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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.

PointLetterLevelling YearRemarksIsland
Cnoc an Lisuail'J'1877Canna
Jetty North of Pollock Rocks and SW St Marys Rock, Douglas Harbour'C'1865Isle of Man
Loch Scresort'G'1875Rhum
New Pier, Hugh Town, St. Marys'K'1887Isles of Scilly
OSBM Bolt, Lerwick, NG Ref HU4783 4129'B'1979Correlated to pre 1900Shetland Islands
OSBM Bolt, Stornoway, NG Ref NB4228 3264'A'1977Correlated to pre 1900Outer Hebrides
Scalasaig Harbour'D'1876Colonsay
St. Johns Church, Liverpool'L'1844Datum prior to Newlyn
Tide Gauge Bolt Newlyn Pier'N'1921Authoritative Datum for UK
Trinity House Landing Place, Lundy'M'1885Lundy Island
Unknown'Z'0Bardsey & North Orkney
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