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How do you measure sea level?

23 Aug 2011
Ordnance Survey
GI Explained

Have you ever looked at the height of an object above sea level on a map and wondered how that figure is worked out? With changing tides across the days and during the seasons, we get a higher tide or a lower tidal point.

The standard way to measure sea level is with an instrument called a tide gauge. These are used in ports and harbours the world over and record the heights of the falling and rising tides. Doing this over a period of time enables Mean Sea Level to be calculated and from this, the difference in height from this point to any other fixed location.

Ordnance Survey set up tide gauges in Felixstowe (1913), Newlyn (1915) and Dunbar (1917). Newlyn was chosen as the single reference datum, largely as it was situated in an area of stable granite rock and the gauge was perched on the end of a stone pier at the harbour entrance where it was exposed to the open Atlantic. This meant it wasn’t liable to be influenced by the silting up of the estuary or river tide delays.

Over the course of six years the Newlyn tide gauge took hourly readings. The gauge worked by recording the rise and fall of the sea with two floats attached by chains. The variations were recorded on paper attached to the rotating drum suspended from its centre.

From these readings the mean sea level was calculated and every height measurement in the country has been calculated based on the work of this machine.

Responsibility for the Newlyn Tide Gauge continued to rest with Ordnance Survey who provided a full time observer until 1983 when the station was handed over to the Institute of Oceanographic Sciences.

And of course, how we calculate the height above sea level has a direct impact on the heights of mountains and hills and the depth of valleys.

Ordnance Survey
By Ordnance Survey
Press Office

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