The Collaborative International Dictionary of English v.0.48:

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]