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.
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| 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.
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| 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.
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| Gathering data at the summit of Pen y Fan |
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| 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.
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| 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.
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| 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.
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| LIDAR image of the rail cutting on left |
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| 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.
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| 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.
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| 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.
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| 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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