What does backsight mean?
Backsight is a technical noun with two established uses. In surveying and levelling, it denotes a reading taken with a level or theodolite onto a point of already known elevation, such as a benchmark. By adding this backsight reading to the benchmark's height, the surveyor establishes the height of the instrument, which then allows foresight readings onto new points to determine their elevations. The term also appears in marksmanship, where it names the rear sight of a firearm, aligned with the front sight when aiming. Backsight belongs chiefly to professional registers — the vocabulary of surveyors, civil engineers, and shooters — and is seldom encountered in casual speech. Its value lies in precision: the word marks one half of the fundamental backsight-and-foresight procedure that underpins accurate measurement of land and construction work.
nounA sight or reading taken backwards to a point of known elevation or position, used in surveying to establish the height of an instrument. In firearms, the rear sight of a gun, through which the shooter aligns with the front sight.
- In surveying, a sighting or reading taken on a point of known elevation or position, typically a benchmark or previously established station, used to determine the height of the instrument.
- In firearms, the rear sight of a gun, aligned with the fore (front) sight when aiming.
"The surveyor took a backsight on the benchmark before measuring the new points."
"Before levelling could begin, the crew recorded a backsight on the nearby benchmark."
"He adjusted the backsight of his rifle until the target sat squarely between the two sights."
The plural is used when referring to multiple readings or to the rear sights of multiple firearms.
"The field book recorded all the backsights taken during the morning's levelling run."
Every accurate map you've ever used depends on someone first taking a humble backsight.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of backsight
Backsight is formed transparently from the English words 'back' and 'sight', following the same compounding pattern that produced related terms such as foresight, hindsight, and insight. It emerged in English as a technical coinage, with its surveying sense developing alongside modern levelling practice and its firearms sense naming the rear element of aiming sights. The compound's components are both native English words of Germanic origin, so the term has no classical root; its relatives are simply the other 'sight' compounds built on the same pattern.
How backsight is actually used
Primarily a technical term used by surveyors, engineers, and marksmen; rare in everyday conversation. In surveying contexts it is often paired with 'foresight' as part of standard levelling procedure.
Easily confused with backsight
A backsight is a reading taken to a previously established point behind the instrument, while a foresight is a reading taken forward to a new point whose elevation is being determined.
Hindsight refers to understanding of past events after they have occurred, whereas backsight is a technical term in surveying and marksmanship.