The details between a
hill’s summit and its critical bwlch are one that is seldom explored outside of
being a list compiler or a surveyor.
This relationship is intrinsic to the hill, as it produces the drop
value, which can be used as a part of the criteria for a hill’s classification.
For the uninitiated the
critical bwlch is the point of least resistance when walking between summits or
between valleys. It is the lowest point
reached when traversing from a hill’s summit to the next higher summit along
the watershed. When doing so, this point
will also be the highest point reached when traversing between opposing
valleys.
Also for the
uninitiated; the drop value of the hill is the vertical height gain between the
critical bwlch and the summit of the hill, with the caveat that each hill only
has one critical bwlch, and no hill can share this point, therefore it is
intrinsically linked to its respective hill’s summit. However, the position of a hill’s critical
bwlch can change and this is dependent upon its known height.
It is the last sentence
above that is all important for the two hills that this article relates to;
Great Rhos and Pegwn Mawr, as the listed height of each hill’s critical bwlch
has been within 1m, and these critical points have also been swapped between
the hills. It was my aim today to finally
determine which bwlch is higher and which is lower and therefore which bwlch is
adjoined to Pegwn Mawr and which to Great Rhos.
Before detailing the
survey result it may be prudent to briefly describe each hill and their
position relating to one another and where each bwlch is also positioned. I will also give brief details to their known
heights and the history of their listed drop values.
Great Rhos is the
highest hill in the Fforest Glud range of hills, which are also referred to as
the Radnor Forest, it is positioned in the eastern part of mid Wales and has
the small community of Maesyfed (New Radnor) and the A44 road to its
south-east.
Pegwn Mawr is the
highest hill in its own hill range and is also situated in the eastern part of
mid Wales; it is positioned between the roads of the A470 to its north-west and
the A483 to its east and has the towns of Llanidloes towards its west
north-west and Y Drenewydd (Newtown) towards its north-east.
The two hills and their
bylchau are orientated in a north-west direction, with the summit of Great Rhos
being the most southerly, a northern traverse then bisects one bwlch which is
positioned close to Llanbister Road Station, a north-westerly traverse then
heads toward the summit of Pegwn Mawr, with a westerly traverse then bisecting
the second bwlch which is positioned beside the A470 road and which has the
town of Llanidloes towards its north-east and the small community of Llangurig
towards its west. Next higher ground is
then to be found in the Pumlumon range of hills which are situated to the
north-west of the second bwlch.
The main classification
of each hill and their listed heights and the history of their listed drop
values are given below:
Great Rhos and Pegwn
Mawr have many classifications, two of which are Marilyn and Uchaf. A Marilyn is a British hill that has 150m
minimum drop irrespective of its height, with the list author being Alan
Dawson. An Uchaf is a Welsh hill that
has a minimum height of 500m and minimum drop of 15m, with the list of Yr Uchafion
co-authored by Myrddyn Phillips and Aled Williams.
When the listing of
Marilyns was first published in 1992 in The
Relative Hills of Britain book no drop values were given, although Great
Rhos was later listed with an estimated c 373m of drop when The Hewitts and Marilyns of Wales
booklet was published by TACit Tables in February 1997. This drop value was based on a 660m map
summit height and estimated bwlch height of c 287m.
Pegwn Mawr was listed in
the TACit Tables booklet with 300m of drop based on a 586m map summit height
and a bwlch height of 286m.
When the listing of Yr
Uchafion was re-evaluated these drop values and the position of their
respective bylchau were examined and as Great Rhos is the higher of these two
hills it should be listed with the lowest of the two bylchau. Therefore either the heights of the bylchau
were incorrect or their positioning had been transposed.
At the time of the
re-evaluation of Yr Uchafion the Marilyns listing had been published on DoBIH
(Database of British and Irish Hills), and at that stage I was working closely
with two of the editors of DoBIH in a survey team, therefore I informed them
that their positioning of these bylchau had been transposed, resulting in these
bylchau then being swapped. This
re-evaluation resulted in the following details being given to these two hills:
DoBIH:
Great Rhos 660m summit at SO 18220 63902 286m bwlch height at SN 927 801, giving
374m of drop.
Pegwn Mawr 586m summit at SO 02394 81245 286m bwlch height at SO 178 717, giving
300m of drop.
