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 th