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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.

Point Letter Levelling Year Remarks Island
Arinagour 'F' 1878 Coll
Cnoc an Lisuail 'J' 1877 Canna
Galmisdale 'H' 1875 Eigg
Jetty North of Pollock Rocks and SW St Marys Rock, Douglas Harbour 'C' 1865 Isle of Man
Loch Scresort 'G' 1875 Rhum
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
Scalasaig Harbour 'D' 1876 Colonsay
Scarnish 'E' 1878 Tiree
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
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