1. backsight - Noun
2. backsight - Verb
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.
Source: Webster's dictionarySet the auto level in the center of 2 staff (having approx length of 60m) and get the reading of Backsight (BS)-point A and foresight (FS)-point B. Ensure that the instrument is well leveled before reading. Source: Internet