Saturday, 18 October 2014

Mapping Mountains – Trimble Surveys – Yr Wyddfa


02.09.14  Yr Wyddfa (SH 609 543)

Yr Wyddfa (SH 609 543)
Following the 5th June reconnaissance of the summit of Yr Wyddfa we kept in communication with Stephen and Aled as we all organised for the day of the survey.  A number of organisations were contacted, including the Snowdonia National Park Authority who as part custodians of the mountain had an obvious interest in the result of its re-survey.  We also had a good contact at hand if needed, who was Dewi Davies; the Head Warden of the National Park who was one of the five people being filmed for ‘The Mountain’ programme.  Stephen was also in consultation with the Snowdon Mountain Railway and the manager of Hafod Eryri; the summit café, whilst we consulted with our contacts at Ordnance Survey.

Two things were evident from our reconnaissance; the first being that we would have to survey the summit either late in the evening or early in the morning, as the multitude of train passengers that visited the area of the summit during the day may hamper data collection.  The second being that the point where people walk up to and stand beside the triangulation pillar is on a man-made plinth, therefore should data be collected from this point or from the highest natural rock still visible on the periphery of the plinth.  We sought guidance from the Ordnance Survey.

The Ordnance Survey consulted their historical height records and informed us that the last known accurate survey of the mountain was in 1961.  The height given the flush bracket was then recorded as 1086.002m.  Although this flush bracket is retained on the current circular trig pillar its retention was an afterthought and it is placed 0.584m above the base of the trig.  In affect its current position has no accurate height meaning.

Prior to the construction of the 1961 trig pillar the Ordnance Survey had taken a measurement to a bolt positioned in the upper bedrock of the mountain.  It was this bolt that the 1961 trig pillar was positioned over, and it is this bolt that is now buried under the current 2009 construction of the summit plinth.  The Ordnance Survey instructed us to gather data from the base of the trig pillar which is the top of the summit plinth; we could then compare this measurement against the height of the bolt in the bedrock.

When assessing the summit plinth and remaining visible high points of natural rock on the reconnaissance we asked a number of people who had walked up the mountain where they thought the summit of Snowdon was.  The consensus of opinion was that it was ‘here, where I’m standing’, and that was the ground at the immediate base of the trig pillar at the top of the summit plinth.  This opinion is interesting as although the question was not a part of a scientific exercise it did at least summarise a layperson’s view on the matter.

Stephen soon gained permission for us to overnight in Hafod Eryri; the summit café and for us to go up the mountain on one of the last afternoon trains and come down the following morning on the goods train.  During our consultation with Ordnance Survey we asked if they would like to send a representative to be present during the survey, they said they would and it was quickly arranged that Mark Greaves, their Geodetic Analyst should be present.  Mark is one of the country’s leading authorities on GNSS technology and had represented the Ordnance Survey during two of our past surveys and one of our Press Conferences; these being Tryfan, Tal y Fan and Glyder Fawr respectively.

The date for the re-survey was set for Tuesday 2nd September; all we hoped was that the weather was fairly decent.  When we met in the car park adjoined to the Snowdon Mountain Railway in Llanberis it was in glorious sunshine and with a high pressure system stabilised over the country the weather for the upcoming evening, night and following morning was going to be fine.

We were booked on the 4.30pm train, the penultimate one to make its way up the mountain on the day.  We carried all our gear on to the platform and stacked it neatly beside the ticket office.  We had enough gear for a grand tour of Europe, with all necessary surveying gear, a multitude of camera gear that Aled had brought, all our sleeping gear and mountain gear making up quite an impressive assortment.  We even took a ready-made meal waiting to be heated up in the summit cafe.

Our gear stacked up prior to transportation to the summit of Wales' highest mountain

The journey to the summit by train is rightly one of the most popular trips for anyone to undertake in north Wales.  It takes the train approximately one hour to make its way up the side of the Llanberis ridge to the summit of Yr Wydda, only breaking out on to the actual ridge near to Clogwyn Station, with dramatic views across the Llanberis Pass to the high Glyderau.

