This evening (Tuesday 14th Oct) at 7.30pm ITV Wales will broadcast a half hour programme about a recent height survey on the roof of Wales, ‘Snowdon – Climbing New Heights’. The upshot of the survey is that the Ordnance Survey will not be changing Snowdon's 1085m map height. But the work did reveal an interesting conundrum. With all the man made bits and bobs littering the summit it is virtually impossible to establish beyond doubt where the natural highest point of England and Wales even is. What we can say for sure is that anyone climbing to the plinth at the base of the trig point will actually have reached a heady 1086m above sea level. John Barnard, Graham Jackson & Myrddyn Phillips of G&J surveys, the amateur team behind recent Munro demotions and the addition of new mountains to England and Wales, tell us more.
Snowdon's summit poses unique difficulties for anyone surveying it. Firstly there are the crowds, far from ideal when placing equipment. But more fundamental is the composition of the summit itself. The top of the mountain has been re-fashioned in a number of ways over the years. Even its Welsh name Yr Wyddfa is associated with a built structure; one translation is ‘the burial mound’, by legend the resting place of a giant named Rhita Gawr. Legend or no, where exactly should a surveyor take their measurements from?
The opportunity for us to re-survey the summit of Snowdon came five months ago when we were approached by TV production company Slam Media, who were filming a series of programmes about Snowdon, due for broadcast on ITV Wales early 2015. 'The Mountain’ looks at how the seasonal change on Snowdon affects the lives of local people, from a hill farmer to a member of Llanberis Mountain Rescue Team.
Snowdon's height and the various constructions on its summit have a convoluted history.
In one of the earliest recorded surveys, undertaken in 1682 as part of the England and Wales survey by Caswell and Adams, the mountain was given an ambitious height of 3720ft (1134m).
In 1773 renowned writer, antiquarian and traveller Thomas Pennant quoted a height ‘above quay at Caernarvon’ of 3568ft (1087.5m). Twenty eight years later Rev William Bingley commented that ‘it rises to a mere point, the summit not being more than 3 to 4 yards in diameter’, the year was 1801 and Bingley’s estimated height for Snowdon was 3570ft (1088m).
When the Royal Engineers visited the summit in 1827 they erected a large cairn on the mountain’s high point. This was added to over subsequent years and by the advent of photography in the mid to late 1800’s the cairn is depicted as a huge neatly stacked pile of stone and rock.
When the Ordnance Survey (OS) brought out their One-Inch ‘Old Series’ map on 1st May 1840, the height given Snowdon was 3570ft (1088m). Since 1901 the map height has been consistent with the present day value, namely 3560ft (1085m).
Meanwhile, builders were at work. The Snowdon Mountain Railway opened in 1896, depositing Victorian passengers just below the summit, which by now sported a number of buildings.
At one time Snowdon had two competing ‘hotels’ facing one another, built under the high point but requiring a substantial levelling of the summit area for them to sit neatly in place. In more recent times a rectangular concrete summit café was built in the 1930’s, then replaced in 2009 with the stylish Hafod Eryri.
Before the latest cafe was built the summit of the mountain was adorned with a standard OS triangulation pillar. In 2009 this was replaced with a sleek circular equivalent with a brass panoramic viewfinder on top. Of most importance to our survey is what was constructed under this newly fashioned trig pillar - a summit plinth which sits atop the natural rock tor of Wales’ highest mountain.
This summit plinth is substantial, and circular in shape, with two built walkways leading up it. The top is cobbled and relatively flat, just wide enough at the base of the trig pillar for three people to edge past one another abreast.
And here was our dilemma. On a summit that is now effectively man made, where exactly do you gather data from?
To prepare for the survey we made a reconnaissance of the summit area. As we approached the high point we had to stand in line to wait our turn: not surprising, given that the summit of Snowdon attracts an estimated 400,000 visitors each year. We quickly realised that the actual survey would have to be conducted either late in the evening or early in the morning, otherwise the weight of numbers would hamper any serious attempt at data collection.
Before departing toward Crib y Ddysgl we took a number of readings with a level and staff around the area of the summit; all were noted and helped us in pinpointing the position of the highest natural rock that is still visible on the periphery of the summit plinth. We also took relative height readings from this rock to the top of the plinth at the base of the trig pillar. These could be compared against old photographs of the summit where we hoped to pinpoint comparable rocks and ascertain by how much the summit plinth had raised the height walkers reach when they stand beside the trig pillar.
We then collected two hours of data from the high point beside the trig on Crib y Ddysgl and this gave the film crew a chance to capture us setting the equipment up and also getting footage of the expanse of high cwm above Glaslyn, the great eastern face of Wales’ highest mountain and the nearby 1000ft cliff of Y Lliwedd. Although we’d experienced this view on many occasions it is always mesmerising.
For the actual survey we decided that we would have to overnight near to the summit to give us the best opportunity of obtaining the minimum two hours of data that Ordnance Survey requires to verify the result. We also decided to discuss the project with our contacts at Ordnance Survey and ask if they would like to send a representative to join us on the day of the survey to put their official stamp of approval on proceedings.
Soon after our recce we gained permission to stay overnight in the café and be transported up the mountain late in the afternoon and come down on the goods train early in the morning. This was vital to the success of the project and we thank the Snowdon Mountain Railway for helping us. We had also arranged with Ordnance Survey for Mark Greaves, their Geodetic Analyst, to attend the survey and represent the organisation. Mark is one of the country’s leading authorities on GNSS technology and had joined us on past surveys of Tryfan and Tal y Fan.
