• In Great Britain, OD for the
Ordnance Survey is
Ordnance Datum Newlyn (
ODN), defined as the MSL as recorded by the
Newlyn Tidal Observatory between 1915 and 1921. • Prior to 1921, OD was
Ordnance Datum Liverpool (
ODL) defined as MSL as recorded in the
Victoria Dock, Liverpool, during a short period in 1844. The first datumestablished in 1840used a benchmark on
St. John’s Church in central Liverpool. • In
Northern Ireland, OD for the
Ordnance Survey of Northern Ireland is
Belfast Ordnance Datum: the MSL at Clarendon Dock,
Belfast Harbour, between 1951 and 1956. • In
Ireland, OD for the
Ordnance Survey of Ireland is
Malin Ordnance Datum: the MSL at Portmoor Pier,
Malin Head,
County Donegal, between 1960 and 1969. • Prior to 1970,
Poolbeg Ordnance Datum was used: the low water of
spring tide at
Poolbeg Lighthouse, Dublin, on 8 April 1837. Poolbeg OD was about lower than Malin OD. Newlyn Tidal Observatory was given
grade II listing on 11 December 2018.
Tunnel datum A
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==