Basic Principle of Leveling

The fundamental principle of leveling lies in finding out the separation of level lines passing through a point of known elevation (B.M.) and that through an unknown point (whose elevation is required to be determined).

With reference to Figure 13.1, let X represents a point of known elevation (H_x) or a B.M. and Y be a point whose elevation is required to be determined. To find out the unknown elevation of Y, a level is set up at L in between X and Y. A leveling staff is first held at X and a reading h_x is observed, by sighting the staff (held vertical to the line of sight of the level). The staff reading at Y, say h_y is then observed. The elevation of the point Y (say H_y) is thus given by H_x + (h_x ~ h_y) i.e., known elevation (H_x ) added to the separation of level lines (h_x ~ h_y) passing through the points.

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