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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.

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