'Backsight' definitions:

Definition of 'Backsight'

From: GCIDE

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. [1913 Webster]