Question

Discuss the differences among HI, BS, and FS in differential leveling. Mention what they are and how they can be calculated.

Answer 1

Backsight (BS) :
It is a sight taken to the level rod placed at a point of known elevation (either a BM or TP) as shown in figure below.

Height of instrument (HI) :
It is the elevation of the line of
sight of the telescope ,it is measured as (Elevation of Benchmark (BM) +BS).

Foresight (FS) :
It is a sight taken on any point at staff to determine its elevation, elevation of this point is calculated as (HI-FS).BS and FS are sighted with the help of theodolite or autolevel. It is reading of staff where middle hair coincides.