Object and Uses of Levelling

As stated in the definition of levelling, the object is
(i) to determine the elevations of given points with respect to a datum
(ii) to establish the points of required height above or below the datum line. Uses of levelling are
(i) to determine or to set the plinth level of a building.
(ii) to decide or set the road, railway, canal or sewage line alignment.
(iii) to determine or to set various levels of dams, towers, etc.
(iv) to determine the capacity of a reservoir.