Tuesday, 20 May 2014

Mapping Mountains – Trimble Surveys – Pumlumon


18.05.14  Pen y Darren (SN 764 942), Creigiau Bwlch Hyddgen (SN 767 931), Foel Fras (SN 765 925), Foel Uchaf (SN 768 925) and Mynydd Lluest y Rhos (SN 765 920).

Creigiau Bwlch Hyddgen (SN 767 931) and Pen y Darren (SN 764 942)
Pumlumon is often celebrated as the source of the Afon Hafren (River Severn), Afon Gwy (River Wye) and the Rheidol and the highest point in mid Wales.  Its ancient name has connotations relating to its five highest peaks, but the beauty of this massif is to the north of these hills, in the escarpment edges, the heather bound hills to the north of the Afon Hengwn, the forgotten valleys and the solitude of the hills to the north of Nant y Moch Reservoir.  It was some of these latter hills we wanted to investigate today.

The hills to the north of Nant y Moch Reservoir form a chain starting with the isolation of Drosgol (SN 759 878) and Banc Llechwedd-mawr (SN 775 898), before the relative ease of Bryn Moel (SN 772 911), Mynydd Lluest y Rhos (SN 765 920), Foel Uchaf (SN 768 925), Foel Fras (SN 765 925), Creigiau Bwlch Hyddgen (SN 767 931) and Pen y Darren (SN 764 942).  Three of these hills are of particular note as they all have the same 529m map spot height given to their summits.  It was these hills that Mark had suggested we prioritise for today’s survey.  I say prioritise as the origin plan forwarded to Mark was to approach from the south and survey seventeen points, Mark thought this too ambitious and suggested concentrating on the priority hills and approaching from the north.  He also suggested that if time and inclination permitted we may visit the summit of Pen Criegiau’r Llan (SN 745 939) towards the end of the walk.

We parked near to the buildings named Cwmyrhaiadr at SN 756 963, where sufficient space can be found for two or three cars.  This is toward the end of Cwm Rhaiadr, a quiet and beautiful valley south-east of Glaspwll (SN 738 975) and south of the town of Machynlleth.

The day’s forecast was good with sunshine and a light breeze, with the possibility of high cloud pushing in from the west toward late afternoon.  There are at least three options for the ascent route toward Pen y Darren from where we parked, we opted for the one signposted ‘Private’.  This led us up the hill on a good forest track beside the Nant Cwm-cemrhiw before height was gained with a series of zig zags.  I couldn’t help myself as every few minutes I stopped and tried to do the luxuriant flora justice with another photograph.




The ascent through the forest





The track soon brought us out above the near conifer plantation, an incomer amongst the lush canopied deciduous tree growth of the small stream valleys below.  We contoured around the western flank of Pen y Darren on the track before branching up over its upper hillside of bilberry to its three summit domes, these are similar to tumuli in shape and appearance but closer inspection shows natural rock formations jutting out from their sides.  We Trimbled the two highest mounds and lay in the sun relaxing next to tiny alpine plants as a male Emperor Moth darted beside us, a blur of colour, an unexpected and rather impressive visitor to such a place.  Once ten minutes of data was collected from each mound we headed south toward the first bwlch of the day.  This consisted of a large bog, once we decided where we thought the critical bwlch was positioned we set the Trimble and Mark’s hand-held GPS to gather data and retired to the drier confines close to the continuation of the track.

Gathering data on the first summit mound on Pen y Darren





















Gathering data on the second summit mound on Pen y Darren
This track would serve us well during the day as although these hills are generally grassy, the bylchau are boggy, and the track made for ease of passage between the points that we wanted to survey.  Our next point was the summit of Creigiau Bwlch Hyddgen, another top consisting of grass and bilberry.

