Friday, 4 November 2016

Mapping Mountains – Trimble Surveys – Bannau Brycheiniog and Fforest Fawr

25.10.16  Craig Gwaun Taf (SO 005 207), Pen y Fan (SO 012 215), Corn Du (SO 007 213), Y Gurn (SN 988 216) and Fan Fawr (SN 969 193, bwlch only)

Corn Du (SO 007 213)

I’d long wanted to visit the high peaks of Bannau Brycheiniog and Trimble them, and doing so was long overdue.  Having visited these and the other Welsh 2,000ft hills on many occasions, re-visiting them is always a pleasure, especially after time between visits being many years, when this happens it is a clichéd expression to say that it is like visiting old friends, but that is exactly how it feels, as although the hills are inanimate objects they have an uncanny presence that on many occasions can form characteristics seemingly all their own.

As well as wanting to visit the higher peaks of Bannau Brycheiniog there was one particular hill that I was extremely interested in Trimbling, this hill is Craig Gwaun Taf, which is an outlier of Corn Du and thrusts upward on its southerly ridge beyond Bwlch Duwynt before descending south-eastward toward the Pentwyn Reservoir.

Craig Gwaun Taf is given an 824m summit spot height on current Ordnance Survey maps and has bwlch contouring between 800m – 810m, with an 815m spot height on the area of the bwlch on the Ordnance Survey enlarged mapping on the Geograph website.  These figures give this hill an approximate c 13m of drop based on interpolation of bwlch contours, or only 9m of drop if based on the 815m spot height.  However, this hill and many others has recently been checked against LIDAR data by Joe Nuttall, who is the son of John and Anne Nuttall, who are guidebook authors and list compilers to the 2,000ft mountains of both England and Wales, and who have kept me busy on the hills for many a year.

LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales, and Joe has been diligently checking a number of hills for qualification to his parents 2,000ft list.  These lists are based on a minimum qualifying drop value of 15m and Joe found that LIDAR data gave Craig Gwaun Taf a drop of 15.783m, Joe then contacted me and asked if I could Trimble the hill.  By doing so it would give a comparison for this hill between LIDAR data and Trimble data, and confirm the hill’s drop value and unless the Trimble result was within an approximate 0.1m of the required minimum drop value of 15m it would also confirm the hill’s status.

I set off from the car park at the top of the A 470 opposite the Storey Arms at 8.50am, having driven from Worcester, the first time I’d ventured into Wales for a hill walk from the recesses of this remote part of deepest, darkest England.  Although the forecast gave settled, dry conditions with little breeze, the cloud base was low and a murky greyness pervaded the hills.

Deciding that I’d leave Y Gurn until later in the afternoon when its moorland path connecting it to the north-westerly ridge of Corn Du may be visible if the thickened layer of mist broke, as predicted, I walked south-eastward adjacent to the busy road on a gravelled track on the periphery of forestry to the car park at Pont ar Daf, this meant doing the higher peaks first, in mist, but as their paths are broad and their ridges easily discernible, route finding was not a problem.

I’d only been on the path from Pont ar Daf to Pen y Fan a couple of times before, once when cycling and pushing mountain bikes to its summit in 1990 during a ten day expedition mountain biking the hill boundary of Powys, on that day it was progressively getting hotter, which was a contrast to today, where when height was gained, a slight chill pervaded the wisps of mist as they gently rolled through the land.

The path at Pont ar Daf leading toward Craig Gwaun Taf, Corn Du and Pen y Fan

By the time I arrived at Bwlch Duwynt, which is the pass between Corn Du and Craig Gwaun Taf, one person had overtaken me and a few people had headed down the hill after making an early morning ascent, all were either heading toward or coming from Pen y Fan, leaving Craig Gwaun Taf quietly forgotten in the mist, which suited me and the Trimble just fine.

