Mean Sea Level – A visit to the Newlyn Tidal Observatory

What exactly is Mean Sea Level and how was this calculated? On a recent trip to Cornwall I stumbled across the rather derelict looking Newlyn Tidal Observatory, the home to Mean Sea Level which is the starting point for levelling in the UK. The Mean Sea Level Datum is the point at which all heights are measured against.

In 1921 the Ordnance Survey decided Newlyn would be the only national datum, chosen for its situation facing the Atlantic and the bed of Cornish granite it sits on. Prior to this there were 3 datums used in the UK (Newlyn, Dunbar & Felixstowe) for collecting Sea Level information. However as discrepancies started to show between them Newlyn was picked as the one datum to be used going forward.

Over a 6 year period from 1915 – 1921 observers recorded the water height every 15 minutes, 24 hours a day, 365 days a year. This data now provides the Mean Sea Level for Great Britain, from which all heights are calculated from.

The Newlyn Tidal Observatory

Tucked in next to the lighthouse that marks the entrance to the busy (and rather charming in its own way) fishing port of Newlyn, The Tidal Observatory is very unassuming and could be easily missed.

Housed inside this small hut was a manual tide gauge clock. During the 1980’s the National Environment Research Council took over the running of this site and replaced the manual gauge with a new electronic tide gauge.

Now declared a Grade II Listed Building by Historic England in 2018 and off limits to the public, the national importance of this small hut at the end of a Cornish pier is still recognised.

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