25.3 : Differential Leveling

Differential leveling is a precise method in surveying used to determine the elevation difference between two points. Its primary goal is to establish accurate vertical measurements to create level surfaces or grade lines critical for designing and constructing infrastructures such as roads, bridges, and buildings.

The procedure for differential leveling begins with setting up and leveling the instrument at a point where the benchmark can be seen. The level rod is held on the benchmark (BM), and a reading is taken, which is entered in the column for backsight (BS) in the leveling notes. The elevation of the instrument's line of sight, represented as the height of the instrument (HI), is calculated using the elevation of the benchmark and the obtained backsight reading, as given in the equation:

The level rod is then moved to an intermediate point, B, between the benchmark and point C, where the elevation must be determined. The reading taken at point B is recorded as the foresight (FS) in the leveling notes. The elevation of point B is calculated as given below:

 
The instrument is now shifted and set up at a new location from where points B and C are visible. The backsight is taken with the level rod placed at point B, and the new height of the instrument is determined (HI’).

The level rod is then placed at point C, and the foresight reading is taken to point C. The elevation of point C is determined using the value of HI’ and the foresight to point C.