Ordnance Datum
The term ‘Ordnance Datum’ refers to the height of mean sea-level, taken from a reference point at Newlyn in Cornwall. This is basis of the national height system for Britain.
The Newlyn Ordnance Datum was preceded by a Liverpool levelling, started in 1840 using a bench mark on St John's Church. In 1844 the datum was changed to the tidal pole in Victoria Dock, but from 1921, this has been generally superseded by the Newlyn Ordnance Datum.
In some remote islands such as Shetland and the Hebrides, datums are based on a local determination of mean sea level.
Ref https://www.ordnancesurvey.co.uk/gps/legacy-control-information/liverpool-to-newlyn
The term Above Ordnance Datum (AOD) refers to a height above the Ordnance Datum. Measurements are given a + or a – sign to signify which side of Ordnance Datum it falls.
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