The Collaborative International Dictionary
Backsight \Back"sight`\ (b[a^]k"s[imac]t`), n. [Back, adv. + sight.] (Surv.) 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. Cf. Foresight, n., 3.
Wiktionary
n. 1 The rear sight of a firearm. 2 (context surveying English) 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 (context surveying English) A measurement of a previously shot point, used to set the angle to zero when occupying a new position. vb. (context surveying English) To shoot a backsight.
Usage examples of "backsight".
He pushed up the leaf of his backsight to its 300-yard adjustment and then settled down on his elbows to peer over it.
For an instant the shaking foresight aligned with the crude vee of the backsight, and he let the shot fly, in flame and a billowing cloud of burnt powder smoke.
Checking the position of Father Sun against the time she'd spent, she looked again at the angle of the blowing snow before backsighting on Heron's ridge.