Ordnance Datum Newlyn - 100 years old

Located near one of the UK's busiest fishing ports, you'll find the Newlyn Tidal Observatory – the home of mean sea level for mainland Great Britain

4 minute read
Whilst you're no doubt familiar with Britain's beautiful Cornish coast, you may not know the national role one of its smallest buildings played in sea level reading 100 years ago…
Exterior of Newlyn Tidal Observatory

The home of mean sea level

Located at the end of one of the UK's busiest fishing ports, you'll find the unassuming Newlyn Tidal Observatory – the home of mean sea level for mainland Great Britain.

Sea level was measured using averages of readings taken using a device known as a tide gauge. Used in ports and harbours worldwide, tide gauges record the heights of the rising and falling tides.

100 years ago, on 30 April 1921, sea level recording here by OS was concluded after six years of measurement. Within these six years, the Newlyn tide gauge took hourly readings.

Conducting and recording these measurements over a set period of time enabled a mean sea level “datum point” to be established, and every height measurement in the country has been determined based on the work of this machine. The resulting height datum is known as Ordnance Datum Newlyn (ODN) and marks height zero on maps in Britain.

In addition, the difference in height from this point to any other fixed location can then also be determined. Spreading out across the country are lines of levelling observations recording the changing height (related back to zero at Newlyn). The many thousands of familiar benchmarks left behind on these lines are permanent markers of a given height above ODN (find out more).

There are many different types of tide gauge, but their design can be quite simple, consisting of a float that sits on top of water in a tube. The gauge worked by recording the rise and fall of the sea, and the variations were recorded on paper attached to the rotating drum suspended from its centre.

Diagram of the Tidal Observatory
An original diagram of the Tidal Observatory indicating the inlet to the stilling well on the harbour side of the South Pier, the Cary Porter tide gauge and its float, and dipping measurements using a steel tape from the Contact Point to the water surface. ‘MSL 1915–1921’ represents Ordnance Datum Newlyn. Copyright details can be found at the end of this blog.
OS brass bolt benchmark


Did you know, how we calculate the height above sea level has a direct impact on the heights of mountains and hills and the depth of valleys? ODN is the level chosen to represent zero height on maps in Britain, so the heights of hills, mountains and buildings are measured as their elevation above ODN.

While ODN was a measurement of mean sea level in 1915-1921, it's important to know that mean sea level has changed since then – it even varies throughout Britain depending on where you are measuring! So, it's best to think about ODN as a height reference point, a measurement of sea level at a specific point in time at a precise location.

(a) The brass bolt benchmark (OS BM 4676 2855) which is located in the Tidal Observatory and from which the ODN national datum is defined as being 4.751 m below the mark, and (b) the cover of the historic mark. Copyright details can be found at the end of this blog.

Newlyn Tidal Observatory

Although OS set up tide gauges in Felixstowe (1913), Newlyn (1915) and Dunbar (1917), Newlyn was chosen as the single reference datum. This was largely because it's in an area of stable granite rock and, with the gauge at the end of a stone pier at the harbour entrance, it's exposed to the open Atlantic. This location meant it wasn't likely to be influenced by the silting up of the estuary or river tide delays.

OS monitored the tide gauge at Newlyn, along with some 29 other tide gauges, until 31 December 1983 when the responsibility of all tidal observations was transferred to the Natural Environment Research Council (NERC). Tide gauges are now the responsibility of the Environment Agency, who use them to monitor rivers and flood water levels.

The Newlyn Tidal Observatory was manned continuously since its construction until the early 21st century, only once being out of reach for tidal observers during the worst storm ever to hit Newlyn in 1962. At the end of 2018, the Newlyn Tidal Observatory was declared a Grade II listed building by Historic England.

Map of the UK showing the locations of 43 tide gauge stations
(a) The 43 stations of the main present-day National Tide Gauge Network (also known as the “A Class Network”) shown by dots, indicating the location of Newlyn in SW England and some other long-term tide gauge stations mentioned in the text. Dunbar is not included in the present National Network and is shown by a triangle. Felixstowe is also not in the present network but is close to the station at Harwich shown by a dot. (b) Map showing the location of Newlyn on the south coast of Cornwall to the south-west of the larger town of Penzance. (c) Sketch of Newlyn harbour showing the North and South Piers, Tidal Observatory, and the locations of benchmarks. Copyright details can be found at the end of this blog.

Height data today

It’s been one hundred years since the measurement of mean sea level at Newlyn, during which time the importance of height data has only increased.

From emergency responders knowing which equipment to deploy to architects visualising designs, height data is relied on by many organisations. It’s also used by sustainable energy companies locating the optimum positions for solar panels and wind turbines, companies managing overhead cable routing, flood modelling, and by mobile phone networks for signal propagation with the positioning of masts.

From the contour lines in OS Explorer and OS Landranger maps to the popular 3D fly through feature in OS Maps, outdoor enthusiasts also rely on height data when exploring the mountains and hills across Great Britain.

The history of the tide gauge at Newlyn reveals the fascinating evolution of levelling - find out more.

BBC News takes a look at the Cornish hut that gave rise to sea level benchmark - read more.

Thank you to Taylor & Francis Online for some of their amazing images. Discover them yourself via their website.

Copyright details for Taylor & Francis Online images.
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