Levelling in surveying. Levelling is a process of determining the height of one level relative to another. It is used in surveying to establish the elevation of a point relative to a datum, or to establish a point at a given elevation relative to a datum.

There are mainly two types of telescope,

(1) the external focusing telescope, and
(2) the internal focusing telescope.

Most of the modern levelling instruments are designed with internal focusing telescope. The level tube is used for the temporary adjustment of the instrument.

The main surveying instruments in use around the world are the theodolite,measuring tape, total station, 3D scanners, GPS/GNSS, level and rod. Most instruments screw onto a tripod when in use. Tape measures are often used for measurement of smaller distances.