backsight

A backsight is a measurement taken from an instrument station back to a previously established point, used to orient the instrument and set a known reference angle. When you move your theodolite, total station, or transit to a new position, a backsight allows you to verify the instrument is functioning correctly and to establish the zero direction (or a known bearing) from which all subsequent measurements are made. Without it, each new station would require you to rely on compass bearings or previous calculations, introducing drift and accumulated error across a survey.

The backsight works because survey points are ideally fixed. You occupy a new position with your instrument and point it back at a point you have already shot and confirmed. The instrument records or displays the angle to that known point. You then compare this angle to what theory or prior measurements predict it should be. If the instrument reads the correct angle, your setup is sound; if not, you have discovered a misalignment or mistake before spending time on forward measurements.

Backsights and Foresights in Sequence

In traverse surveying, the workflow is backsight, then foresight. You set up at a station, shoot a backsight to the previous point, shoot a foresight to the next point, then move forward and repeat. The angle between backsight and foresight is the turning angle at that station. This method is rigorous because the backsight immediately validates the instrument orientation before you measure anything new. In leveling, backsights and foresights serve a different purpose: they measure height differences by reading a staff held at known and unknown points, but the principle of checking against a prior point still applies.

Angle closure (the accumulated error in a closed traverse) is one reason backsights matter. If you drift slightly at each station without verifying your setup against a known point, errors compound. A backsight caught a setup error early; a missed backsight compounds it forward. Many surveyors take a second backsight to a different previous point, or even shoot a backsight at the end of a day to verify the whole traverse has not tilted or twisted.

The term backsight reflects direction: you are sighting back, behind, toward where you came from. It is the opposite of foresight (a measurement looking ahead to where you are going). In older literature you may also see it called a back-bearing or rear sight, though backsight is the standard in modern English-language surveying. The nomenclature is clear once you picture the surveyor facing the direction of progress: ahead is foresight, behind is backsight.