Wednesday, 22 November 2017

Mapping Mountains – Trimble Surveys – Mynyddoedd Duon

28.09.17  Crib y Garth (SO 274 348), Hay Bluff (SO 244 366), Twyn Llech (SO 255 353) and Pt. 637.1m (SO 267 322)

Crib y Garth (SO 274 348)

The Olchon Valley penetrates the eastern ridges of Mynyddoedd Duon, it nestles amongst grazing pasture where the greens of fields butt up against the browned bracken of descending hill slopes.  It is a quiet valley with few houses or farms, this is one of its qualities as it gives the valley peace and tranquillity.

Getting to the Olchon Valley requires miles of driving negotiating narrow country lanes, today I approached from Y Gelli Gandryll (Hay-on-Wye) with the intention of doing a circuit taking in Crib y Garth and ending atop a recently reclassified Uchaf that is still listed as a Nuttall, on route I planned on surveying anything and everything.  It was 11 miles from the outskirts of Y Gelli meandering around narrow lanes with high hedgerows and hopefully few vehicles, to the car park at the base of the south-easterly ridge of Crib y Garth.  I had my boots on and was walking by 9.05am; not bad considering the drive south had taken over two hours.

With a forecast of light breeze and sunny conditions the day was set fine.  Above, the steepening slopes of Crib y Garth were coloured in dulled bracken and yellowed gorse interspersed with the greens of closely cropped grass, whilst the eastern main ridge of Mynyddoedd Duon glimmered against a backdrop of early morning sun, away to the east the Malvern Hills shone in a bluish tinged slender silhouette set against a manicured green foreground, it was a beautiful day to be on the hill.

The path leading up Crib y Garth

The southern part of the main easterly ridge of Mynyddoedd Duon

The Malvern Hills toward the east

To my west across the upper reaches of the Olchon Valley the path for my descent route skimmed down steep slopes with the moorland dulled greens merging with the browned bracken.  It was the first time I had approached these hills from this direction and it was good to see the opposing side of the hill listed as Black Mountain South Top in the Nuttall’s guide, having approached its summit from the westerly side on many an occasion.

Pt. 637.1m (SO 267 322) and my descent route off the hill

Crib y Garth is also known as the Black Hill and the Cat’s Back Ridge, and its south-easterly ridge is highly recommended as it heads up on steep slopes to a narrow slender ridge giving expansive views.  A good path leads from the car park to the summit of the hill and nowhere could the going be described as dangerous, as although the ridge is narrow it is also wide enough to always feel safe on.  In its upper section the ridge broadens and the trig pillar on the summit area comes in to view, today the area around the trig was festooned by army personnel out on early morning manoeuvres.

The central section of Crib y Garth

The upper section of Crib by Garth

The trig pillar atop Crib y Garth

Many of the points I planned on surveying had been analysed from LIDAR data by Aled, and I had come well prepared with their respective ten figure grid references.  The high point of Crig y Garth is a few metres south-westward of the trig pillar amongst heather and tussock grass, as I zeroed in to the LIDAR grid reference I said my hello’s to a few of the throng on top, they were split in to two groups with one group preparing to leave in the next five minutes, the previous day they had been on the Bannau Brycheiniog (Brecon Beacons) in foul weather which one in their party described as good conditions for testing navigational skills.  By the time the Trimble had gathered its first five minute data set they were away, walking fast and then running with large and no doubt heavy rucksacks heading down the path toward Hay Bluff.

Gathering data at the summit of Crib y Garth

Heading off toward Hay Bluff

The second group left the summit shortly after I set off toward this hill’s connecting col; I stood by the side of the path as they zoomed past me, saying hi, hello and good morning as they sped past.

LIDAR gives the col position for Crib y Garth beside a small pool next to the path, another five minute data set was gathered from this point before I continued on the path heading north-westward toward my next summit; Hay Bluff.

Gathering data at the critical col of Crib y Garth

It felt good to be out, and although the summit of Crib y Garth had been festooned by the military and more were appearing descending a near hillside, there was a sublime beauty to the land with open skies and early autumnal colour and warmth to the sun that dissipated the slight morning chilled breeze.

Before continuing toward Hay Bluff I took another data set from the furthest north-westerly point of the area of the col close to where a path emerges on the hill from the upper reaches of the Olchon Valley, from this point Crib y Garth rose in what visually could be judged to be a minimum of 15m, however the eye is rather deceptive in judging height, and I suspect this is a case in point.

The white trig pillar atop Hay Bluff was on view a long time before I reached it, thankfully although the path contouring up toward the connecting bwlch was a little boggy in places it helps progress, otherwise the moorland surrounds would take their toll.  Grazing horses dotted the landscape munching on tasty titbits as I reached the main north – south path that follows the broad ridge crest of the main eastern ridge.

