetic needle
4) Adjustment of the pivot
To set the pivot point at the geometric center of the graduated ring