The Penwith Papers

Newlyn Tidal Observatory

Anyone visiting the rather remote village of Newlyn, just west of Penzance, and looking around the harbour might glance at the end of the south pier and notice an inconspicuous low building just before the small lighthouse. It appears to be something to do with fishing, perhaps somewhere to sit, although further investigation would find access to the small pier closed off by a locked gate. Few would give it a second thought, but passers-by might be amazed to learn that it had played a vital role in a national development - and was now connected to even more important international networks.

View to the north-east, from Newlyn Harbour to St. Michael's Mount, showing the lighthouse and Tidal Observatory to its right, both painted red and white. The white GPS antenna on top of the lighthouse platform can be clearly seen. (Source Wikimedia Commons.)

This little building is in fact the Newlyn Tidal Observatory. Its creation was part of the huge nineteenth-century  growth of interest in counting and measuring almost everything. This was partly because of the disastrous numbers of shipwrecks in bad weather. Another factor was the apparent connection between  adverse conditions and dangers to public health - until actual causes of infections were identified, it was believed that many illnesses, such as various fevers, were carried around in the air, hence the origin of the name malaria for one fever. Which way the wind was blowing could therefore be seen as significant in the spread of disease.

As a result of these concerns, one of the earliest investigations of the new Statistical Society in London was to seek the collection of exact and regular facts about the weather all over the country. By1871 these were being reported by telegraph - from West Cornwall, to what would later become the Meteorological Office.

Fig A above shows the 43 stations of the present day National Tidal Grid Network. Fig C is a sketch of Newlyn harbour showing the piers, Tidal Observatory and benchmark locations.

Fig A above shows the 43 stations of the present day National Tidal Grid Network. Fig C is a sketch of Newlyn harbour showing the piers, Tidal Observatory and benchmark locations.

This study was soon to be joined by a national interest in the exact levels of tides. New technology was facilitating the construction of larger and larger ships, with necessarily bigger draughts (depth of water needed for ship to float freely). This made tidal information essential knowledge for ships using ports and harbours. And at about the same time, the Ordnance Survey was considering problems with the heights of landscapes shown on their maps. The relative height of one hill compared to its neighbour was fairly simple to measure; what was needed was a base line from which all heights, all over the country, could be more precisely recorded.

The concept of ‘sea level’ seemed to provide the answer, and as one of the busiest ports in Britain, Liverpool was chosen as the place to establish Mean Sea Level - that is, the average taken from careful records of high and low tides.

This worked well for about 80 years. Meanwhile, however, other UK coastal towns had been taking measurements for their own purposes. It was gradually discovered that the records, when combined, showed the mean sea level was not the same everywhere - a concept difficult to grasp at first.

This revelation did not please the Ordnance Survey and in 1912 a re-survey of the country - called the Second Geodetic - was undertaken. As part of this enterprise, Newlyn and two other UK tidal observatories were fitted with automatic tidal recording machines. It was soon confirmed that each had a different MSL, and Newlyn was selected as the final location for the official measurement of the national mean sea level.

Original diagram of the Newlyn Tidal Observatory. 'MSL 1915 -1921' indicates Ordnance Datum Newlyn.There were several reasons for this choice. Newlyn received the global tide coming in unimpeded, directly from the Atlantic. It was located in a stable area on granite, with only rare and minor earth tremors from a small, offshore fault. It is not near a major river estuary whose output could fluctuate (as compared with Liverpool, for example). Newlyn is also the closest to the continental shelf edge. All this ensured that the readings would not be affected by major external factors, and would most accurately monitor the global tide circling the planet.

On 14 January 1915 the Cornish Telegraph reported the imminent completion of the south pier and concrete shed, which had been started two years previously. Although the article did point out briefly that Newlyn was now the principal tidal station for the whole of the UK, the main part of the article enthused about the popular local reception of the pier itself, which was seen as a very material improvement to the harbour’s functionality in heavy southerly gales. The pier ensured that even when there were very heavy seas in the Bay, the sheltered harbour waters were almost calm. The Harbour Commission had clearly succeeded in pleasing both the local and visiting shipping. But no one appeared very interested in the Tidal Observatory and its function.

(a) The brass bolt benchmark (OS BM 4676 2855) located in the Tidal Observatory and from which the ODN national datum is defined as being 4.751 metres below the mark, and (b) the cover of the historic mark. (photos by Les Bradley)The Ordnance Survey however, was very pleased and ran the Observatory until 1984. Over several years following 1915, the whole of the UK was finally surveyed and its heights above sea level measured. The resulting OS maps, produced at many scales, have been acknowledged as probably the best in the world. Their accuracy was established in Newlyn between 1915 and 1921 when visual observations of the water level were made every 15 minutes, 365 days a year. Finally, a brass bolt was fixed to the side of the pier, which became the benchmark for the whole country, to which all land heights and all other tidal monitoring points were referenced.

Other countries soon recognised the importance of a ‘datum plane’ from which all their national heights could be calculated, and set up their own tidal observatories, such as the important one at Marseille. All these would eventually be connected - by means of conventional spirit levelling - through the United European Levelling Network.

The long, unbroken series of records from Newlyn makes it possible to note and study relatively long-term developments such as a rise in sea level accompanying climate change, a very current concern. This data shows that the real rate of sea level change in the 20th century has been about 1.8mm per annum, but that the rate is now increasing. The Newlyn data can also be used to study changes in land level that may result from, for example, major isostatic movements.

On a recent visit to the Observatory, members of Penwith Local History Group were reminded by Mr. Turner how coastal land will sink a little each day as the tide comes in, and then recover, which was something of a surprise to us, although obvious once pointed out. It was also impressive to learn that a recent simple mudslide on the seabed (quite common along the edges of continental shelves) registered very clearly on the Newlyn sensing equipment - as do undersea earthquakes as far away as the Atlantic coast of Portugal, which can give rise to tsunamis.

Uses of modern technology at Newlyn include the placing of a GPS antenna on the lighthouse, which is monitored by the Observatory. Data is sent to several other international data banks from which it is possible, for example, to co-ordinate components for Newlyn relative to the centre of the earth, identifying any change in location of Newlyn itself, in any dimension. Full technical explanations, plus photographs,are available here.

This includes the original Cary Porter gauge from 1915, still operable (above, photo Les Bradley).

This includes the original Cary Porter gauge from 1915, still operable (above, photo Les Bradley).

The Newlyn Observatory is at the moment (2018) funded by the UK Environmental Agency. The British Oceanography Centre in Liverpool (previously called the Proudman Oceanographic Laboratory) is responsible for quality control and archiving of the data, all of which is freely available to download. Current information from the latter is that leaving the European Union should not affect Newlyn’s work; it should remain in operation as long as both organisations are funded to look after it. It is to be hoped that frequent upgrading will keep the Observatory at state-of-the-art into the foreseeable future so that its immensely valuable contribution to world scientific knowledge will continue.



Communication from BODC 28.2.18

Hewitt, Rachel Map of a Nation: A Biography of the Ordnance Survey, 2011

More, Steve, Fading Benchmarks 2016

Newlyn Life 1870-1914 The Village that inspired the artists, Penwith Local History Group 2003.



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