height of instrument in surveying
The height of the instrument (101.815 m) is then calculated by adding backsight reading ( 0.685m) with the elevation (R.L.) of point S1 (101.130m). Foresight is taken at S2 and its elevation (98.245m) is determined by subtracting the foresight (3.570m) from the height of the instrument (101.815 m).