Yr Ucahfion:
Great Rhos 660m summit at SO 18217 63896 c 285m bwlch height at SN 92702 80073,
giving c 375m of drop.
Pegwn Mawr 586m summit at SO 02394 81245 c 287m bwlch height at SO 17837 71756,
giving c 299m of drop.
The above resulted in
DoBIH swapping the positions of the bylchau but listing them as the same
height, whilst the Yr Uchafion list differentiated each bwlch by height. I do not know if Alan Dawson as the Marilyns
author was informed of this by DoBIH, I did not inform him at the time but have
done so with the results of my survey with the Trimble GeoXH 6000.
Before detailing the
survey results I will give a brief synopsis of each bwlch, as the surrounds of
each has undergone intrusion by man and until recently it was not known how
this had affected their respective heights, or if the natural bwlch still
survived. The latter point is important
as it is generally accepted that if the natural bwlch of a hill is still intact
even if a cutting reduces the height of the hill to hill traverse, then the
height of the natural bwlch is taken as a part of the drop value of the hill.
When the positions of
these bylchau were swapped list compilers were in the main reliant upon spot
heights and the 10m contour intervals on contemporary Ordnance Survey 1:25,000
Explorer and 1:50,000 Landranger maps.
However, the series of Ordnance Survey Six-Inch maps were now available
online and these include many benchmark and levelled heights which are more
accurate than the beige spot heights that appear on contemporary maps.
The Ordnance Survey
Interactive Coverage Map hosted on the Geograph website was also now available
and this enlarged mapping gave many spot heights not shown on any other
publicly available map and the facility of producing a ten figure grid
reference for any given position.
These two forms of
mapping, one old and one new, helped in determining the height of each
bwlch. The southerly bwlch has two
options for its critical point, both of which are between 280m – 290m in
height on contemporary maps, one of these is placed near a gravel track that
heads to a farm named Maylord, the other is placed on or beside a rail track
which had been laid in a cutting on the eastern side of the bwlch
surrounds. An old benchmark helped in
estimating the height of this bwlch, although it was not placed where the bwlch
was considered to be positioned and therefore its height was not agreed,
however there was consensus that its critical point was positioned near the
track that leads to Maylord.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map for the bwlch of Great Rhos |
The northerly bwlch has
a disused railway cutting that bisects the surrounds of the bwlch in an east to
west direction, with an elevated road that bisects this disused railway in a
north-east to south-west direction, both have grazing fields on either
side. The 10m map contouring also gives
this bwlch between 280m – 290m in height with contemporary Ordnance Survey
1:25,000 maps giving a 286m spot height on the road toward the north-east of
the land that makes up that between the 280m – 290m contours, this spot height
has been rounded up from a 285.3m and 285.5m (metric equivalent) height that
appears on old Ordnance Survey base maps.
There are also a series of heights given to the old railway and road
prior to when the rail bridge was demolished, however consensus could not be
reached between the height or position of this bwlch, and the ones used by the
then author of Yr Uchafion and DoBIH are quoted above.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map for the bwlch of Pegwn Mawr |
Since the time when the
position of these bylchau were swapped there has been dramatic changes in how
such things are determined as people are now using the facility of Google cars
to examine bylchau, and 5m contouring is now available on OS Maps, however the
most revolutionary progress is that of LIDAR.
The LIDAR (Light
Detection & Ranging) technique produced highly accurate height data that is
now freely available for much of England and Wales, and this process has
resulted in an almost endless stream of hill reclassifications. With one of its major advantages being the
facility to create coloured contouring at 0.001m intervals, this facility can
be used to determine if natural bylchau still exist where intrusion by man has
created either a road or railway that has been cut or elevated.
