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 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.

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