backsight

Backsight is a measurement in surveying taken with an instrument toward a staff held at a point of known elevation. It is the reading made in the direction opposite to the area of measurement, and it is used to establish the height of the instrument (HI) relative to a datum. After the instrument is set up at a temporary station, the surveyor places a graduated staff on a benchmark with known elevation and reads the vertical scale—the backsight, B. The instrument height is then computed as HI = elevation of the known point + B. Readings taken on the staff at unknown points in front of the instrument are called foresights; their readings are used with HI to determine reduced levels (RL) via RL = HI − F. Backsight readings also help verify proper instrument setup and maintain a consistent datum during traverse or leveling networks. The term is contrasted with foresight, which measures elevations ahead of the instrument. In modern surveying practice, including the use of digital levels and total stations, the concept of backsight remains central to establishing and transferring vertical control, even when calculations are automated.