Aled filming as we pass Llechog
Once the train arrived at the summit platform we ferried all the gear out of our small compartment and assembled it on the platform before storing it in an out building.  A photo opportunity then presented itself, one which was not to be missed, dependent of course upon the final result.  Peering down from above was a neatly calved block of granite proclaiming the mountain’s height of 1085m. 

The granite facade of Hafod Eryri.  Would the height need updating?
As the last train down the mountain was not departing for another half an hour we relaxed in the café and were introduced to Jonathan Tyler; the manager of Hafod Eryri, Jonathan would be overnighting with us in the café and proved a very good and understanding host.

We were also introduced to Dewi Davies, the Head Warden for the Snowdonia National Park who had spent the majority of the day at or near to the summit filming with Bear Grylls, the famous adventurer.  Dewi explained that the filming with Bear had concentrated on the geology of the mountain.  When Dewi accompanied us outside as we walked toward the summit, he showed us fossils of small shells next to the stepped path leading up to the high point.  These were rather beautiful in design, and their position probably unknown by the vast majority of the estimated 400,000 people who visit the summit each year.

Before heading outside we were filmed with Dewi looking through a series of old photographs of the summit of Yr Wyddfa.  Some were from recent times when the old concrete café was still in situ; others were from the late 1800’s, early 1900’s when two buildings, described as Hotels were positioned beside a gigantic summit cairn.  All of these photographs helped us interpret what the area of the summit had looked like during the last 150 years or so.   

As the last train departed the summit of the mountain was now free of the multitude and we ventured out to visit the high point.  Most of our movements around the summit area were being filmed by Aled, who occasionally asked us to walk this way or that, and to repeat sequences until he was satisfied with the outcome. 

Considering how tranquil the late afternoon and early evening was it was amazing the summit had no one on it except for us, this helped tremendously as we could go about our work undisturbed. 

Since our reconnaissance we had considered a number of ways to survey the high point of Wales’ highest mountain and one of them was now tested.  Mark had brought a ‘plate’ that fitted in to the groves of the ‘spider’ at the top of the trig pillar.  This was put in place and our Leica GS15 was then attached to it, the data set from this point would give us an accurate position to the centre of the trig and an accurate height to the position of the equipment.  The offset between its position and the high point at the base of the trig pillar could then be ascertained by the use of a level and staff and subsequently taken off the processed result to obtain the final measurement. 

The pillar plate on top of the trig pillar

The Leica GS15 fixed on the pillar plate
However, as the plate and Leica GS15 was assembled Mark discovered that the hooks that are positioned on standard triangulation pillars were missing from this re-fashioned one.  We experimented with how stable the apparatus would be if we tied it down around protruding rocks on the side of the circular trig, we soon realised that this was not ideal and decided against using this method to re-survey the mountain.

We discussed our options and decided that as the wind speed was only 15-20mph that the equipment could be assembled on its two metre pole and left in place gathering data beside the trig pillar at the top of the summit plinth.  This would also enable any summit visitors to visit the trig and their heads to still be below the antenna, thus not disrupting any signals received by the antenna from orbiting satellites.

We then used the level and staff to take a series of readings around the summit plinth at the base of the trig, this gave us an average reading for its relative height and also pin pointed the high point of the plinth. 

Using the level and staff to determine the high point of the summit plinth
The Leica GS15 was soon in place on its two metre pole, we used a number of rocks on the lower part of the tripod’s legs to weigh it down just in case the wind speed increased.  Once happy with its position and stability the button was pressed and data started to be collected at 8.00pm.  During this time the sun sank ever downward and cast out magical colours across the land. 

