Tuesday, 14 October 2014

Mapping Mountains – Trimble Surveys – Yr Wyddfa


05.06.14  Yr Wyddfa (SH 609 543)

Yr Wyddfa (SH 609 543)
Yr Wyddfa stands as an iconic sentinel to a nation, the highest point in an ancient land, centred in Eryri where mountain and sea both meet.  It is bathed in myth and legend and holds the distinction of being the most visited summit in the British Isles with many hundreds of thousands of people either clambering up its ridges or arriving at its summit after being transported by train every year.

As befits such an iconic mountain its height is viewed as important.  After all without such knowledge part of this iconic status would not be given.  The mountain’s current given height is 1085m, this height and its imperial equivalent of 3560ft has been constant from the early part of the 1900’s. 

Earlier height documentation quotes 3570ft (1088m [Ordnance Survey One-Inch 'Old Series' 1st May 1840]), this height matches that given by the Rev William Bingley in 1801 who estimated the height from local sea level as 3570ft (1088m), saying that “It rises to a mere point, the summit not being more than 3 to 4 yards in diameter”.  Earlier still are two other height recordings, one by the renowned naturalist, traveller, writer and antiquarian; Thomas Pennant, who quoted an estimate of 3568ft (1087.5m), the other was by Mr Caswell from a survey in 1682 with instruments made by Flamstead who estimated the summit to be 3720ft (1134m).

The summit area of the mountain has withstood numerous man-made structures including hotels, huts, café’s, stone walls and a nearby railway terminus.  It is currently crowned by a summit plinth adorned with a circular triangulation pillar with a brass panoramic viewfinder on its top, there to distinguish the view for the many thousands who peer out in to the fog of mist when days aren’t especially clear.  When those days are clear and the view is savoured, one can be mesmerised by the light and shade as the play of light highlight mountain top, lake and valley, all at the foot of Yr Wyddfa as the great bulk of its eastern face plunges down toward Glaslyn and Llyn Llydaw, two timeless blue enhanced lakes.

We had wondered about the accurate height of Yr Wyddfa for a number of years, partly due to a flush bracket height quoted as 1086.002m in the OS Trig Database.  This height was taken to the top of a flush bracket that was on the side of the old triangulation pillar that once stood on the summit.  The year given for its completion and levelling is 1961.  As flush brackets are usually positioned 30 – 35cm above the base of the trig pillar, it would mean that the height to the base of the trig pillar would be approximately 1085.70m, and as heights are usually rounded up or down when they appear on the map, it seemed that this height was anomalous as it appears as 1085m on current maps.

There is also the unwanted dilemma of what constitutes the high point of the highest mountain in Wales; few would say it is the top of the trig pillar as this is man-made.  But if man-made objects should not count, isn’t the ground immediately below the base of the trig pillar also man-made?  Surely the high point of this most iconic of Welsh mountains should be taken to its natural high point and not something that man has built?  Advice on this specific subject would have to be sought from Ordnance Survey.

It was our intention to accurately re-height Yr Wyddfa with the latest GNSS technology and do our utmost in determining the remaining natural high point of the mountain.  During this process we would also gather height data for the ground immediately at the base of the triangulation pillar to satisfy those who enjoy such debates between ‘natural’ and ‘man-made’ summit structures.

The opportunity to re-survey Yr Wyddfa was prompted by Stephen Edwards who is the organiser of the Snowdon race and who operates CREAD Cyf, a company specialising in aspects of radio and TV coverage, centred on the outdoor world.  Stephen contacted us and we arranged a meeting with him to discuss the project.  Stephen had been present on some of our past surveys, including the ones that determined Glyder Fawr to be 1000.8m high (999m on then current maps) and Tryfan to be 917.5m high (915m on then current maps).  Since our last meeting he had teamed up with Aled Llŷr who runs Slam Media productions, which is an independent film company.

The project Stephen outlined was exciting as he knew of our intention to one day re-survey Yr Wyddfa, and he wanted this to be a part of a six programme TV series entitled ‘The Mountain’ which followed five people; Head Warden of the Snowdonia National Park, Hill Farmer, Train Driver on the Snowdon Mountain Railway, member of the Llanberis Mountain Rescue Team and the owner of the Halfway House café.  The series would follow these people and the seasonal impact the mountain has on their lives with Yr Wyddfa as backdrop but also intrinsic in their daily occupation.  Stephen envisaged that our re-survey of the mountain would form part of the programmes based around the summer months. Stephen suggested at least three visits were necessary, two to recce the summit area and the final one to measure the height and gather summit data.

The first recce was arranged for 5th June 2014, we met Aled and Stephen in the car park adjacent to the Snowdon Mountain Railway and caught the 8.00am goods train to the summit along with the staff from the café and an entourage of cream cakes, various drinks and buttered scones, all neatly stacked for the hundreds of visitors due to disembark from the train over the course of the next nine hours.

Aled and John waiting to depart on the 8.00am goods train
This was a treat for us and one that we are very privileged to be offered, and thanks should be expressed to Stephen for arranging it and for the Snowdon Mountain Railway in letting us go to the summit in the train.  The major advantage with this was the ease of carriage for the weight involved, as a surveying team our equipment takes three people to carry it and Stephen and Aled also had their cameras and mountain gear.

The journey by train also fulfilled a life time’s ambition for Graham who had never previously travelled on the mountain railway, neither had John, I had; but have no memory of it as I was aged about eight or nine and went up with my parents and brother when on a family holiday in north Wales.

