Newlyn Tidal Observatory
Newlyn Tidal Observatory is a Grade II listed tide gauge hut on the South Pier in Newlyn, Cornwall, England. Built in 1915, it is a small concrete hut painted red and white, located behind the lighthouse at the end of the pier.
Inside the hut is an Ordnance Survey benchmark brass bolt and a 1.6-metre diameter stilling well that opens to the sea. The observatory was set up by the Ordnance Survey in 1915 to help measure mean sea level and define a national vertical datum for Great Britain, known as Ordnance Datum Newlyn.
From 1 May 1915 to 30 April 1921, six years of measurements gave mean sea level as 4.751 metres (15.59 feet) below the brass bolt inside the hut. Because Newlyn sits on stable granite bedrock close to the open ocean and away from rivers, it was chosen as the reference point for elevations across Britain. About 200 fundamental benchmarks were created and linked to the Newlyn bolt.
From 1915 to 1983 the site used a float in the stilling well and a gauge supplied by Cary and Porter. In 1981 a Aanderaa pressure gauge was added; since 1983 the primary sensor has been a bubbler gauge. The Ordnance Survey ran the tide gauge until 1983, after which the Natural Environment Research Council took over. By 2023 the observatory is operated by the National Oceanography Centre, with funding from the UK Environment Agency.
These measurements continue to define the height reference system for Great Britain and are used in flood planning, infrastructure design, and even UAV operations. In 2016 a paper celebrated 100 years of data, and in 2021 its ongoing importance for ocean and climate studies was noted.
This page was last edited on 3 February 2026, at 19:50 (CET).