Basic Principle

A levelling exercise is required when difference between levels of two points is to be determined. This could be simple levelling, when both the points are visible from a single instrument setting (Figure 2(a)) or differential levelling (Figure 2(b)) when the two points are quite far apart, the level difference is large or there are obstacles in the line of sight.

                       a) Simple Levelling                         (b) Differential Levelling

In simple levelling, instrument is set approximately at midway (at O) between stations A and B. Staff is held at A then at B and corresponding stages taken and recorded. Now let these be ra and rb respectively. The difference among ra and rb gives the level difference among stations A and B. It may be remember that when station is lower the staff reading is greater and vice-versa. The instrument's telescope must remain horizontal although taking the readings. This implies that bubble of the spirit level attached to instrument must be kept to mid position during the whole levelling exercise at position of instrument setup. If the levelling of instrument is required during observation period, it will denote in which a few error is introduced in the measurement process.