Over recent years there has been extensive repairs to the path hereabouts, I remember it being a literal bog trot that over many years expanded outward as a progression of hill walkers sought dry refuse away from the worst of the black oozing slime, nowadays it is paved and gravelled and gives quick and easy access to the summit trig atop Hay Bluff.

This summit must be one of the most popular in the hill range as there is easy access from the car park close to the top of Bwlch yr Efengyl (Gospel Pass), and although there were a number of people beside the trig as I made my way on the gravelled path, they were all gone by the time I reached the summit, and as the Trimble gathered data perched atop a small grassy knoll ten metres from the trig pillar all was quiet.

Gathering data at the summit of Hay Bluff

The next point to survey was the critical bwlch of Hay Bluff, this proved to be close to the path in a morass of puddles and boggy ground.  As the Trimble collected its customary five minutes of data another large group of kaki clad army personnel gathered at the meeting of paths close to the bwlch and sped off down to the east, their orange topped rucksacks parading in a fast moving line through the moor and bog.

Gathering data at the critical bwlch of Hay Bluff

Heading down from the area of the Hay Bluff bwlch

To my immediate south the path led up to the high point of this eastern main ridge, the summit of Twyn Llech consists of tussock grass interspersed amongst rogue heather and is plateau like, with a northern and a southern rise with a slight dip between, I wanted to gather data from what I judged to be the high point of each.  The northern high point proved relatively easy to pinpoint as I assessed the lay of land from a number of directions and also asked a couple of walker’s what point they thought the highest, as one of them crouched down and peered across the tussocks he confirmed my placement.  Once 300 datum points were collected I proceeded to the southern high point and repeated the process, although this second point proved a little more difficult in judging where the high point lay, but happy with my chosen spot the Trimble was set up and another data set gathered.

Gathering data from the northerly high point of Twyn Llech

Gathering data from the southerly high point of Twyn Llech

Only one hill remained and this had been reclassified approximately twelve months ago from an Uchaf to a 500m Sub-Uchaf due to LIDAR data analysed by Aled.  This analysis had given the bwlch as 622.669m high and three points at the summit all 637.369m high, with the latter three points close to one another.  I now wanted to gather a number of data sets from both the bwlch and the summit.

The path repairs are continuing as although much of the summit and upper southern section of Twyn Llech is now flagged, the path gives way to its wide bare and boggy nature lower down, and grouped all around the path were large black bags full of gravel awaiting their dispersal and path repair.

Bags full of gravel for imminent path repairs

The ten figure grid reference for the next bwlch produced by LIDAR data gives the critical point to be a couple of metres from the gravelled path on its eastern side, I zeroed in to this point using the Trimble as a hand-held GPS and set it on top of my rucksack to give it a semblance of elevation above its immediate surrounds and measured the offset between the internal antenna and the ground at the base of the rucksack, and pressed ‘Log’ once the required 0.1m accuracy level had been attained.  Once five minutes of data were gathered I switched the equipment off, and repeated the survey for a second data set from the exact same point with the intention of using an average of the two data sets for the height from this point.

Gathering data at the first set-up position at the bwlch of Pt. 637.1m

However, during my wait in the afternoon sunshine for these two data sets to be gathered I assessed the lay of land and decided to gather another data set from where I judged the low point of the gravelled path to be, this was only a few metres from where the Trimble had previously been set up and was on the hill to hill traverse.  I again gathered two data sets from this point.

Gathering data at the second set-up position at the bwlch of Pt. 637.1m

All that remained was to gather a number of data sets from the summit, during the last bwlch data set a walker approached and I asked him if he would be good enough to wait three or four minutes until the Trimble had done its stuff.  After packing the Trimble away we walked together to the summit, chatting about the hills and surveying.

The summit area of this last hill consists in the main of tussock grass and is plateau like, almost like a tussock grassed football field it sprawls out barely undulating, the small cairn signifying where the summit could be was a good few metres from where LIDAR gives the central position of the three identical high points to be.  Having zeroed in on the LIDAR position I set the Trimble up and proceeded to gather another two data sets.

Gathering data at the first set-up position at the summit of Pt. 637.1m

Whilst waiting for the Trimble to gather its second summit data set I decided that whilst here I should gather two data sets from beside the small cairn for comparison to the LIDAR position.  Therefore as the western sky gave delicate colour to the high cloud I set the Trimble up and measured a 0.49m offset to the peat below the moor grass covered lower small rocks of the cairn and again gathered two data sets.  Happy that I had completed all I could, I packed the Trimble away and headed south toward a crossroads of paths and then started my eastward descent that I had looked out toward earlier in the day.

