Ordnance Datum

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 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, a tunnel datum of ODN − 100m is used;[2] thus a depth of -60 m is 40m ATD (above tunnel datum)
  • for the Channel Tunnel, a tunnel datum of ODN − 200m is used;[3] thus a depth of -60 m is 140m ATD


  1. ^ a b Irish Grid Reference System from OSI website
  2. ^ "Victoria Station Upgrade Environmental Statement: Main Report" (PDF). Transport for London. 2007-11-15. pp. p.2–6; §2.5.4, fn 2. http://www.tfl.gov.uk/assets/downloads/victoria-Environmental-Statement-Main-Report.pdf. 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 0419179208. 

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