Switching the Leica GS15 on for the three hour evening data set
As the evening was clear the warmth of the sun soon disappeared as it sank ever lower on the western horizon, and the wind speed added to the increasing chill.  Thankfully as evening turned to night the wind decreased in strength.

Evening turning to night as the sun sinks on the western horizon
When John, Graham, Mark and Stephen headed in to the café, I remained at the summit with Aled.  Both of us were happy enough looking out as the sun sank in to a layer of thin cloud balanced above the western horizon.  Aled filmed whilst I took photos, the red orb slowly cast out its last light and we were left in a world where all colour slowly drained from the landscape.  I’d wanted to experience the sun set from Yr Wyddfa for most of my adult life and it did not disappoint.

The sun setting over the trig pillar from the summit of Yr Wyddfa
As the last light ebbed from the evening sky Aled set his camera up to capture the scene.  I stood beside the trig pillar on the summit plinth and framed him silhouetted against a night of radiant darkened blue with the last slither of evening orange disappearing beyond the horizon.  Next to me the coloured leds of the Leica GS15 shone as data continued to be gathered.

Aled capturing the scene from just below the summit of Yr Wyddfa
At 9.30pm I ventured in to the café as there were rumours that Jonathan was cooking the world’s largest pasty.  Prior to this I had taken ten minutes of data with the Trimble from the top of the trig and had been joined in the darkness by Graham.

The Trimble GeoXH 6000 and the Leica GS15 both gathering data atop Yr Wyddfa

Gathering data from the top of the trig pillar atop Yr Wyddfa
When I entered the café it was ablaze with light and I quickly stripped my outer layer of clothing off, as the air conditioning made it similar to a tropical climate when compared to the night time vigil outside in the chill air on the summit.

When delivered the pasty was truly ginormous and required serious thought in how to tackle it!  We added cooked spicy bean burgers and small cakes made by Graham’s wife; Janet, to make the meal a sumptuous affair.  As we indulged ourselves the equipment remained on the summit collecting data, we had decided to leave it there unattended as no one had visited the summit for over three hours since we first emerged from the café at about 6.00pm.

Mmmmm yummy, a hugely ginormous pasty, this took serious consideration in how to eat the beastie
Earlier in the evening we had discussed with Mark what timeframe of data set was ideal for the Ordnance Survey, Mark advised us to collect data in two time frames, one in the evening and one the following morning, both of them the same length, we all decided that two data sets of three hours a piece was achievable.  And once the pasty had had its come-upance we ventured out and back to the summit, it was approaching 11.00pm and as we waited for the equipment to log its last data during the first three hour data set, Graham looked up to the heavens and pointed out the Milky Way as it cascaded across the sky.

We left the tripod and two metre pole in place overnight and retired to the comfort of the café, I slept in the shop which was a little surreal, but the word ‘slept’ may be a misnomer in this instance as any sleep was intermittent and by 4.15am I was up and about, quietly getting my gear ready for the morning’s data set.  Soon everyone else was up and sorting and packing gear.

By 5.15am the Leica GS15 had been put back on top of the two metre pole and by 5.20am the equipment was gathering data.  We now had another three hours of data to collect.  Some of us headed back inside the café whilst others remained outside watching the sky slowly become coloured in its morning awakening.

Looking toward Moel Siabod as we waited for the sun to rise
The previous evening’s attention had been westward toward the sea and the ever dimming colour as the sun set, this morning our attention was turned eastward toward Moel Siabod as the slither of orange hue above Capel Curig progressively expanded. 

The next hour was sublime as the sun cast beautiful colour around the eastern sky
The following two hours were sublime as darkness slowly evolved into dawn.  Pushing westward from the heartland of inner Wales was a bank of grey cloud, never lower than any summit, but nevertheless it slowly crept toward Moel Siabod, behind which the sky was turning its customary orange heralding the rising of the sun.

