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 In 1844 the datum was changed to the tidal pole in Victoria Dock, but from 1921, this has been generally superseded by the Newlyn Ordnance Datum.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.
The importance of early engagement
Construction lessons from the Trillium HealthWorks Experience Centre.
Mayors are to be given planning call in powers
Mayors across England will be able to make the most important planning decisions.
The Master Builder: William Butterfield and his times. Book review.
Why construction can't afford to ignore the skills gap.
Building Safety Regulator, 19 August
Gill Kernick appointed Independent Chair of Residents’ Panel.
Connecting knowledge, technology and conservation
Building competence for the future of built heritage.
Building Regulations and Building Safety Act
CIOB publishes free advice for non-domestic clients.
Building Safety Newsletter from MHCLG.
An extraordinary record of steel engineering worldwide.
Households living near new pylons to save on bills.
ECA warns growth must not outpace grid capacity or skills.
The role of the client and decision making
CIOB response to built environment professions call for evidence.