Sunday, 22 May 2016

Mapping Mountains – Trimble Surveys – Pumlumon

13.05.16  Bryn yr Ŵyn (SN 839 925), Bryn Llwyd (SN 835 920) and Fedw Ddu (SN 856 917)

Bryn Llwyd (SN 835 920) centre left of photo and Bryn yr Ŵyn (SN 839 925) on right of photo

Having visited Bryn Llwyd (this is a locally known name, used in preference to Esgair Greolen which appears on Ordnance Survey maps) and Bryn yr Ŵyn eight days ago the resulting data for each summit and the drop attained for the higher of the two was of sufficient interest that a second survey was warranted. 

I approached from the south-east from where a minor road heads south from the small community of Penffordd Las (Staylittle).  A passing place proved convenient to park with a few other such places also available near to where a track heads north toward the farm of Nant-yr-hafod.

The forecast for the day was good with sunshine and dry conditions predicted and as I walked up the track the morning’s blue sky pieced the landscape.  Just before the farm the track veers west toward the 488m map heighted summit of Fedw Ddu, and it was here that I met Gareth Griffith who came chugging around the bend with three farm dogs scampering around his vehicle.  I waved a ‘hello’ as I lifted the catch on a gate and he pulled up for a chat.

Gareth Griffith

Gareth had lived at Nant-yr-hafod for forty years and was heading up the hill to feed the sheep.  I explained where I was hoping to go and as some of this land was not a part of open access or did not have a right of way on it, I asked if he minded me using the continuation of the track, he was happy enough for me to do this and we continued our conversation, he told me that he still goes out shepherding on horseback and soon the topic of hill names was mentioned.  He proved extremely knowledgeable and rolled off name after name of the hills and streams above where he farmed.  One name in particular was of interest, it related to the hill that Ordnance Survey maps name as Esgair Greolen, which is the hill I wanted to concentrate the morning’s surveying activities on.  He knew this hill by a different name; Bryn Llwyd, and after mentioning it he said that ‘this name doesn’t appear on the map.’ 

Gareth and his dogs

As Gareth was heading up to the end of the track which is close to the summit of Fedw Ddu I asked if I could have a lift, he kindly said yes and within a couple of minutes I was happily sitting next to him in his vehicle as it chugged its way up the track.  He stopped three times to unload sheep feed and put his dogs in the cage on the trailer at the back.  Rarely do you have an opportunity to make such swift progress up a hill, and as he pulled up at the end of the track I clambered out, put my rucksack on, shook his hand and thanked him in Welsh.

Time to feed the sheep

As Gareth turned his vehicle round I waved my thanks and he disappeared down the hill, leaving me in the early morning sunshine smiling at the thought of how little energy I’d used to get so high.  Within a few minutes I was assessing the land at the bwlch of Fedw Ddu and soon had the Trimble positioned on my rucksack gathering data.

Gathering data at the critical bwlch of Fedw Ddu

Beyond this bwlch a path on the open hillside made good progress toward the summit of Bryn yr Ŵyn.  I planned to take a further two data sets from this first summit, giving five in all when combined with my previous visit, I also wanted to take three data sets from the summit of Bryn Llwyd which would make four in all when combined with those previously taken, and at least seven data sets from the area of the critical bwlch.  All accumulated summit data could be averaged to give the absolute height of each hill, whilst the bwlch data would give an evaluation of the lay of land on the hill to hill traverse, when the data were combined the higher summit would be clarified and its drop obtained.

Once on top of Bryn yr Ŵyn I quickly placed the Trimble on the same point as I had eight days previously and stood a safe distant from it and below it as the first of what proved to be a further 19 data sets were gathered from these hills.  Away to my south south-west Bryn Llwyd looked distinctly lower, as it does from almost every angle that I’ve viewed these hills from.

Gathering data at the summit of Bryn yr Ŵyn

Once a further two data sets were gathered from the summit of Bryn yr Ŵyn I walked over the moor toward Bryn Llwyd and took a further three data sets from its summit.  I placed the Trimble on the ground for two of these data sets and once on top of my rucksack.

Gathering data at the summit of Bryn Llwyd

All that remained was to gather data from the bwlch, this proved fun as a further 13 data sets were taken.  The first of which was from where contour interpolation suggests the critical bwlch to be placed on the ground, I’d taken the third bwlch data set from this position on my last visit and judged this to be visually on the downward slope of the valley to valley traverse, with the traverse from the hill to hill direction just a few metres to the north of this point.

During the remainder of my time at the bwlch I took 12 data sets on what I judged to be on the hill to hill traverse, this heads in a west to east direction.  On every bwlch data set I positioned the Trimble atop my rucksack to give it elevation above the surrounding moor and measured the offset between its internal antenna and the ground with a steel tape.  The furthest point from where I gathered bwlch data from to the west and to the east was at a point that was visually on the upward slope of the hill to hill traverse, with the remainder of the positions approximately 8-10 metres apart, and on the land that I judged to be where the low point of the hill to hill traverse was situated I took data sets five metres apart.  Each point was marked with a flag which proved ideal to visually assess the lay of land.  Once all data sets were complete I packed everything away and started the walk back toward my car.

The march of flags across the bwlch

The positions surveyed at the bwlch are indicated with flags with Bryn Llwyd in the background

Leaving the bwlch I followed a path which crests the connecting bwlch between Bryn Llwyd and Bryn yr Ŵyn and contours around the latter’s southern slopes before joining the inward path and track toward Fedw Ddu.  I’d visited this hill once before and wondered if summit data were possible to collect as its high point is immersed in a conifer plantation.

A forest break conveniently passes over where the spot height appears on the map, and I followed the moss laden ground between the trees until the up seemed to have ended and the beyond seemed to consist of down.  I aimed for a high point at the base of trees and happy that I’d visited what could be construed as the summit I positioned the Trimble to try and collect data, remarkably its 0.1m accuracy level before data should be logged was attained relatively quickly, and as it beeped away collecting its customary 300 data points I stood behind one of many conifer trees, out of sight of the equipment.

Gathering data at the summit of Fedw Ddu

After packing the Trimble away all that remained was to re-trace my inward route, this time on foot, and 6 ½ hours after setting off I arrived back at my car with 19 data points stored in the Trimble. 

Survey Result:

The result of the processed data for the first Trimble survey of Bryn Llwyd and Bryn yr Ŵyn was of sufficient interest to warrant another survey, and therefore the following detail takes in the combined results from each survey:

Bryn yr Ŵyn

Summit Height:  499.9m (converted to OSGM15, and average of five surveys and summit relocated to Bryn Llwyd [SN 83574 92022])

Summit Grid Reference:  SN 83919 92571

Drop:  9.9m

Dominance:  1.99%

Summit Height:  501.4m (converted to OSGM15, and average of four surveys and Dewey and Uchaf status confirmed and summit relocated from Bryn yr Ŵyn [SN 83919 92571] and new Pellennig hill qualifying on distance) 

Summit Grid Reference:  SN 83574 92022

Bwlch Height:  471.2m (converted to OSGM15)

Bwlch Grid Reference:  SN 83220 91901

Drop:  30.3m (500m Twmpau status remains)

Dominance:  6.04%

Fedw Ddu

Summit Height:  487.7m (converted to OSGM15)

Summit Grid Reference:  SN 85672 91803

Bwlch Height:  434.8m (converted to OSGM15)

Bwlch Grid Reference:  SN 85176 91727

Drop:  52.9m

Dominance:  10.85%

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