The Leica GS15 gathering its three hour early morning data set
When the sun appeared it was only for five minutes as it quickly disappeared behind the cloud bank, but although an initial disappointment this heralded a delicate show of colour as the land around Moel Siabaod was transformed into gentle and impressive colours with crimson hues merging into reds, scarlets and orange, all subtly changing as the early morning sky evolved in mood.

Sunrise from the top of Yr Wyddfa
Across Glaslyn the pointed peak of Crib Goch was framed against a darkened sky, but it was Moel Siabod that caught the eye as its bulk of shape lent itself to the forever changing light of morning.  The oranges and reds had gently evolved in to colours of rusty crimson’s.  Quite enchanted I stood and watched.

Crib Goch framed by delicate colour
As with all natural light shows there comes a time that the pinnacle of wonderment is replaced by an ebbing of colour, when this happened and the scarlet crimson’s diminished in intensity the eye was struck by the silver edged mist on Llynnau Mymbyr.

Llynnau Mymbyr emerging out of early morning mist
This had been somewhat neglected as the colour wash dominated, but the fragile gracefulness of mist now predominated as light cast out from behind cloud highlighted its flow from lake surface to lower hillside, all was accomplished in an elegant way as wisps of mist crept upward to be swallowed by the warmth of day.  It seemed as if the early morning awakening on the land of Eryri had been transformed into a Japanese silk painting, one of wonder and myth.
 
As the mist dissipated the dull ache of afterglow turned adjacent hillsides from radiant reds to blended blues and the world around Yr Wyddfa took on one of norm.

Radiant reds turned to blended blues
The equipment was switched off at 8.20pm and as we headed back inside the café we were happy in the knowledge that six hours of data had now been collected.  During the latter part of the morning’s data gathering Mark was busy processing the evening data set on his laptop, this gave us a provisional result of 1085.63m for the height of the summit plinth.  Soon afterwards we packed all our gear and waited for the journey down the mountain on the 9.00am goods train.

Mark processing the evening data set to obtain the provisional result
Once precise ephemeris data for the satellite positions had been published the two three hour data sets from the survey of Yr Wyddfa were combined into one which Mark Greaves later post-processed using Ordnance Survey state of the art Bernese software.  This gave an official height for the top of the summit plinth at the base of the triangulation pillar of 1085.67m, an increase of 4cm from the provisional result.

During this project we had studied a number of old photographs of the summit area dating from the late 1800’s to the early 2000’s, by doing so we distinguished comparable rocks and took height readings to these on our recce in early June.  From these measurements we estimated that the summit plinth had raised the height walkers reach when they stand beside the trig pillar by 0.8m when compared to the high point of the natural bedrock.  Our approximate 1084.87m height compares favourably with the 1084.74m height obtained during the 1961 survey.

The Ordnance Survey will adhere to the protocol of listing the height of Yr Wyddfa by the highest natural ground that is known and in this case, that is the rounded up figure of 1085m from the survey of 1961, therefore the current map height is unaltered.

However, for all those many thousands of people who visit this mountain of myth and legend; Yr Wyddfa / Snowdon the highest mountain in Wales, and who contemplate all those small steps ascending those 1085m to its top, well, the task just got a little bit harder as in affect they are ascending a 1085m high mountain and when that extra 1m of summit plinth at the top is gained they are peering down on land and sea that is 1086m below them!


The following are heights based on the Trimble data from the top of the trig and staff readings taken to the summit plinth:


Survey Result:

Yr Wyddfa

Top of Trig:  1086.771m (converted to OSGM15)

Base of Trig:  1085.716m (converted to OSGM15)

Summit Grid Reference:  SH 60985 54374

Drop:  1039m

Dominance:  95.79%


Ordnance Survey Result from Leica GS15 Survey:
Base of Trig:  1085.716m (converted to OSGM15)


Ordnance Survey Result from 1961 Survey to High Point of Bedrock:
Natural High Point:  1084.74m (not converted to OSGM15)



For further details please consult the Trimble survey spreadsheet click {here}

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