The Trimble GeoXH 6000 collecting data on the summit of Creigiau Bwlch Hyddgen with (L-R) Foel Uchaf, Mynydd Lluest y Rhos and Foel Fras in background
The southern ridge of this hill led us down to the next bwlch, the map indicates there are two possibilities for the position of the critical bwlch, map contouring suggests the first of these can be discounted and so it proved when we arrived at this spot, as the second option was easily seen to be lower.  Again this consisted of a bog, relatively dry due to the sunshine and fine weather of the last week or so, but no doubt a squelchy affair during wet spells.

Foel Uchaf (SN 768 925) from the bwlch between Creigiau Bwlch Hyddgen and Foel Fras
From this bwlch we made our way to the summit of the second of the 529m map height hills; Foel Fras, a central pivot on this ridge, an enlarged bulk and mound of grass, the summit is sponge like and quite flexible when bounced up and down on.  We sat on grass in a convenient peat channel and waited for the customary ten minutes of Trimble data to be gathered, ate a butty and chatted.

Mynydd Lluest y Rhos (SN 765 920) from the summit of Foel Fras
We then headed eastward down the hill toward what proved to be a problematical bwlch, not in the choice of placement for the Trimble, but in the time it took for the equipment to achieve the required 0.1m accuracy before being activated to log data.  The warnings were there from the start as it showed 0.80m when first logged on to a number of satellites and it then spent over 30 minutes creeping down to the magical 0.1m.  This was probably due to the tussock terrain of the bwlch but probably more so with the close proximity of steepness of hill side.  This critical bwlch is just west of the continuation of the track and not east of the track as shown on the Ordnance Survey 1:50, 000 and 1:25,000 maps.  During our prolonged wait we were joined by Adam Lewis who was three or four days in to a 350km off road cycle journey heading south from Conwy to Port Talbot, as we chatted we were joined by Maldwyn Lewis and his canine companions; Mot and Pero, Maldwyn was on a quad bike and was checking a number of gates to make sure that they were closed.  As Maldwyn waved us goodbye as he sped off on his quad bike with Mot and Pero somehow balanced on the back, Adam zoomed off north on the track heading for Nant yr Arian and a quiet spot to pitch his tent.

Adam Lewis on his way from Conwy to Port Talbot


Maldwyn Lewis with Pero and Mot



We spent around 45 minutes waiting for the Trimble to attain its 0.1m accuracy and to gather data

Our next survey was the summit of Foel Uchaf which is positioned directly above the previously surveyed bwlch.  Once data had been collected we headed south toward the next bwlch which proved remarkably easy to find considering the land hereabouts.  Another ten minutes of data was collected whilst we sat in the late afternoon sun.  By now we’d been out over six hours and the prospect of including Pen Creigiau’r Llan at the end of the walk was quickly dismissed.

(L-R) Mynydd Lluest y Rhos (SN 765 920) and Foel Grafiau (SN 759 920) from the summit of Foel Uchaf

Mark heading towards the last bwlch to survey between Foel Uchaf and Mynydd Lluest y Rhos 


Pen Creigiau'r Llan (SN 745 939) would have to wait for another day
Our last summit to survey was Mynydd Lluest y Rhos, a hill to the east of the Sub-Pedwar named Foel Grafiau.  Another convenient grassy seat above layered peat gave us time to relax as the Trimble gathered the all precious summit data.  Mark lay on the summit ridge and looked northward toward the other two 529m map height hills of Pen y Darren and Foel Fras, the two distinct summit mounds of the former could be seen over the slightly lower western summit ridge of the latter.  We both thought that the most aesthetically pleasing hill out of these three 529m map height hills was the one we were on as it has a fine profile when viewed from the direction we walked these hills, but the indication was that Pen y Darren may just be the higher of the three.  Our belly down on the summit ridge and looking out to the other hills approach to surveying was not scientific, but when I tried it I thought the grass smelled rather nice.

Once data had been stored the Trimble was packed away and as Mark quite justifiably observed; we could now go for a hill walk.  After all, surveying many points is not conducive to what most people regard as a hill walk, I do find it fun and it’s great to see the figures appear on the screen during post processing, but surveying does somewhat break the momentum of a walk.

