ordnance datum

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ordnance datum

n
(Surveying) mean sea level calculated from observation taken at Newlyn, Cornwall, and used as the official basis for height calculation on British maps. Abbreviation: OD
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
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It appears to stand at 85m AOD (above ordnance datum) which takes into account the ground level being above sea level.
New arrivals helping to change capital's skyline
The benchmarks are used by land surveyors to record the height above Ordnance Datum (mean sea level).
Readers solve sign riddle
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