Saturday, 19 April 2014

Mapping Mountains – Trimble Surveys – Pumlumon

18.04.14 and 20.04.14  Bryn y Tail (SN 916 874), Waun y Gadair (SN 918 887), Dinas (SN

904 885), Mynydd Derw Llwydion (SN 907 900), Foel Fawr (SN 900 904), 

Carreg y Big (SN 902 909), Mynydd Dyrnol (SN 901 917) and Gamallt (SN 895 925)

Yr Allt (SN 908 875)
Pedwar bagging, and a walk I’d wanted to do for a number of years, but as many of the hills are not dramatic and are no more than high grassy fields, albeit quite lovely high grassy fields, and as I guessed that there would be a number of barbed wire fences to negotiate and non-access land to venture on to, I wondered if it was appropriate to do the walk with another person.  Having now got the Trimble and wanting to survey the eight summits and eight bylchau en route it was definitely a walk to do on my own.  I remember John Fitzgerald (pioneer of Trimble mountain surveys) saying that he was finding some of his hill walking friends were mysteriously disappearing and no longer visiting the hills with him as they didn’t want to stop every few minutes and wait whilst he gathered another data set.  I’ll have to be aware of this, especially so on walks that include a number of summits.  After all there seems to be only a select few hardened surveying souls who are willing to twiddle their respective fingers time after time; Barnard, Jackson, Trengove and Williams to name but four are ones that swing to mind.

I parked in the car park adjacent to the Llyn Clywedog dam (SN 921 881) at 8.40am and was soon standing beside the road with a small sign saying ‘Staylittle Please’ on one side and ‘Three Miles Please’ on the other.  The intention being to hitch to Staylittle and walk back over the hills to the Clywedog dam, half an hour later only about ten cars had passed and none had stopped.  Therefore I drove to Staylittle, parked and smiled at a woman who was walking up the lane to her car, within a few minutes I was happily sitting in her and her husband’s motor home heading back down the road to the Clywedog dam.  They were on their way to mainland Europe for a week travelling around.  I thanked them as I was dropped off at the top of the road above Llyn Clywedog, next to a gate and stile that gave easy access to the first hill of the day.

It was just past 9.35am as I made my way up toward the summit of Bryn y Tail, the hill didn’t live up to its name (the manure hill or the dung hill) as there wasn’t a cow pat in sight.  The last upper section is on a small, broad grassy ridge; I placed the Trimble on the high point and gathered the customary ten minutes of data.  Bryn y Fan dominated the view to the north-east, with Dinas (the third hill of the day) dominating the north-western view across the waters of Llyn Clywedog.

The Trimble GeoXH 6000 on the summit of Bryn y Tail (SN 916 874) with Bryn y Fan (SN 931 884) in the background
Bryn y Tail (SN 916 874)
The northerly ridge down toward the dam is a delight; soon it brought me to a gate and access on to the road and the dam.  I wanted to investigate the land below the dam on its eastern side and see if any remnants of the natural bwlch still remained, before doing so I gathered five minutes of data from the stone parapet of the dam wall and input a 1.03m height offset in to the Trimble. 

Gathering data on the stone parapet of the dam at Bwlch y Gle
The Reservoir was completed in 1967 and dammed the Afon Clywedog; a tributary of the Afon Hafren (River Severn), its concrete dam wall is the tallest of its kind in the UK with a height of 72m and a length of 230 metres.  The catchment water supplies Birmingham and the English midlands.  When looking on Ordnance Survey maps that pre-date 1967 the Afon Clywedog contours its way around Dinas, the finger of north-easterly land delicately descending from the summit of Yr Allt and the southerly land of Bryn y Tail before heading south-east to join the Afon Hafren at Llanidloes.  These old maps also give the area of land where the natural bwlch once lay.  It was this that I now wanted to go and look for.

Just below the dam road is a path / track that makes its way toward the summit of Bryn y Fan, this path is also on the hill to hill traverse between Bryn y Tail and Waun y Gadair.  I placed the Trimble on a suitable spot and gathered another five minutes of data.  It was very hard to judge if the natural bwlch still remained; it could well be buried under the road or the waters of the reservoir.  However the processed data will at least give us an accurate height for the ground below the road and this can then be compared to the contours on old maps.

Is this the natural critical bwlch for Bryn y Tail?
By now I’d taken my one skin summer walking jacket off as the slight chill of the morning had been replaced by the strengthening heat of the sun.  The next hill was Waun y Gadair (SN 918 887); a Pedwar with a listed drop of 39m.  Once summit data was collected I spent quite some time getting data from a number of points to see which the true critical bwlch was.  Altogether five points were surveyed, some beside the road, which proved rather daunting considering the last time I did this a car ran over the Trimble, this time it was placed a metre or so from the road and I stood near to it, facing oncoming traffic, so cars had to pass me first.

Waun y Gadair (SN 918 887)
Dicing with Trimble death, one of the five points surveyed for the critical bwlch position of Waun y Gadair
The continuation toward Staylittle was via Dinas (SN 904 885), another Pedwar, and one that had a grassed vehicle track making its way up toward the summit, the high point overlooks the blue waters of Clywedog.  Once summit data had been collected I retraced my steps and joined the road to survey the critical bwlch for Dinas.

Dinas (SN 904 885)
Or so I thought, that is, as far as visiting the summit of Dinas is concerned, having processed the data the day after the visit I found out that I only got as far as a small subsidiary summit a few hundred metres from the higher top - oooppppssss.  So I revisited early in the morning on the 20th in slightly different conditions, as it was mightily chilly, murky and with wisps of mist passing over the summit.  At least I got some summit data this time!  Right the interlude is over, on with the blog post about the walk..............

