Ordnance datum

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

Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level.

  • 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. 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. 15 November 2007. pp. 2–6, §2.5.4, fn 2. Retrieved 28 July 2008.
  3. Radcliffe, Eric (1995). "Control and Construction Surveys". In Colin J Kirkland (ed.). Engineering the Channel Tunnel. Taylor & Francis. p. 53. ISBN 0-419-17920-8.

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