Leveling of an instrument is done to make the vertical axis of the instrument truly vertical. Generally, there are three leveling screws and two plate levels are present in a theodolite instrument. Thus, leveling is being achieved by carrying out the following steps (Figure 20.6):
Step 1: Bring one of the level tube parallel to any two of the foot screws, by rotating the upper part of the instrument.
Step 2: The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. The bubble moves in the same direction as the left thumb. [Figure 20.6(a)]
Step 3: The bubble of the other level tube is then brought to the centre of the level tube by rotating the third foot screw either inward or outward [Figure 20.6(b)]. [In step 1 itself, the other plate level will be parallel to the line joining the third foot screw and the centre of the line joining the previous two foot screws.]
Step 4: Repeat Step 2 and step 3 in the same quadrant till both the bubble remain central.
Step 5: By rotating the upper part of the instrument through 180°, the level tube is brought parallel to first two foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.
Otherwise, repeat the whole process starting from step1 to step5.