The reading of the leveling staff in its unchanged position when the leveling instrument has been taken to a new position; a sight directed backwards to a station previously occupied
A backsight is a reading taken on a position of known coordinate(s) Since a survey progresses from a point of known position to points of unknown position, a backsight is a reading looking "backward" along the line of progress
{i} rear sight on a firearm; (in leveling) reading on a rod which is held on a point of known elevation
A rod reading taken on a point of known elevation, a benchmark or a turning point Backsights are added to the known elevation to arrive at the height of the instrument With a known height of the instrument, the telescope can be used to determine the elevation of other points in the vicinity