Today the critical bwlch consisted of an elongated, shallow muddy puddle which stretched over the bwlch from the valley to valley direction.  I checked the ten figure grid reference for the bwlch that Joe had given me when he assessed this hill’s drop via the LIDAR data, and chose a position within a few metres of this grid reference for the first of three Trimble positions at the bwlch.  This position was central on the valley to valley traverse and I gathered two five minute data sets from the same position.  I then collected a further five minute data set from either end of the shallow puddle, firstly on its western side and secondly on its eastern side.  These four data sets will give a good value for the height of this bwlch.

Gathering data at the bwlch of Craig Gwaun Taf

Gathering data at the first of three positions Trimbled at the bwlch of Craig Gwaun Taf

Having taken a number of photos I packed the Trimble away and headed to the summit of Craig Gwaun Taf, which I found was the second rise along the ridge when ascending from its connecting bwlch with Corn Du.  The summit consists of a small grassed hummock a couple of metres east of the path, which continues following the course of the hill’s south-easterly ridge.

I placed the Trimble on the high point and waited patiently for it to reach its 0.1m accuracy level before data should be logged, once one five minute data set was collected I switched the equipment off, then switched it back on and repeated the process until another five minute data set was gathered.  I then positioned the Trimble on top of my rucksack and measured a 0.23m offset between the high point of the hill and the Trimble’s internal antenna and waited until another five minute data set was gathered.  These three data sets will give a good value for the height of this hill and when combined with the four data sets taken at its bwlch, will give sufficient data to give Craig Gwaun Taf an accurate drop value and ascertain its status.

Looking north from the summit of Craig Gwaun Taf with the Trimble gathering data

Looking south from the summit of Craig Gwaun Taf with the Trimble gathering data

As I left the summit of Craig Gwaun Taf a number of voices cast from the mist and drifted my way in the slight breeze that occasionally sprang up, and by the time I reached the path toward Pen y Fan a multitude of people were heading in all directions, some downward, others up towards Corn Du and more toward Pen y Fan, I suspected the summit of south Wales’ highest mountain may be a little busy.

Glimpsing Corn Du through the mist

The summit of Pen y Fan once had a trig pillar at its high point, nowadays it has a constructed cairn with an embedded stone marker giving the hill’s name and height, and it also has many visitors who throng to its summit in all weathers.  When I arrived I wondered how on earth I was going to keep so many people away from the high point when the Trimble was gathering its all-important summit data.  Surprisingly this proved relatively easy and I only had to explain to a few people what I wanted to do and all were extremely compliant with the wants of a mountain surveying Trimbling person.

The Trimble holding back the multitude as it gathers data at the summit of Pen y Fan

Having gathered data at the top of the solid cairn I wanted another five minute data set at the highest land at the periphery of the cairn, this again proved relatively easy to obtain. 

Gathering data at the high point on the periphery of the summit cone

Gathering data adjacent to the coned cairn atop Pen y Fan

Leaving the throng of people beside the summit of Pen y Fan I back-tracked to its connecting bwlch with Corn Du, the position of this bwlch proved easy to identify, and within a few minutes the Trimble was balanced over the northern precipice gathering data.

Gathering data at the critical bwlch of Corn Du

My next objective was Corn Du and I hoped that its summit would give a little peace and quiet when compared to that of Pen y Fan, however it didn’t want to be outdone by its higher neighbour and another multitude of people were gathered around its constructed cairn.  This summit also proved relatively easy to survey as everyone gathered around its periphery giving the Trimble time and space to gather its data.  Before heading down the north-westerly ridge of Corn Du I wanted another five minute data set from the highest natural ground remaining on the hill, I judged this to be one of the small grass hummocks on the edge of its very steep northerly drop, I placed the Trimble on it and secured it in place with a few small rocks before pressing ‘Log’ and hoping that it wouldn’t fancy a quick descent in to the cwm 250m – 300m below.

