It took some time for the changed definition — Liverpool to Newlyn — to work through the system: by 1950, some 40% of the lower secondary and tertiary levellings were still using the Liverpool datum. But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum. Tunnel datum Tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level. for the London Underground, a tunnel datum of ODN −100 m is used; thus a depth of −60 m AOD is 40 m ATD (above tunnel datum) for the Channel Tunnel, a tunnel datum of ODN −200 m is used; thus a depth of −60 m is 140 m ATD See also Trinity High Water Chart datum Vertical Offshore Reference Frames References External links Ordnance Survey webpage on datum height differences Vertical datums