Friday, 22 May 2015

Mapping Mountains – Trimble Surveys – Pumlumon

16.05.15  Bryn y Tail (SN 916 874) (bwlch only)

Bryn y Tail (SN 916 874)
In April of last year I did a walk from the dam that crosses Llyn Clywedog at SN 921 881 to Staylittle across a number of Pedwar hills in glorious spring sunshine.  I surveyed 21 points in all that day; two of them were for the bwlch of Bryn y Tail which is listed as a Pedwar and Hump.  I took data from the stone parapet that runs adjacent to the pavement beside the busy B4518 and noted the measurement offset to the road, I also took data from below the dam on land that some may constitute as the remains of the natural bwlch for Bryn y Tail.  Having had this on-site visit I concluded that it was not possible to state definitely that the natural bwlch remained, therefore I now wanted to take a further data set for comparison of height to the water level of the Reservoir.  I had waited just over a year to re-visit and do this and with an evening meal invitation waiting for me at 6.30pm in Llanidloes, I just had time to re-visit and gather the all-important data.

Fortunately access to this dam is easy as there is a large parking area next to it; from here I walked across the road and looked down at the choppy waters being blown against the stone dam as the wind blew in from the west.  I had wondered what the access would be like down to the water’s edge; from this direction it didn’t look very hopeful.  Ideally I would need the Trimble to be placed above the water, safe from its depths and out of harm’s way, but the ground over the parapet was not ideal, neither was the ground to the south which entered the water over a small vertical rock.

Deciding that my only option was to the north of the dam I walked the small distance on the road to where a fence gave access to the water’s edge, I clambered over and dropped down to a flat concrete walk way at the base of the stone surrounds of the dam, this was level and wide enough to walk two abreast for a number of metres before it started its downward route into the water.  I stood and watched the water as it frothed its way toward the walkway as it blew in on a strong wind.  At the base of this concrete walkway were a number of large rocks which broke the water swell and even though the water was choppy it did not spill over onto the concrete walkway, I decided that this was ideal for the Trimble to laze in the afternoon sun gathering data!

The main problem was that the stone surround of the dam was immediately behind the Trimble, but I had confidence in its working capability to gather adequate data even though satellite coverage was not ideal.  Before placing the Trimble down on the concrete I measured the offset between its position and the water level which was becalmed against the concrete amongst a jumble of twigs and debris, the offset came to 0.71m.

This position even had a concrete seat for me to sit on, and as what I thought would be a long wait for the Trimble to attain its accuracy commenced, I sat shielded from direct sunlight by a small tree and watched traffic pass above me as it zoomed off to Machynlleth and Llanidloes.  I checked the Trimble every few minutes and thought my wait would be a long one, but after about ten minutes the figure of 0.1m showed on its screen and I quickly pressed ‘Log’ and retired to my concrete seat to happily scoff another butty.

Gathering data beside the water of Llyn Clywedog
I left the Trimble gathering data for ten minutes as I replenished my energy levels before switching it off.  I now wanted to re-examine the land at the base of the dam on its eastern side, this is where the natural bwlch lay if it had not been swamped under the waters of the Reservoir or interned in the stone of the dam, having inspected this land before I do not think that the natural bwlch still remains, but I wanted another look and take a few more photos and see if an examination of the land from the south as it descends from Bryn y Tail toward the road over the dam could throw any light on where the bwlch lay.

Looking south across the road with the Reservoir on right of photo and the earthen facade of the dam on the left
The land from Bryn y Tail meets the road where I had looked down at the small vertical rock entering the waters of the Reservoir, from here one can imagine where the continuation of the natural land went as it bisected the road and continued down to meet the northerly land which descended from the summit of Waun y Gadair.  The land from Waun y Gadair descends to the bottom of the grassed eastern façade of the dam which is 7.7m lower than the road.

Looking down from the top of the earthen facade of the dam to the connecting land between Bryn y Tail and Waun y Gadair
Looking south up the graveled path that descends from the car park with the point where Trimble data was previously gathered just to the left of the wooden fence
As I stood near to the spot where I had collected Trimble data from thirteen months previously I wondered if anyone could definitely say that the natural bwlch still existed, to me this would be hard to justify as the land hereabouts had been upturned and drastically altered.  Although the descending Waun y Gadair land seemed easier to follow down beside the grassed eastern façade of the dam, it would be difficult to say for certain if the natural course of the hill to hill traverse at the critical bwlch still remained, as the southerly land seemed non-existent as it had been upturned for the construction of the dam.  Happy in the knowledge that this on-site visit had confirmed my thoughts from my previous visit I walked up the path to the car park and headed off to Llanidloes for a lovely evening meal.

Survey Result:

Bryn y Tail

Bwlch Height (beside water in Reservoir):  282.6m (converted to OSGM15)

Bwlch Grid Reference (point at which survey for water level was taken):  SN 92061 88376

Dominance:  31.16%

See below for results of summit survey and the drop for the hill:

Bryn y Tail

Summit Height:  403.1m (converted to OSGM15) (Pedwar status confirmed)

Summit Grid Reference:  SN 91690 87470

Bwlch Height:  277.5m (converted to OSGM15) (ground below road passing over dam)

Bwlch Grid Reference:  SN 92150 88255 (ground below road passing over dam)

Drop:  125.6m

Dominance:  31.16%

The Ordnance Survey Historical 1:25,000 map has bwlch contouring between 925ft – 950ft (282m –290m).  As the height of the ground below the dam is 277.5m (converted to OSGM15) it indicates that the natural bwlch no longer exists.  However, the ground surveyed with the Trimble below the dam is likely to be closer in position and height than any other feature still remaining, therefore these details are taken for the bwlch of this hill.  

For the post detailing this hill’s previous summit and bwlch survey click {here}

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

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