In the
British Isles
The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles (O ...
, an ordnance datum or OD is a
vertical datum used by an ordnance survey as the basis for deriving altitudes on
maps. A
spot height
A spot height is an exact point on a map with an elevation recorded beside it that represents its height above a given datum.Whittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984, p. 506. .
In the UK this is the Ord ...
may be expressed as AOD for "above ordnance datum". Usually
mean sea level
There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set.
For a data set, the '' ari ...
(MSL) is used for the datum. In particular:
* In Great Britain, OD for the
Ordnance Survey
Ordnance Survey (OS) is the national mapping agency for Great Britain. The agency's name indicates its original military purpose (see ordnance and surveying), which was to map Scotland in the wake of the Jacobite rising of 1745. There was ...
is ODN (Ordnance Datum Newlyn), defined as the MSL as recorded by the tidal gauge at
Newlyn
Newlyn ( kw, Lulyn: Lu 'fleet', Lynn/Lydn 'pool') is a seaside town and fishing port (the largest fishing port in England) in south-west Cornwall, UK.Ordnance Survey: Landranger map sheet 203 ''Land's End''
Newlyn lies on the shore of Moun ...
in Cornwall between 1915 and 1921.
**Prior to 1921, OD was defined as MSL as recorded in the
Victoria Dock, Liverpool, during a short period in 1844 (ODL).
* In
Northern Ireland
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label=Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. North ...
, OD for the
Ordnance Survey of Northern Ireland is Belfast Ordnance Datum, the MSL at Clarendon Dock,
Belfast
Belfast ( , ; from ga, Béal Feirste , meaning 'mouth of the sand-bank ford') is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 12th-largest city in the United Kingd ...
, between 1951 and 1956.
* In the
Republic of Ireland
Ireland ( ga, Éire ), also known as the Republic of Ireland (), is a country in north-western Europe consisting of 26 of the 32 Counties of Ireland, counties of the island of Ireland. The capital and largest city is Dublin, on the eastern ...
, OD for the
Ordnance Survey of Ireland is Malin Ordnance Datum: the MSL at Portmoor Pier,
Malin Head,
County Donegal
County Donegal ( ; ga, Contae Dhún na nGall) is a county of Ireland in the province of Ulster and in the Northern and Western Region. It is named after the town of Donegal in the south of the county. It has also been known as County Tyrconn ...
, between 1960 and 1969.
[Irish Grid Reference System](_blank)
from OSI website
** Prior to 1970, Poolbeg Ordnance Datum was used: the low water of
spring tide at
Poolbeg Lighthouse, Dublin, on 8 April 1837. Poolbeg OD was about lower than Malin OD.
Ordnance Datum Newlyn and its antecedents
The First Geodetic Levelling of England and Wales (1840-1860) needed to define a datum plane from which to specify spot heights. At first it was specified as a horizontal plane 100 feet below an arbitrary benchmark on St John's Church, Liverpool. Subsequently, however, it was redefined as mean sea level (MSL). To establish MSL, tidal observations were taken at the
Victoria Dock, Liverpool, over a short period in 1844.
By the time of the Second Geodetic Levelling (1912-1921) the importance of stability was better appreciated and so it was decided to use Fundamental Bench Marks (FBMs) installed in solid rock, rather than on buildings as before. To measure average MSL around Great Britain three tide gauges were employed: at
Dunbar
Dunbar () is a town on the North Sea coast in East Lothian in the south-east of Scotland, approximately east of Edinburgh and from the English border north of Berwick-upon-Tweed.
Dunbar is a former royal burgh, and gave its name to an ec ...
, Newlyn and
Felixstowe
Felixstowe ( ) is a port town in Suffolk, England. The estimated population in 2017 was 24,521. The Port of Felixstowe is the largest container port in the United Kingdom. Felixstowe is approximately 116km (72 miles) northeast of London.
H ...
. However, it was found that the measured difference between the Dunbar and Newlyn stations was 0.81 feet (0.247m), far larger than could be accounted for by error. The difference was real. Accordingly, it was decided to abandon the attempt and fix on one site: MSL Newlyn. Newlyn has certain practical advantages: it is set in granite bedrock, is far from major rivers, and it better represents deep ocean sea levels.
The difference between ODL (Liverpool) and ODN (Newlyn) was found to be 0.13 feet. It took some time for the changed definition — Liverpool to Newlyn — to work through the system: by 1950, some 40% of the lower secondary and tertiary levellings were still using the Liverpool datum. But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum.
Tunnel datum
Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level.
* for the
London Underground
The London Underground (also known simply as the Underground or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in England.
The U ...
, a tunnel datum of ODN −100 m is used; thus a depth of −60 m AOD is 40 m ATD (above tunnel datum)
* for the
Channel Tunnel
The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone ( Kent, England, UK) with Coquelles (Hauts-de-France, France) beneath the English Channel at the Strait of Dove ...
, a tunnel datum of ODN −200 m is used;
[
] thus a depth of −60 m is 140 m ATD
See also
*
Trinity High Water
*
Chart datum
*
Vertical Offshore Reference Frames
References
External links
Ordnance Survey webpage on datum height differences
Vertical datums
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