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: 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.