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.