The Trimble GeoXH 6000 on the subsidiary summit of Dinas (must remember to check the map next time)
Early morning and now back at the higher summit which can be seen in the background in the above photo
My route now took me away from the B4518 as I headed north up a minor road to the summit of a 387m map heighted hill at SN 907 900 which is listed as Mynydd Derw Llwydion as the land where the summit of this hill is situated is on the sheepwalk of the farm named Derw Llwydion.  As the Trimble gathered data atop the grassy top I stood and watched a Red Kite majestically pass overhead with a radiant blue as a backdrop.  This hill is listed as a Sub with 29m of drop in the Welsh 300m P30 list, the drop figure based on the 387m summit spot height and a 358m spot height at SN 903 902 that appears on a road junction on OS maps.  The critical bwlch proved to be higher than the road junction and just to its north-east.

Such a majestic bird - Red Kite
Summits and bylchau were now appearing in quick succession, with the summit of Foel Fawr (SN 900 904) being another attractive high point atop a rock outcrop.  The next bwlch was in a bog (SN 900 906) which thankfully had dried somewhat due to the sunshine over the last week, whilst collecting data a well camouflaged hare scampered off, I hoped it wasn’t on a collision course with the Trimble as it was heading that way!  The following summit was Carreg y Big (SN 902 909) a Sub-Pedwar with 23m of listed drop based on 417m summit and 394m bwlch spot heights, the bog at the bwlch was higher and to the west of where the 394m spot height appears on the map, doubtful that it’ll loose its Sub-Pedwar status though.

The summit of Foel Fawr (SN 900 904)
The next bwlch (SN 901 913) has a 379m spot height on a road junction on OS maps, when I reached this point I looked, and looked, and did a bit more looking and approached the area from various angles and still ended up not knowing where on earth to place the Trimble.  I think the heat had caused a serious case of bwlch mind muddle!  The path of the bwlch seemed to curve its way around the central grass and gravel triangle that separated the roads and then it seemed to curve its way back.  I gave the margin of uncertainty applied to the Trimble position as 0.5m, placed it next to the road and hoped a car would not try and kill it again.  I think it was on a safe bet as no car had passed on this minor road for at least half an hour.

Bwlch mind muddle, such a small area of land, but I just could not work out where the Trimble should be placed
Only two summits remained and the next was Mynydd Dyrnol (SN901 917) another Sub-Pedwar with a 28m listed drop.  The summit area was quite expansive when compared to previous ones of the day; I picked my spot, logged ten minutes of data and proceeded down to another bog which is next to a raised stone track and the bwlch of the hill.

All that remained was the slow plod up to the highest summit of the day; Gamallt (SN 895 925).  The high point is a few metres away from a fence junction that has an unusual pillar next to it.  The last and 21st data point (a record for me, beating the 16 data points taken on the walk around the Breiddin on 18.12.13) of the day was beside a fence separating rough ground from furrowed land that beyond comprised a conifer plantation.  After five minutes of data from the point, which is the critical bwlch for Gamallt, was collected, I joined a track that swings around the hill and down in to Staylittle.  A great day, eight hills, eight bylchau, 21 data points taking over eight hours, these hills are seldom visited and are situated in beautiful land overlooking Llyn Clywedog and the wilds of the high Pumlumon range to their west, a good little ridge walk, albeit a bit convoluted.

The summit of Gamallt (SN895 925)

Survey Result:

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.  

Post detailing the later survey beside the water in the Reservoir

Waun y Gadair

Summit Height:  403.8m (converted to OSGM15) (Pedwar status confirmed)

Summit Grid Reference:  SN 91823 88721

Bwlch Height:  367.0m (converted to OSGM15)

Bwlch Grid Reference:  SN 91460 88895

Drop:  36.8m

Dominance:  9.12%


Summit Height:  445.3m (converted to OSGM15)

Summit Grid Reference:  SN 90409 88538

Bwlch Height:  342.9m (converted to OSGM15)

Bwlch Grid Reference:  SN 90377 89417

Drop:  102.4m (Hump status confirmed)

Dominance:  23.00%

Mynydd Derw Llwydion

Summit Height:  388.1m (converted to OSGM15)

Summit Grid Reference:  SN 90721 90069

Bwlch Height:  358.1m (converted to OSGM15)

Bwlch Grid Reference:  SN 90391 90264

Drop:  30.0m (Trichant addition confirmed)

Doninance:  7.73%   

Summit Height:  422.9m (converted to OSGM15)

Summit Grid Reference:  SN 90029 90416

Bwlch Height:  378.8m (converted to OSGM15)

Bwlch Grid Reference:  SN 90119 91400

Drop:  44.1m

Dominance:  10.43%

Carreg y Big

Summit Height:  417.0m (converted to OSGM15)

Summit Grid Reference:  SN 90241 90949

Bwlch Height:  397.3m (converted to OSGM15)

Bwlch Grid Reference:  SN 90124 90659

Drop:  19.7m (400m Sub-Pedwar deletion confirmed)

Dominance:  4.72%

Mynydd Dyrnol

Summit Height:  412.0m (converted to OSGM15)

Summit Grid Reference:  SN 90242 91672

Bwlch Height:  383.0m (converted to OSGM15)

Bwlch Grid Reference:  SN 90145 91971

Drop:  28.9m (400m Sub-Pedwar status confirmed)

Dominance:  7.02%


Summit Height:  450.8m (converted to OSGM15)

Summit Grid Reference:  SN 89552 92558

Bwlch Height:  415.0m (converted to OSGM15)

Bwlch Grid Reference:  SN 89529 92861

Drop:  35.8m (Pedwar status confirmed)

Dominance:  7.94%

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

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