Ordnance Datum

All mapping created by the Ordnance Survey within the British Isles shows heights above mean sea level. Until 1921 the datum used to calculate these heights (known as the Old British Datum) was in the Victoria Docks, Liverpool, and was based on observations in March 1844. Since 1921 a new datum has been used (the New British Datum) which is based on the mean sea level at Newlyn Tidal Observatory in Cornwall, and is derived from measurements made between May 1915 and April 1921.