Part of the team assembled (L-R) Graham Jackson (Surveyor), John Barnard (Surveyor) and Aled Llŷr (Director)
The journey to the summit took about an hour, we left in the morning’s sunshine and when past the rocky peak of Llechog the mist enveloped us.  The remaining journey was done in the knowledge that it was going to be a wee bit chilly on the top, so much so that when we disembarked we instantly put on extra gear and headed straight in to the café for a hot cup of tea.  No sooner had the café doors been opened and one or two walkers came in.  Over the next couple of hours a whole multitude of people spilled out of the trains wearing all manner of exotic clothing.  We busied ourselves with conversation and timed our first sojourn out to the misty peak for when the train passengers had mostly departed.

Into the mist as we pass the peak of Llechog
Nearing the final ascent, Jonathan Tyler (Hafod Eryri manager) in red top and Stephen Edwards (Producer) on right of photo
Aled filmed us as we left the café and walked up the steps toward the summit, having visited the top more than 30 times I was pleasantly and somewhat relieved to find that what we had started to refer to as the summit plinth was not wholly man-made, as large embedded rocks make up the bottom few metres of summit structure.  These rocks are part of what is the summit tor and are the rocks that in 1801 the Rev William Bingley described as “It rises to a mere point, the summit not being more than 3 to 4 yards in diameter”.  It is surprising how little one notices when just visiting a summit for pure enjoyment, today we had a specific purpose and it was great to get to grips with the summit tor, examining the lower rock and seeing where the rock strata changed in position, this happens toward the top of the ‘mound’ and is continuous around it.  The lower section is made up of the ‘natural’ tor and has embedded rock usually slanting upwards, whilst the upper one metre – two metres has smaller rock arranged pointing inwards encircling the ‘mound’.  Further investigation proved these upper rocks to have been placed by man, as a number of them either moved when wriggled or came out when pulled; those that did were carefully replaced.  We then concentrated on these upper rocks and found small crevices between them, some large enough for us to insert our arms up to the elbow.  We then inserted a flexible metal tape measure in a number of these crevices and found that the length it went in matched that between the circular plinth at the base of the trig and where these upper rocks had been positioned.  This indicated that the circular plinth was solid, rather substantial and no doubt man-made, it was this that the metal tape was reaching when it would not go further in to each crevice, this indicated that it was the circular plinth that had been constructed over the ‘natural’ summit tor. 

John getting to grips with the outer section of the mound that surrounds the summit plinth
Somewhere in here is the high point of the summit tor
Our next objective was to set up the level on the circular plinth beside the trig pillar and attain readings to a number of the higher rocks of the tor, this meant that Graham had to position himself delicately on the outside of the ‘mound; whilst holding a surveyors staff, vertically aligned for John to read out figures on the staff from the level, which I noted.  This was repeated on a number of ‘natural’ rocks around the upper section of the ‘mound’.  We then took readings to the top of the trig pillar, its base and also to its flush bracket.  These we could compare to data that had been collected from the top of the trig pillar using the Trimble GeoXH 6000.  

The Trimble gathering data from the top of the trig on Yr Wyddfa
All of this was being filmed by Aled, whilst a multitude of people marched up the steps either side of the summit ‘mound’.  The realisation soon dawned on us that the survey would probably have to take place in the evening or early morning as we had difficulty operating the level purely through the amount of people visiting the summit.

John with the level as another summit visitor poses for a photo
Graham balances on the periphery of the summit plinth with the staff as Stephen looks on
Once satisfied that our first recce had accomplished more than we had hoped, we headed for the café and more tea.  By this time there were signs that the predicted break in the weather with the mist forecast to disperse and sunshine breaking through was just about to happen, and as we left the café on our way toward Crib y Ddysgl the mist was ripped from the summits, blue sky appeared and the world outside showed itself.

Aled filming a passing train as we head toward Crib y Ddysgl
Although our priority for the day was the reconnaissance of the summit of Yr Wyddfa, we wanted to take the opportunity and also survey Crib y Ddysgl, which is listed as the second highest mountain in Wales with a summit height of 1065m.  However, to its north in the range of mountains known as the Carneddau lies Carnedd Llywelyn, which has a map height of 1064m, and with a margin of uncertainty of + / - 3m applied to the surveying method that attained the latter height, it meant that Carnedd Llywelyn may in fact be higher than Crib y Ddysgl.

The team assembled (L-R) Stephen Edwards (Producer), Aled Llŷr (Director), Graham Jackson (Surveyor) and John Barnard (Surveyor) and there's another surveyor behind the camera; me
Once on its summit we found the highest point with the aid of the level and staff.  The Leica GS15 was positioned over it and we gathered two hours of summit data.  By now blue sky shone from above with the clarity of colour enjoyed and although I’d seen the view across to Yr Wyddfa from this position on many occasions, it still proved awe inspiring as its great wedge of eastern face shot up to the pointed summit, all enhanced by the perspective of tiny figures on its summit and the upper part of the Llanberis path.

The Leica GS15 on the high point of Crib y Ddysgl
Aled filming as Graham explains the intricacies of surveying
As the equipment was packed away we headed down and on to the Llanberis path with Llechog in front of us and the Glyderau across the Afon Nant Peris to our right.  We had a thankful rest and cup of tea in the Halfway House café, meeting Alwena Jones who is one of the five people who Stephen and Aled are following for ‘The Mountain’ programme.  I stayed a few minutes chatting with Alwena as Stephen, Aled, John and Graham headed down.  My descent was on my own, as however fast I tried to walk there seemed no way of catching the others, after a few minutes I settled in to a comfortable pace and spent the next hour admiring the greens on the Moel Eilio ridge as the late afternoons light shone vivid, accentuating the beauty of the hills. 

Heading down from the hill

   
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:  1086.728m (converted to OSGM15)

Base of Trig:  1085.673m (converted to OSGM15)

Summit Grid Reference:  SH 60986 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}

No comments: