Levelling, a process in land-surveying for the determination of the variation in level over a given area. The instrument employed is the surveyor's level, consisting of a telescope mounted on a tripod stand and arranged so that it can be adjusted accurately horizontal by means of a spirit-level of the ordinary type. If two poles, graduated in feet and inches, are held vertical at two points, one some distance in front of the telescope and the other behind, the readings on them when viewed through the telescope will only be identical when the two points are at the same level. The difference of level will be the difference in their readings. On this principle a, series of observations may be taken, which however, require correction for the curvature of the earth when taken over a large area.