By now the predicted high cloud had milked the sky with the sunshine turning a lazy summered haziness, we descended west over Foel Grafiau and dropped down to a track to the north-west of this hill’s summit.  We followed this as it gained a little height toward the edge of the conifer plantation at the head of Llyn Penrhaeadr.  Just before the conifer plantation we walked out in to a morass of flat bog, relatively dry and becalmed.  Once to the north of the lake we picked up a track and green path that took us to the top of Pistyll y Llyn.

Pistyll y Llyn is one of the tallest waterfalls in Wales and takes the waters of the Afon Llyfnant on a cascading journey from their sedentary passage north of Llyn Penrhaeadr to a downward drop of approximately 160 metres.

Once on the opposing bank of the steam we found the start of the path that descends in to the cwm.  As we started our downward passage we stood and tried to stare over and in to oblivion, problem was that the ground fell away so steeply that we couldn’t see the waterfall from our airy perch, the sound of water was heard and the view was dramatic but the waterfall was completely out of sight until low down in the valley.

Looking down the Afon Llyfnant valley
The path we were now on proved quite dramatic in its upper section as it clings on to the edge of a vertical drop, lower down we lost the main path and walked through copses of fire trees as the sun sank lower in the sky, casting sublime colours on the land.

Evening light followed us down toward the secluded valley at the base of Pistyll y Llyn
Slowly we lost height toward the valley bottom where yellowed reed grass stretched out in a continuous silken bed, green fields with white specks of bleating lambs overlooked by trees heavy with May blossom and the ever present bird song gave an unending quality to the scene, with Pistyll y Llyn as backdrop.


Pistyll y Llyn almost hidden amongst the crags


May Blossom


Pistyll y Llyn at the head of the Afon Llyfnant valley
The last few minutes back to the car were spent walking past the buildings of Cwmyrhaiadr with their golden mature trees and narrow country lanes, a wonderful walk.


Journeys end










Survey Result:


Pen y Darren

Summit Height:  529.4m (converted to OSGM15)

Summit Grid Reference:  SN 76431 94297

Drop:  105m (non-prioritised Twin Hump reclassified to Hump)

Dominance:  19.83%




Creigiau Bwlch Hyddgen

Summit Height:  506.1m (converted to OSGM15)

Summit Grid Reference:  SN 76740 93201

Bwlch Height:  482.5m (converted to OSGM15)

Bwlch Grid Reference:  SN 76715 92865

Drop:  23.6m

Dominance:  4.66%




Foel Fras

Summit Height:  528.7m (converted to OSGM15)

Summit Grid Reference:  SN 76544 92604

Bwlch Height:  461.2m (converted to OSGM15)

Bwlch Grid Reference:  SN 76454 93672

Drop:  67.6m (prioritised Twin Hump deletion confirmed)

Dominance:  12.78%




Foel Uchaf

Summit Height:  522.0m (converted to OSGM15)

Summit Grid Reference:  SN 76844 92519

Bwlch Height:  499.1m (converted to OSGM15)

Bwlch Grid Reference:  SN 76808 92229

Drop:  22.9m

Dominance:  4.39%




Mynydd Lluest y Rhos (significant name change)

Summit Height:  528.3m (converted to OSGM15)

Summit Grid Reference:  SN 76543 92029

Bwlch Height:  498.3m (converted to OSGM15)

Bwlch Grid Reference:  SN 76752 92522

Drop:  30.0m (non-prioritised Twin Hump deletion confirmed, continued Dewey status is speculative)

Dominance:  5.68%




As the 30m (30.009m) drop value has an accuracy of + / - 0.1m the result is within the margin of uncertainty of the technology used.  Therefore, it is hoped that a line survey can be conducted to determine the absolute drop for this hill.



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

1 comment:

dels arty party said...

stunning photography as always Mr P