A century ago, scientists in this remote Cornish hut established height zero. Here's why it matters.

Every height on a map in England and Wales is based on the mean sea level at Newlyn, Cornwall. But what will rising seas mean for every height above it?

By Jonathan Manning
Published 25 Aug 2021, 13:24 BST
A modern view of Newlyn lighthouse, with St Michael's Mount visible through the haze in the ...

A modern view of Newlyn lighthouse, with St Michael's Mount visible through the haze in the distance. The hut immediately to the right of the lighthouse was the Newlyn Tidal Observatory, which a century ago established the measurement used by Ordnance Survey to calculate what is now known as Mean Sea Level (MSL.)

Photograph by Andrew Ray, Alamy

For anyone who has ever stood on top of a mountain in Britain and wondered what height they had reached, the answer can be traced back to a modest concrete hut built a century ago at the end of a stone pier in Cornwall.

The Newlyn Tidal Observatory set the benchmark for Britain’s sea level, height zero on Ordnance Survey maps, from which all other heights are measured. The heights of Ben Nevis’s lofty 1,345m (4,413 ft) summit, Snowdon’s dramatic 1,085m (3,560ft) peak, and Scafell Pike’s 978m (3,209ft) top are all based on ground zero, or sea zero, as measured at Newlyn. These, as well as just about every elevation measurement you can think of.

Charity challengers reaching the 1,345m summit of Ben Nevis, in Scotland's central Highlands. The highest ground in Britain, this - like all summits like it - base their relative heights from a consistent baseline – commonly called 'sea level.'  

Photograph by Ian Wray, Alamy

A trigonometrical (trig) pillar on the top of Cliff Hill, near Poolewe, Scotland. Pillars such as this were constructed by the Ordnance Survey not just to ascertain heights, but principally to map the country; from each trig pillar – upon which a theodolite was mounted – two more could be seen, allowing precise piecemeal mapping of the country using trigonometry.  

Photograph by Andy Sutton, Alamy

Called Ordnance Datum Newlyn (ODN), this baseline took six years to establish, during which an automated system recording the water level on an hourly basis. The first measurement, taken from a float in a tube, anchored inside a manmade well in the pier, was noted on May 1, 1915 and the last on 30 April 30, 1921. This data was then number-crunched to establish Mean Sea Level (MSL), relative to the height of nearby land.

Strictly speaking, the observatory should have recorded measurements for 18.6 years to take account of the fullest range of tidal constituents, such as the gravitational interactions of the sun and moon, says Mark Greaves, senior production consultant (Geodesy) at Ordnance Survey. But six years’ data was more than sufficient to create a highly accurate measure, he adds. Even today, a brass bolt in the floor of the observatory, 4.75m above MSL, sets the reference benchmark for all heights in England and Wales.

The right location

Prior to Newlyn, the First Geodetic Levelling of England and Wales, which took place between 1840 and 1860, established sea level as an arbitrary point, 100ft below a benchmark on St John’s Church in Liverpool. This was finessed in 1844 to a mark on a post in Victoria Dock, Liverpool following nine days’ of tidal measurements.

“Between 1993 and 2014, sea level at Newlyn rose by 3.8mm per year – although the original, 100-year old ODN has not changed.”

“But as science advanced and techniques improved, it was recognised that a proper measure of MSL was required,” says Greaves. “Part of that realisation was that somewhere like Liverpool was not suitable for an accurate measurement of MSL. It’s not exposed to clean tides and the location was prone to silting up, which makes the measurement noisy and inaccurate.”

The Second Geodetic Levelling of England and Wales favoured Newlyn, a small fishing village on a rocky peninsula that juts out into the Atlantic, about six miles from Land’s End. Due to its location, Newlyn is subject to ocean tides that are not influenced either by river estuaries or by the more extreme storm surges that impact the North Sea. Newlyn also sits on a granite bedrock, which provides the stable foundation essential for a measure of this kind.

Inside the Newlyn Tidal Observatory in 1970. The device is a Munro gauge, which is attached to a well open to the sea, in which a float records the fluctuations of the tide. 

Photograph by Ordnance Survey

Similar tidal observatories were built at the same time at Felixstowe and Dunbar, but bizarrely their mean sea level calculations were out of kilter with Newlyn. Felixstowe was close, with about 1.2cm discrepancy, but Dunbar recorded a significant 24.7cm variance to the Cornish observatory. Geodesists, who study the geometric shape of the Earth, still debate whether calculation errors have created an artificial slope in Britain or whether there truly is a difference in MSL around the British Isles – contrary to the logical notion that water is supposed to find its own level. Their overriding objective, however, was to establish a single MSL from which all heights could be referenced, and Newlyn won the day.

Radiating from Newlyn, a series of 86 base marks and thousands of trig points could then be used to measure the height of any point in Britain, using triangulation. A century on, ODN still provides the MSL reference point for the heights marked on map contour lines; it’s an arbitrary level, universally accepted as zero.

The modern measure

These days, geodesists rely on satellites and tiny variations in gravity to measure heights. Their measurements are less about the altitude that might interest a mountaineer, and much more about relative heights, says Greaves.

“For a large construction project you need to know the slope across the site – if you’re laying a large sewer drain you’ll want to know that it’s going to run downhill,” he says. This type of height data is vital for deciding where to install wind turbines and where to locate mobile phone masts for maximum reach, as well as flood modelling. It’s also vital information for drones to fly safely along aerial corridors without crashing into each other.

A sea level gauge beneath the quay at Newlyn. The Cornish location was chosen due to its oceanic tides free from the influence of estuaries or storm surges.  

Photograph by Jay Sturdevant, Alamy

Today’s satellite-based measurements serve as a compliment to the precision of early 20th century geodesists and their techniques, although a re-survey of Ben Nevis in 2016 found Britain’s tallest mountain to be slightly taller than originally thought. Rounding up its measurements saw the peak gain one metre on the Ordnance Survey’s contour line.

Rising seas, sinking mountains

For other summits, rising sea levels potentially threaten their mountainous status. Between 1993 and 2014, sea level at Newlyn rose by 3.8mm per year, although the original, 100-year old ODN has not changed. But if the difference between mean sea level and ODN becomes too large, then the Ordnance Survey would have to consider a new mean sea level for ODN, says Mark.

“A rise in the level of ODN would mean that all heights relative to it would go down,” he adds. “However, the current difference between mean sea level and ODN is nowhere near large enough to warrant revaluing ODN and, at a current rate of change of approximately 0.2m per century, it’s going to be a long time before a re-valuation will be considered,” adds Mark.

While ODN has proved a priceless reference point for land heights in Britain, it is plays a key role in ocean studies, too, says Professor Phil Woodworth, Emeritus fellow at the National Oceanography Centre.

“One example of needing to know ODN at different places is across the Strait of Dover,” he says. “From knowledge of ODN at Dover and Calais, thanks to levelling through the tunnel, we can compare sea levels on the two sides with reference to ODN, and so estimate the current through the strait.”

Observing the ocean is also important for tracking and understanding climate change, with rising sea levels one of the major symptoms of global warming. But the century-old ODN is not playing a significant role in this work, explains Woodworth.

“We know sea level is rising by about 3mm per year at the moment, which is monitored from space using satellite altimetry and at the coast by tide gauges,” he says. “We need to know if those measured changes are consistent with global warming, which is where climate models and the work of the IPCC [Intergovernmental Panel on Climate Change] comes in.”

All advanced stuff – and all owing at least a little to a modest hut on the Cornish coast.


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