Ordnance datum

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Vertical references in Europe

In the British Isles, 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 may be expressed as AOD for "above ordnance datum". Usually mean sea level (MSL) is used for the datum. In particular:

Tunnel datum[edit]

Tunnel datum is a datum based on an ordnance datum and 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, a tunnel datum of ODN +100 m is used;[2] thus a depth of −60m AOD is 40m ATD (above tunnel datum)
  • for the Channel Tunnel, a tunnel datum of ODN −200 m is used;[3] thus a depth of −60 m is 140 m ATD


  1. ^ a b Irish Grid Reference System Archived 2008-06-14 at the Wayback Machine. from OSI website
  2. ^ "Victoria Station Upgrade Environmental Statement: Main Report" (PDF). Transport for London. 2007-11-15. pp. 2–6; §2.5.4, fn 2. Retrieved 2008-07-28. 
  3. ^ Radcliffe, Eric (1995). "Control and Construction Surveys". In Colin J Kirkland. Engineering the Channel Tunnel. Taylor & Francis. p. 53. ISBN 0-419-17920-8. 

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