Below is a list of all the known local levelling datums within Great Britain. The local datums arose because the Ordnance Datum (Liverpool or Newlyn) could not be transferred across to the offshore island as they were beyond the range of levelling methods.
On these remoter islands Ordnance Datum is based on a local determination of mean sea level, which, being usually based on a short period of observations, can only be regarded as approximate.
|Cnoc an Lisuail||'J'||1877||Canna|
|Jetty North of Pollock Rocks and SW St Marys Rock, Douglas Harbour||'C'||1865||Isle of Man|
|New Pier, Hugh Town, St. Marys||'K'||1887||Isles of Scilly|
|OSBM Bolt, Lerwick, NG Ref HU4783 4129||'B'||1979||Correlated to pre 1900||Shetland Islands|
|OSBM Bolt, Stornoway, NG Ref NB4228 3264||'A'||1977||Correlated to pre 1900||Outer Hebrides|
|St. Johns Church, Liverpool||'L'||1844||Datum prior to Newlyn|
|Tide Gauge Bolt Newlyn Pier||'N'||1921||Authoritative Datum for UK|
|Trinity House Landing Place, Lundy||'M'||1885||Lundy Island|
|Unknown||'Z'||0||Bardsey & North Orkney|