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Meaning of backsight

back•sight

Pronunciation: (bak'sīt"), [key]
— n. Survey.
  1. a sight on a previously occupied instrument station.
  2. (in leveling) the reading on a rod that is held on a point of known elevation, used in computing the elevation of the instrument.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.