Gathering data at the second set-up position at the summit of Pt. 637.1m

The path led down on grassed slopes beside bracken and gave excellent views across the valley toward Crib y Garth and south to patch worked fields interspersed with the occasion up thrust of a small hill. 

The view south from above the Olchon Valley

As I descended I spotted the person who I had asked to wait whilst the last of the four bwlch data sets were gathered, he was sitting having a bite to eat with a stunning view in front.  I stopped and chatted and asked about the footpath marked on the map that crosses the stream in the valley below, he’d been this way a number of times and offered to direct me to a lower footpath which was his favoured descent route.  As he was quicker than me, I set off down the steepening path and he caught me up just as I reached the narrow lane, he lived in Ross-on-Wye and he’d chosen what proved to be a good route, we walked and chatted our way down the lane and across a foot stile leading down a field to a small bridge crossing the steam followed by a steep ascent the other side toward the lane and the car park, he left me on the last section as I was flagging, I slowly plodded up content to be out in the hills and looking forward to a sit down in the car, as I approached the last section of uphill lane he waved as he drove past.  It was good to rest at the car, I now had to decide whether to drive to Bwlch yr Efengyl (Gospel Pass) to try and gather a data set for the critical bwlch of Twyn Llech.  This point had also been analysed with LIDAR data by Aled and I had looked at the road the previous evening via a Google car and it looked as if it was tight and may prove difficult to survey with any passing car, but being a completist I succumbed to the temptation and set off on my inward route of miles and miles on narrow lanes, and further on still toward the highest public road in the whole of Wales.

Bwlch yr Efengyl (Gospel Pass) is also the critical bwlch of Twyn Llech and the highest public road in Wales

Arriving late in the afternoon at the top of the road with low sunlight casting illuminated colour after miles and hours on the hill was an uplifting experience.  I left the car squeezed on the side of the road in red splattered muddy puddles with just enough space for any vehicle to pass and proceeded to assess the lay of land from both directions on the narrow road before activating the Trimble to gather data perched safely on top of my car’s roof with a 1.44m offset already measured.  Two cars passed during the next five minutes and I pottered around taking a few photographs and wondering if it was possible to gather another data set from the opposing edge of road.

Gathering data at the first set-up position at the bwlch of Twyn Llech

Once the Trimble had gathered five minutes of data I switched it off, and immediately switched it on again and left it above the road on a small grassy knoll to log into satellite coverage, and reversed my car to a small pull in spot just below the critical bwlch.  By now traffic was less frequent and I quickly positioned the Trimble atop my rucksack on the opposing side of the narrow road and stood back as it gathered its last data set of the day.

Gathering data at the critical bwlch of Twyn Llech

Only one car approached as the Trimble gathered this last data set and I flagged it down and explained what I was doing, and asked if they would mind waiting three minutes until the allotted five minutes of data were collected.

Gathering data at the second set-up position at the bwlch of Twyn Llech

Once data were stored, I packed the Trimble away and drove down the road a short distance to the roadside car park and changed before heading the two hours home.  In all 17 data sets had been gathered and four hills surveyed, and as per usual there will no doubt be some interesting results.

Survey Result:

Crib y Garth

Summit Height:  640.6m (converted to OSGM15) 
Summit Grid Reference:  SO 27475 34800

Col Height:  632.9m (converted to OSGM15)

Col Grid Reference:  SO 27008 35057

Drop:  7.7m

Dominance:  1.20%

Hay Bluff

Summit Height:  676.3m (converted to OSGM15)  

Summit Grid Reference:  SO 24421 36624

Bwlch Height:  663.8m (converted to OSGM15)

Bwlch Grid Reference:  SO 25030 36098

Drop:  12.5m (non Uchaf status confirmed)

Dominance:  1.85%

Summit Height:  703.6m (converted to OSGM15, average of two Trimble GeoXH 6000 surveys) (significant height revision) 
Summit Grid Reference:  SO 25520 35383 (summit relocation confirmed)

Bwlch Height:  549.1m (converted to OSGM15)

Bwlch Grid Reference:  SO 23542 35192

Drop:  154.5m

Dominance:  21.96%

Pt. 637.1m

Summit Height:  637.1m (converted to OSGM15, average of two surveys)  

Summit Grid Reference:  SO 26709 32261

Bwlch Height:  622.5m (converted to OSGM15, average of two surveys)

Bwlch Grid Reference:  SO 26710 32627

Drop:  14.5m (non Uchaf status confirmed and potential Nuttall deletion)

Dominance:  2.28%

For details on the second summit survey of Twyn Llech

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