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 In 1844 the datum was changed to the tidal pole in Victoria Dock, but from 1921, this has been generally superseded by the Newlyn Ordnance Datum.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.
[edit] Related articles on Designing Buildings
Featured articles
Check out some of the best features and news from Designing Buildings as well as key stories from around the web.
Construction Management, 10 June
24 months to 14: CITB launches accelerated apprenticeships.
Bridging the gap between clients and contractors
Concerns remain around contractor quality, capability, and delivery.
Construction Management, 10 June.
Heat pumps beat boilers in new home tests.
Building Safety Act implementation in Wales
CIAT to host industry panel on 26 June.
New and updated CLC building safety guidance.
New UK National Buildings Database.
Building Safety Wiki Interviews
Chief executive of the British Woodworking Federation.
Planning condition discharge in England and Wales
A brief explanation from a building compliance expert, with further links.
Overheating guidance and tools for building designers
Guidance for dealing with element of building fabric control that have increasing importance.
Shading for housing, a design guide
From the Good Homes Alliance and British Blind and Shutter Association.
UK Standard Skills Classification (SSC)
A shared framework for describing skills needs.