When the theodolite has a four-screw levelling head, the following procedure is adopted.

1.  After setting up and centring the theodolite, bring the plate level parallel to any one pair of diagonally opposite foot screws. Operate these foot screws to centre the bubble (shown in figure below)

2.  Swing the instrument to bring the plate level parallel to the other pair of foot screws. Centre the bubble.

3.  Swing it back to the previous position. Check whether the bubble traverses. If it does not, centre it with

 the foot screws to which the level is parallel.

4.  Swing it back, check the position of the bubble, and repeat the procedure.

5.  Once the bubble traverses in the two orthogonal positions, swing it through 180°. The bubble should

 traverse in this position or in any other position.

If two plate levels are provided, the procedure is the same. Bring one plate level parallel to a pair of

opposite foot screws. The other pair will be parallel to the remaining pair of foot screws. There is no need to

swing the instrument. Bring the bubble to the central position alternately and check in the other positions.