As the southern bwlch
was not covered by LIDAR the area was examined from the confines of a Google
car and this quickly determined that the critical point was not placed beside
the gravel track that leads to Maylord farm, as land from this point
consistently went downhill toward the rail track on the hill to hill
traverse. This was substantiated by
examining the 5m contours on OS Maps, this mapping has the bwlch position
beside the gravel track leading to Maylord farm between 285m – 290m, whilst the
bwlch on or beside the rail track is between 280m – 285m.
 |
| Extract from the 5m contouring on OS Maps |
The northern bwlch is
covered by LIDAR and extensive analysis determined that its critical natural
point was intact and had not been obliterated either from the construction of
the road or disused railway. LIDAR
analysis also determined that another natural bwlch still existed for this
hill, but as this was higher on the hill to hill traverse it is not the
critical point for this bwlch. However,
a lower point does exist on the hill to hill traverse and its position was
determined by LIDAR analysis, this is placed in the remains of what is
now an overgrown disused rail cutting.
 |
| LIDAR image for the bwlch of Pegwn Mawr with the joining of the white contours on bottom left the meeting of the valley to valley traverse if using the disused rail cutting, the near joining of the white contours in the centre of the image being the intact natural bwlch, and the meeting of the white contours toward the top right of the image being a remaining natural bwlch of the hill that has been slightly altered in recent times due to widening of access tracks |
Two points were surveyed
with the Trimble GeoXH 6000 at the southern bwlch, the first of which is where
the 5m contouring on OS Maps place the critical bwlch, this is on the rail
track. OS Maps give the opposing valley
to valley 280m contours to be positioned between SO 17712 71770 and SO 17871
71838, a mid-way point is conveniently placed where a gated track passes over
the rail line, and this is where Trimble data were gathered from.
 |
| Approaching the bwlch of Great Rhos for the 1st survey |
 |
| Gathering data at the bwlch of Great Rhos on the 1st survey |
The second point
surveyed at the southern bwlch is where the position of this bwlch had
previously been listed, and this is beside the gravel track that leads to
Maylord farm. Visual confirmation that
this is higher than the rail track substantiated examination of the land via a
Google car and the 5m contouring on OS Maps.
 |
| Gathering data at the 2nd survey for the bwlch position of Great Rhos with the Trimble set up on top of my rucksack at the top of the gravel track leading to Maylord farm, and the rail track in the valley below to the right with a fence line indicating its position |
 |
| The Trimble set-up position at the 2nd survey for the bwlch position of Great Rhos |
Three points were
surveyed with the Trimble GeoXH 6000 at the northern bwlch, the first of which
is where LIDAR analysis places the natural critical bwlch, this is to the north
of the A470 road and the disused rail cutting, and is placed in a grazing
field.
 |
| Gathering data at the critical and intact natural bwlch of Pegwn Mawr during the 3rd survey (1st survey for Pegwn Mawr) |
The second point
surveyed is where LIDAR analysis places a remaining natural bwlch, but not the
remaining natural bwlch, this is beside a gravelled track that has either been
newly laid or widened, and therefore this ground has been recently disturbed by
man.
 |
| Gathering data at a remaining natural bwlch, but not the remaining natural bwlch for Pegwn Mawr duting the 4th survey (2nd survey for Pegwn Mawr) |
The third point surveyed
is where LIDAR analysis places the high point of the valley to valley traverse
that takes in the old disused rail cutting, this is also on the northern side
of the A 470 road, and it is now overgrown and on the day I visited was being
grazed by three cows.
 |
| Gathering data in the overgrown and disused rail cutting during the 5th survey (3rd for Pegwn Mawr) |
The Trimble GeoXH 6000
results for each of these five surveyed points are given below:
1st
survey: 281.3m at SO 17776 71803
2nd
survey: 287.5m at SO 17877 71735
3rd
survey: 285.9m at SN 92693 80074
4th
survey: 286.7m at SN 92767 80103
5th
survey: 283.8m at SN 92601 80035
As the natural point of
the northern bwlch is intact and is higher (3rd survey) than the
southern bwlch (1st survey), this northern bwlch should be adjoined
to the lower hill; Pegwn Mawr (585.2m summit at SO 02394 81245). This gives Pegwn Mawr 299.3m of drop using the natural bwlch, or if using the disused rail cutting bwlch this gives Pegwn Mawr 301.5m of drop.
As the southern bwlch (1st
survey) is lower than the northern bwlch (3rd survey, natural bwlch or 5th survey, disused rail cutting bwlch) this should be
adjoined to the higher hill; Great Rhos (659.9m summit at SO 18213 63896). This gives Great Rhos 378.6m of drop.
Myrddyn Phllips (July
2018)
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