Backsight

Origin

From the back + sight

Full definition of backsight

Noun

1. The rear sight of a firearm.
2. (surveying) A measurement or reading taken back towards a point of known elevation, used to calculate the height of the surveying instrument (theodolite, transit, total station).
3. (surveying) A measurement of a previously shot point, used to set the angle to zero when occupying a new position.

Verb

1. (surveying) To shoot a backsight.