Friday, 6 December 2019

Mapping Mountains – Trimble Surveys – Bannau Brycheiniog

13.09.19  Pen y Fan (SO 012 215), only bwlch surveyed

For many hill classifications the connection between the summit and bwlch of the respective hill is all important, as knowledge of each height gives the drop value of the hill.  This sounds simple, and usually in practice it is, however there are examples where the nature of the summit and that of the connecting bwlch come to the fore, and for Wales there are few other hills that can compare with Pen y Fan for the sometimes fraught nature of what constitutes a summit and also a bwlch.

Pen y Fan (SO 012 215)

Pen y Fan is the highest hill in south Wales and as such one can expect its connecting bwlch to the next higher summit to be a great distance from its high point, and this is so, with its bwlch situated towards the north close to or in the village of Talerddig.

For many years the summit of Pen y Fan had an Ordnance Survey triangulation pillar on its top.  Details in the OS Trig Database give a completion year of 1939; presumably this is when the pillar was installed.  The pillar was still in situ in 1990 but it was soon removed.  The summit of Pen y Fan now has a neatly arranged conical cairn at its high point, and adorning its top is a large upright flat rock with a fixed plaque giving the name and map height of the hill.  The old trig pillar had a flush bracket height of 886.358m and was set on a concrete base approximately 0.20m high, with natural ground at its base approximately 885.8m in height.

The trig pillar that once adorned the summit of Pen y Fan

I had visited and surveyed the summit of Pen y Fan on the 25th October 2016, taking two data sets from the summit area, one at the top of the man-made conical cairn and the other on what I deemed to be the highest remaining natural ground of the hill.  These two data sets came to 885.739m at SO 01208 21582 and 884.601m at SO 01210 21587 respectively.  Dependent upon one’s view the 885.7m height from the top of the conical cairn can be taken as that for the hill as although the cairn is man-made it is nearer to the natural height of this hill when compared to the 884.6m height to remaining natural ground.  However, neither option is perfect.

Gathering data at the summit of Pen y Fan

Gathering data at the base of the man-made conical cairn at what can be deemed the remaining natural high point of the hill

The bwlch of Pen y Fan is to its north and placed close to, or in Talerddig, dependent again on one’s view of what constitutes a bwlch.  I had endeavoured to survey this bwlch on the 18th August 2015 arriving in Talerddig early in the morning and proceeded to take a data set from where contour interpolation and an on-site inspection suggested the critical point to be positioned.  This point was in a field close to a house whose drive led toward the main road that passes through the village. 

Prior to data collection I had assessed the lay of land for a number of minutes, and had also done so after closing the equipment down and heading toward a gate which gave access to a drive leading to a house and which was beside the main road and a minor road where my car was parked.  When I reached the gate I looked back on my set-up position and wondered if it was correct, but with a big surveying day ahead that did not end until sunset I decided I was happy enough with the placement and headed south to examine the bwlch of Allt yr Esgair.

Gathering data at the 1st bwlch survey of Pen y Fan

Between then and now LIDAR has become available.  This has revolutionised hill classification in ways that those interested in such things could only dream of a few years ago.  Subsequently the bwlch of Pen y Fan was analysed via LIDAR by Aled Williams and the critical point was found to be beside the gate where I had looked back to the Trimble set-up position.

However, LIDAR also gives the opportunity to examine the lay of land in detail by building up contours at 1mm intervals if the inclination to do so takes a hold.  It also gives an opportunity to place accurate heights to such things as road and rail cuttings, something that contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps seldom do.  And just to add complication to an already fraught situation of what constitutes a summit and a bwlch, Talerddig just happens to have a rail cutting that when built was the deepest in the world.

LIDAR image of bwlch highlighted in white on right with the northerly part of the rail cutting highlighted in yellow on left

The rail line passing through Talerddig joined Machynlleth to Newtown and was constructed in the early 1860s.  The cutting is 37m deep (120ft) deep and must have been a difficult engineering task to complete.  Using LIDAR gives an opportunity to add an accurate height to this cutting as it makes its way southward up valley to the line’s high point before descending down valley toward Caersŵs.  The high point of the rail line to the south of the cutting would also be on the hill to hill traverse for anyone making their way from Pen y Fan to the next higher hill, and dependent upon one’s view should be taken in to account when calculating the drop value of this hill.  However, there’s no fundamental rights or wrongs with whether such things as cuttings should constitute a part of the drop value of a hill,  all one can do is assess the options and form an opinion, and if a different conclusion is formed by another person; respect it even though one may not agree with it.

LIDAR image of the rail cutting on left

LIDAR close up image of the rail line and the critical point of the bwlch highlighted in white

Therefore, we have a summit whose conical cairn is now man-made, although its height is nearer that of the natural height of this hill when compared to the highest remaining natural ground at the periphery of the conical cairn, and a bwlch that has a rail cutting which is lower on the hill to hill traverse compared to what may constitute the natural bwlch.  I say ‘may constitute the natural bwlch’ as when building up the contours with LIDAR I became aware that ground beside the house named Hafan that overlooks the field and gate where the Trimble GeoXH 6000 was previously set-up and where LIDAR places one position for the bwlch, looked as if it has been terra-formed as contours appear slightly ragged compared to their usual form.  The descending hill to hill contours also indicate that the bwlch should be placed near to where the Trimble was previously set up, although this is now on a descending route down valley from the gate where LIDAR places this option for the bwlch.  Having examined LIDAR in detail I concluded that the natural bwlch had been terra-formed and although a natural bwlch can still be followed up valley to ground at the base of the gate, the building of the house named Hafan had altered the lay of land sufficiently to disturb land where the natural bwlch was situated.

LIDAR close up image of the land leading to what could be deemed the remaining natural bwlch with the meeting of the white contour lines being the critical point; note jagged contours indicating the land has been terra-formed

Sometimes it is the surveying that is the simple task, with the dilemma of what constitutes a summit and bwlch being the difficult part.

Having surveyed what I thought to be the bwlch and with LIDAR indicating this option to be beside the gate I wanted to re-visit and as Talerddig was on route from the bwlch of Newydd Fynyddog which I had just surveyed to the bwlch of Aran Fawddwy which was my next port of call, I made my way over the hill road toward Talerddig. 

Having parked in the village, I walked the short distance to the gate with Trimble and surveying steps in hand and placed the Trimble atop the solid stone pillar adjacent to the gate and at the end of the drive leading to the house of Hafan.

The gate where LIDAR gives the remaining natural bwlch of Pen y Fan

I smiled as I looked over the gate toward where I had taken data from on my previous visit and quietly opened the gate to venture in to the field to take a measurement offset between the Trimble’s internal antenna and the ground in the field at its base.  Having noted a 1.51m offset I went back through the gate entrance, locked it behind me and stood on the surveying steps to set the file up in the Trimble, and once the 0.1m accuracy level was attained I pressed ‘Log’ and marched off toward the main road where I stood and took a few photos.

The Trimble set-up position at what can be deemed the remaining natural bwlch of Pen y Fan

Once the allotted data were gathered and stored I headed back to the surveying steps,  clambered up on them and pressed ‘Done’ and closed the equipment down, packed it away and headed back to my car, such a simple survey considering the complications of what constitutes the bwlch and summit of Pen y Fan. 


Survey Result:

Pen y Fan

Summit Height:  885.7m (converted to OSGM15, from previous Trimble GeoXH 6000 survey)

Summit Grid Reference:  SO 01208 21582

Bwlch Height:  215.0m (converted to OSGM15, remaining natural bwlch)

Bwlch Height:  209.3m (LIDAR, rail cutting)

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

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

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

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

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

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


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