Determining the position and height for the Great Rhos (SO 182 638) and Pegwn Mawr (SO 023 812) bylchau

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:

1^st survey:  281.3m at SO 17776 71803

2^nd survey:  287.5m at SO 17877 71735

3^rd survey:  285.9m at SN 92693 80074

4^th survey:  286.7m at SN 92767 80103

5^th survey:  283.8m at SN 92601 80035

As the natural point of the northern bwlch is intact and is higher (3^rd survey) than the southern bwlch (1^st 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 (1^st survey) is lower than the northern bwlch (3^rd survey, natural bwlch or 5^th 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)