Datum height differences

The difference between the Liverpool and Newlyn Datums varies across the country. This is due to levelling of the day only being fit to form a framework to control lower order levelling and contouring.

The original Liverpool levelling was started in 1840 using a bench mark on St John's Church. In 1844 the datum was changed to the tidal pole in Victoria Dock and tidal observations taken place over a nine day period.

Due to the imperfections with the levelling, it was decided to undertake a second geodetic levelling (1912 to 1921). It was at this time that mean sea level was fixed at Newlyn in Cornwall. Fixed points throughout the country were established fundamental bench marks (FBMs).

In 1950 it was decided to undertake a third geodetic levelling, still based on the Newlyn tidal observations. This started in 1951 and was completed in 1956.

The conversion factors between the two datums vary sometimes between kilometre squares. The rule of thumb is that the conversion factor is negative in the south of the country and positive in the north. The conversion figures are only given as a guide and cannot be given exactly, but can be given with sufficient accuracy for most practical purposes to one decimal place of a foot.

In remoter islands such as Shetland and Hebrides and so on, datums are based on a local determination of mean sea level. Being based on short observations, the figures should be regarded as only approximate.

The height difference returned from the search below should be applied to a Liverpool altitude in order to realise an altitude relative to Ordnance Datum Newlyn (I.e. Liverpool datum + correction = Newlyn datum)

e.g. for km square TQ4880, the given difference is -1.3 feet, or the height relative to Newlyn is 1.3 feet below the height relative to the Liverpool datum.

Results returned will be limited to a maximum of 100 results.

Download the complete list of offsets (CSV)

Datum

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
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'1979Correlated to pre 1900Shetland Islands
OSBM Bolt, Stornoway, NG Ref NB4228 3264'A'1977Correlated to pre 1900Outer Hebrides
Scalasaig Harbour'D'1876 Colonsay
Scarnish'E'1878 Tiree
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'1885 Lundy Island
Unknown'Z'0 Bardsey & North Orkney

Download local levelling datum (CSV)