In the British Isles

British Isles

The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...

, an Ordnance Datum or OD is a vertical datum used by an ordnance survey as the basis for deriving altitude

Altitude

Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

s on map

Map

A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....

s. A spot height may be expressed as AOD for "above ordnance datum". Usually mean

Mean

In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....

 sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 (MSL) is used for the datum. In particular:

• In Great Britain
  Great Britain
  Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
  
  , OD for the Ordnance Survey
  Ordnance Survey
  Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
  
   is ODN (Ordnance Datum Newlyn), defined as the MSL at Newlyn
  Newlyn
  Newlyn is a town and fishing port in southwest Cornwall, England, United Kingdom.Newlyn forms a conurbation with the neighbouring town of Penzance and is part of Penzance civil parish...
  
   in Cornwall
  Cornwall
  Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
  
   between 1915 and 1921. Prior to 1921, OD was taken from the level of the Victoria Dock, Liverpool.
• In Northern Ireland
  Northern Ireland
  Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...
  
  , OD for the Ordnance Survey of Northern Ireland
  Ordnance Survey of Northern Ireland
  Ordnance Survey of Northern Ireland was the official mapping agency of Northern Ireland. The agency ceased to exist separately on 1 April 2008 when it became part of Land and Property Services Northern Ireland, an executive agency of the Northern Ireland Department of Finance and Personnel, along...
  
   is Belfast Ordnance Datum, the MSL at Clarendon Lock, Belfast
  Belfast
  Belfast is the capital of and largest city in Northern Ireland. By population, it is the 14th biggest city in the United Kingdom and second biggest on the island of Ireland . It is the seat of the devolved government and legislative Northern Ireland Assembly...
  
   between 1951 and 1956.
• In the Republic of Ireland
  Republic of Ireland
  Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
  
  , OD for the Ordnance Survey of Ireland is Malin Ordnance Datum: the MSL at Portmoor Pier, Malin Head
  Malin Head
  Malin Head , on the Inishowen Peninsula, County Donegal, is usually given as the most northerly headland of the mainland of Ireland . In fact, the most northerly point is actually a headland named Banba's Crown on the Inishowen Peninsula about 2 km to the northeast...
  
  , County Donegal
  County Donegal
  County Donegal is a county in Ireland. It is part of the Border Region and is also located in the province of Ulster. It is named after the town of Donegal. Donegal County Council is the local authority for the county...
  
  , between 1960 and 1969.
  • Prior to 1970, Poolbeg Ordnance Datum was used: the low water of spring tide at Poolbeg lighthouse
    Poolbeg Lighthouse
    Poolbeg Lighthouse in Dublin Bay was built in 1768 and initially operated on candlepower but changed to oil in 1786...
    
    , Dublin, on 8 April 1837. Poolbeg OD was about 2.7 m lower than Malin OD.

Tunnel Datum

Tunnel Datum is a datum based on Ordnance Datum used in designing tunnels which pass below sea level. By using a lower datum as the base point, negative numbers can be avoided in calculations, eliminating a possible source of mistakes.

• for the London Underground
  London Underground
  The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...
  
  , a tunnel datum of ODN − 100m is used; thus a depth of -60 m is 40m ATD (above tunnel datum)
• for the Channel Tunnel
  Channel Tunnel
  The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...
  
  , a tunnel datum of ODN − 200m is used; thus a depth of -60 m is 140m ATD