Ordnance datum

Vertical references in Europe
View from Newlyn harbour showing the lighthouse and Newlyn Tidal Observatory to its right, both painted red and white.

An ordnance datum (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 above ordnance datum (AOD). Usually mean sea level (MSL) at a particular place is used for the datum.