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ordnance datum

noun
  1. mean sea level calculated from observation taken at Newlyn, Cornwall, and used as the official basis for height calculation on British mapsAbbreviation: OD
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012