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

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 −60 m is 40 m 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

References

  1. 1.0 1.1 Irish Grid Reference System from OSI website
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External links

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