Backsight and Foresight (BS and FS)

The staff reading taken at a point of known or predetermined elevation, e.g. a bench mark is termed backsight or plus (+) sight. It is the first staff that reading taken after setting the instrument at specified survey station. The foresight is the staff reading of the point whose elevation is needed to be acquired, particularly at a change point. It is the last staff reading at the station before the instrument is shifted to a new station. All another staff readings taken at various points of interest of unknown elevations from one instrumental set up among the back sight and fore sight are known as intermediate sights (IS).