Source : Webster's Revised Unabridged Dictionary (1913)

Backsight \Back"sight`\, 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.