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.
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