Gathering data at the summit of Corn Du

Gathering data at what I judged to be the remaining 'natural' high point of Corn Du

Once the Trimble was salvaged from its precarious position I plodded down in continuing mist, following the broad path to the obelisk commemorating Tommy Jones, who at the age of five lost his way from Cwm-llwch farm and whose body was found on this spot 29 days later.  I knew that a path went from near this obelisk toward the bwlch and then the summit of my last hill of the day; Y Gurn, but I also knew that the path was relatively narrow and that other paths probably existed heading off in a number of directions.  Finding one that took me onward I happily followed it until it seemed to be heading too far south, I stopped and checked the grid reference in the Trimble against the map and back-tracked across the moor until at the critical bwlch of Y Gurn, by now the misty conditions that had pervaded the hills started to break up and sunlight pierced out from behind the cloud, brightening the land and giving a partly hazed autumnal glow.

After gathering data at the critical bwlch of Y Gurn I followed a good path to its summit and placed the Trimble beside the small cairn that sits atop a peat hagged hummock, once the customary five minutes of data were gathered I followed the path and fence line down to connect with the main path that heads from Storey Arms to Pen y Fan.

Gathering data at the critical bwlch of Y Gurn

Gathering data at the summit of Y Gurn

There were many people on this main path, with the majority heading down to the Storey Arms, but some were heading up in what was proving to be brightening conditions with glimmers of the higher peaks tantalisingly on show as their straight edged ridges gleamed out of the cloud.

Craig Gwaun Taf above afternoon mist with people heading back to the Storey Arms on the main Pen y Fan path

Only one survey remained and that was for the critical bwlch of Fan Fawr which proved to be adjacent to the car park opposite the Storey Arms.  This was on ground next to a ditch which I followed in both directions, and which seemed to be continuous and therefore probably man-made for drainage, because of this I chose a spot that was conveniently on solid ground above the ditch beside one of the duck boards laid across it, once five minutes of data were collected I headed back to my car happy in the knowledge that 15 surveys had been conducted and that the status of Craig Gwaun Taf would be finally determined.

Gathering data at the critical bwlch of Fan Fawr

Survey Result:

Craig Gwaun Taf

Summit Height:  826.4m (converted to OSGM15) (significant height revision) 

Summit Grid Reference:  SO 00548 20720

Bwlch Height:  810.3m (converted to OSGM15)

Bwlch Grid Reference:  SO 00555 20862

Dominance:  1.95%


Pen y Fan

Summit Height:  885.7m (converted to OSGM15)

Summit Grid Reference:  SO 01208 21582

Bwlch Height:  215.0m (converted to OSGM15, from subsequent Trimble GeoXH 6000 survey to remaining natural bwlch)

Bwlch Height:  209.3m (LIDAR, disused rail cutting)

Bwlch Grid Reference:  SH 93136 00074 (remaining natural bwlch)

Bwlch Grid Reference:  SH 93041 99532 (LIDAR, disused rail cutting)

Drop:  670.7m (cairned summit and remaining natural bwlch)

Drop:  676.45m (cairned summit and disused rail cutting bwlch)

Dominance:  75.72% (cairned summit and remaining natural bwlch)

Dominance:  76.37% (cairned summit and disused rail cutting bwlch)

Corn Du

Summit Height:  872.6m (converted to OSGM15)

Summit Grid Reference:  SO 00718 21332

Bwlch Height:  843.6m (converted to OSGM15)

Bwlch Grid Reference:  SO 00911 21394

Drop:  29.0m (Subsimm and 800m Sub-Twmpau status confirmed)

Dominance:  3.32%


Y Gurn

Summit Height:  618.9m (converted to OSGM15)

Summit Grid Reference:  SN 98895 21607

Bwlch Height:  591.5m (converted to OSGM15)

Bwlch Grid Reference:  SN 99173 22100

Drop:  27.4m (Subsimm and 600m Sub-Twmpau status confirmed)

Dominance:  4.42% 

Fan Fawr

Bwlch Height:  439.5m (converted to OSGM15)

Bwlch Grid Reference:  SN 98223 20289

Drop:  295m

Dominance:  40.19%


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