backsight

[bak-sahyt]
noun Surveying.
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.

Origin:
1840–50; back2 + sight

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
backsight (ˈbækˌsaɪt)
 
n
1.  the sight of a rifle nearer the stock
2.  surveying Compare foresight a reading taken looking backwards to a previously occupied station

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Example sentences
Simply add the backsight reading to the elevation of the bench mark to get the height of the instrument above gage datum.
Requires observing backsight multiple times for large number of foresights.
At the conclusion of each set up, the direction to the backsight should be
  reobserved.
Backsight requirements are unaffected by the number of foresights.
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