On survey day, the evening of 2nd September and morning of the 3rd, the weather forecast was remarkably settled.
We met in Llanberis and squeezed onto the 4.30pm train. Between us we had all necessary film, surveying and sleeping gear: seemingly enough for a grand tour of Europe. Once the last train down had departed the number of people around the summit dramatically decreased. We expected some to visit the summit during our time on its top as the evening was so clear, but remarkably only one person came anywhere near during the whole evening.
We had discussed the options for surveying the summit with Mark and had been instructed by Ordnance Survey to take the data set from the summit plinth, as this measurement could then be compared against the height obtained from the last accurate survey of the summit which took place in 1961.
We used a level and staff to take a series of readings on the summit plinth and assembled the equipment over the highest point we found, all under the enthusiastic scrutiny of Mark. Once the Leica GS15 was in position we put rocks on the tripod legs to weigh it down and keep it in place for the data collection. The more data collected, the better the accuracy. We are only talking a centimetre or so here, but nevertheless we decided on three hour data sets in both the evening and the following morning - six hours of data collection in all.
At 8pm the long vigil commenced. By now the sun was low in the western sky, hanging over the sea and illuminating the land with magical colour. Slowly it sank and cast out its final light as the land turned a blue tinge of grey and the chill of the night set in, enhanced by the wind. Above us the stars shone bright, and cascading across the night sky the Milky Way could easily be picked out. The re-survey gave us this fantastic chance to sit on the summit of Snowdon watching evening ebb into night.
As no one had visited the summit during the evening we left the tripod and the two metre pole in place for the early morning data collection. It was nearing midnight when we bedded down in the café, some sleeping in the shop which proved quite a surreal experience.
By 5.20am next day we were back out gathering data again.
The next two hours were sublime as darkness slowly evolved into dawn. Pushing westward from the heartland of inner Wales was a bank of grey cloud, creeping slowly towards Moel Siabod. Sunrise heralded a delicate show of colour as the land around Moel Siabod was transformed into gentle reds and oranges. This early morning awakening was rather stunning, framed by mist on Llynnau Mymbyr. Mist delicately wisped its way up adjacent hillsides and it seemed as if Eryri had been transformed into a Japanese silk painting.
At 8.20am the equipment had gathered its three hours of data. We bade our farewells to the summit and headed down on the early goods train.
Mark later processed the two data sets once precise ephemeris data for the satellite positions had been published. The result confirmed by Mark is that the height of the summit plinth at the base of the trig pillar atop Wales’ highest mountain is 1085.67m. We had calculated from old photographs and the measurements that were taken on the recce in early June that the plinth stood an approximate 0.8m above the highest natural rock of the summit tor. This compares favourably with the 1084.74m height obtained by the Ordnance Survey in 1961 (rounded up to 1085m).
Wales’s highest mountain, Yr Wyddfa / Snowdon, that mountain of myth and legend, will retain its existing map height, since Ordnance Survey protocols dictate that the height in this instance should be taken to the high point of the bedrock. However, for the many thousands who walk up Snowdon each year to reach the trig pillar, the ascent turns out to be a little bit further than it looked. In effect they are climbing that extra 1m of man-made summit plinth at the top, to stand at 1086m.
So How do you measure a mountain?
Two processes are involved in measuring the height of a mountain. Firstly one has to locate the highest point and secondly one has to measure it. The location of the highest point is often not obvious visually and therefore instruments have to be used. Typically we would use an automatic optical level (Leica NA730) and staff (expandable up to 5 metres). The former is essentially a type of telescope that can be mounted on a tripod and set so that it is horizontal. The staff is a giant ruler which can be held vertically over the point to be surveyed. Looking through the level, a measurement from the staff can be obtained. By taking staff measurements from all possible candidate points, the lowest measurement will be for the highest point of the mountain.
The height measurement can then be done in a number of ways. Historically optical equipment was used, but nowadays photogrammetry (aerial photography) or GNSS (Global Navigation Satellite System) are employed. All of these methods employ the mathematical principle of triangulation. In a nutshell, this means that if you know the position and height of two different points you can calculate the position and height of a third point if you can measure the distances and angles of this third point from the other two.
In order to measure absolute height for mountains we use a Leica Viva GS15 GNSS unit; the same units as used by Ordnance Survey. This operates on the same general principle as hand-held GPS units and SATNAV systems. Since the positions of satellites with respect to the earth are known at all times, the GNSS unit can measure the time taken for a signal to be received from the satellite and hence the distance calculated. As with the principle of triangulation mentioned above, the position and height of a survey point can be calculated if signals from a range of satellites are processed at the same time.
There are a few important differences between a survey grade GNSS unit and a typical SATNAV or handheld GPS unit. The GS15 receives signals from both the US GPS and the Russian GLONASS satellite systems. It can also receive the signals transmitted at different frequencies from each satellite, and that allows inaccuracies which result from distortion as the satellite signals pass through the earth’s atmosphere to be reduced. As a stand-alone instrument, the GS15 is still only capable of giving precision in position and height to about two and five metres respectively. So to achieve the ultimate accuracy, height and position is measured relative to other GNSS units set up and operated by Ordnance Survey as a national network of Base Stations whose positions and heights are known accurately. This network continuously monitors the satellite signals and therefore can compare measured with known coordinates for each Base Station. These data can be used to create “corrections” to the survey point position and height, leading to precision of a few centimeters in height.
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