Friday, 25 September 2015

Mapping Mountains – Trimble Surveys – Mynydd Epynt

18.08.15  Banc y Celyn (SO 047 464, bwlch only - SO 016 445)  

The critical bwlch of Banc y Celyn (SO 016 445)

For those people who enjoy aspects of hill data Banc y Celyn (summit at SO 047 464) is an interesting hill.  It has a 472m spot height given to its trig pillar which is based on the 472.440m height given to its adjoined flush bracket in the OS Trig Database.  The highest ground at the summit is approximately three metres from the base of the trig pillar but is not significantly higher than it.  If the flush bracket height is accurate the highest natural ground would be approximately 472.1m high.

The hill is listed as a Pedwar with a current drop of 100m based on its 472m summit height and a 372m spot height that appears on the area of this hill’s bwlch at SO 01621 44533.  But the hill is not listed as a Hump by Mark Jackson and is only given a drop value of 97m and therefore it is listed as a Sub-Hump.  This latter drop value is based on its 472m summit height and a 1,230ft (374.9m) height that appears on the area of this hill’s bwlch on the Ordnance Survey Six Inch map.

I had a brief email exchange with a member of the DoBIH editorial team about this hill and its 372m bwlch spot height and the 1,230ft Six Inch height, and we decided to agree to disagree, which in those days seemed to be the order of the day.  Ever since this email exchange I’d always wanted to visit the area of the bwlch and try and ascertain a height and position for the critical bwlch.  However, current maps suggest that the area of this hill’s bwlch is very close to that of the Military Firing Range, and if the bwlch is on this land it may prove problematic to survey.

As I drove north from Upper Chapel the evening was spreading out to night with the sun still radiating illuminated colour on the land but I hoped that this bwlch could be surveyed with the Trimble and that it did not lie in the land that is out of bounds for much of the year.  However, I knew that this land is out of bounds when the red flag flies and that on occasion the military turns a blind eye to evening forays when firing and training exercises have ended for the day.  I had already decided that if the bwlch lay on Military land and if the red flag was flying and / or exercises were taking place I would not venture any further, but if no flag was in evidence and if all was quiet I may have a potter about and see if the bwlch position could be Trimbled.

As I arrived at my spot I parked, grabbed all my gear and headed toward a fence, there was no red flag flying and no evidence that training exercises were taking place, there was also an open gate giving access onto the Military land.  It is debatable if I should have ventured onto this land but I once had the pleasure of accompanying a local farmer in his Landrover around these hills in the evening time when the military were packing up for the day and they waved us through, so I hoped that a late evening wander when all was quiet and the sun sinking ever deeper into the western horizon would be OK.

Sign on fence

Sign next to open gate beside track

This area of the bwlch is complicated by the addition of two intervening 380m ring contours that are plonked on it and resemble small islands adrift in a sea of 370m contour land.  This meant that three points would need to be Trimbled for an adequate comparison of height.  Two of these were positioned on the Military land and therefore needed a clandestine visit, with the easterly of the three positions being positioned beside the narrow lane that I had just driven on to get to my parking spot, I decided to leave this easterly option as the last one to survey.

As I walked through the fern and reed grass toward where the 372m spot height appears on the ground I was all too aware that at any minute a truck or soldier may appear over the horizon, as the open gate gave access on to a gravelled track that led up toward where I was now wandering.

I headed straight for the 372m spot heighted bwlch first, and as this is positioned between the two 380m ring contour islands I was sufficiently out of view to at least feel partly comfortable with where I now was.  I assessed the land which was not difficult as the low point of the hill to hill traverse was relatively easy to pinpoint, whereas the valley to valley traverse continued almost flat like for a number of metres, but happy with where I decided to place the Trimble I quickly set it up on top of my rucksack, measured a 0.43m offset between its internal antenna and the ground and hoped that the 0.1m accuracy level would be attained quickly, thankfully it was and as it quietly beeped away as each data point were collected I hid behind large clumps of reed grass and fern, occasionally sticking my head up to scour the land, thankfully all continued to be quiet.

Gathering data at the critical bwlch of Banc y Celyn where the 372m spot height appears on the ground

Once the allotted five minute data collection had ended I switched the equipment off and took a series of photos from different directions and quickly packed everything away, grabbed my rucksack and quietly moved around the intervening lump that constituents one of the 380m ring contours and walked up toward where the furthest westerly option of the three possible positions for this bwlch lay.

At the critical bwlch with the land to the north in the background

This westerly position was visually higher than where the 372m spot height appears on the ground but it still needed Trimbling.  Again I set the equipment up on my rucksack and this time measured a 0.44m offset between the Trimble’s internal antenna and the ground.  As it collected data I crept off and hid behind more fern and reed grass.  This second position was more in the open when compared to the first, but it now looked as if I would succeed in getting data sets from these two important points, I just hoped that a tank would not appear as my rucksack was positioned in the centre of vehicle tracks that crossed this part of the moor so large vehicles certainly came this way.

The second Trimble survey was on the most westerly of the three potential bwlch positions

Gathering data at the second of the three positions surveyed for the bwlch of Banc y Celyn

At the second and most westerly of the three positions surveyed with the land to the north in the background

Once five minutes of data were collected I quickly sneaked back up to the Trimble, turned it off, took a series of photos, packed everything away and headed quietly toward the open gate and safety – phew!!

All that remained was to get a data set from the easterly option for the position of the bwlch, I drove a couple of hundred metres toward this and parked the car and spent five minutes or so assessing the lay of land and decided where the Trimble should be placed.  This third option for the bwlch of Banc y Celyn is visually higher than the centrally positioned bwlch where the 372m spot height appears on the ground.  Once the Trimble had done its stuff I headed back to the car happy in the knowledge that all possibilities for the bwlch of Banc y Celyn had now been surveyed.

Gathering data from the most easterly and the third position surveyed for the bwlch of Banc y Celyn

Soaking up the last of the sun's rays

With the last of sixteen data sets complete it was time to head home

It had been a long day as I had set off from home at 7.40am and surveyed in fields full of Maize, visited two Pedwarau and two 300m Sub-Pedwarau and completed 16 surveys in all.  It was nearing 8.15 pm as I walked back to my car and as I got changed the sun sank behind a ridge off to my west, a fitting end to the day.         

Survey Result:

Banc y Celyn

Bwlch Height:  369.9m (converted to OSGM15)

Bwlch Grid Reference:  SO 01618 44530

Drop:  102m (Subhump reclassified to Hump confirmed)

Dominance:  21.61%

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

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