Thursday 17 October 2024

Mapping Mountains – Trimble Surveys – Glyder Fawr

 

11.08.24  Craig Cwrwgl (SH 615 615) 

One of the most obscure and esoteric of Welsh summits is a great pillar of rock that forms part of the evocatively named Craig Cwrwgl, ‘coracle crag’.  These rocks front the north-eastern face of Elidir Fawr, and eventually give way to steep slopes that plunge into the dark waters of Marchlyn Mawr.

Craig Cwrwgl (SH 615 615).  Photo: Aled Williams

The detachment of the pillar from the bulk of Elidir Fawr is unusually complex.  In addition to the original bwlch (col, pass) that consists of bedrock and defines the traditional separation of a top from its parent mountain, the gap above the bwlch is also spanned by two rock-bridges, one being lower than the other.  These rock-bridges are the suspended remnants of towers that fell from the pillar and were held from falling further by the adjacent hillside.  Remarkably, the higher of the two towers fragmented upon impact into three large boulders and the cracks between each of them are permeated by small holes.

The upper rock bridge above the natural bwlch.  Photo: Aled Williams

Typically, a bwlch between mountains, hills or tops represents both the apex in the valley-to-valley traverse as well as the trough in the hill-to-hill traverse.  In this case and at the present time, it may be argued that the original bwlch only represents the valley-to-valley apex, while the higher of the two rock-bridges represents the hill-to-hill trough.  Conversely, it is fact that the rock-bridge does not, and never did, represent the valley-to-valley apex, whereas the original bwlch has unequivocally represented both at one time in natural history.  Does this matter?  Well, it does if the prominence of the pillar is to be measured, and it was for this purpose that I intended to climb Craig Cwrwgl.

Gearing up for the climb.  Photo: Aled Williams

The first-recorded ascent of Craig Cwrwgl was by J. M. Archer Thomson and W. J. Williams in April 1900, and because a name had not been recorded by the Ordnance Survey, Thomson initially referred to it as the ‘Pillar of Elidr’.  However, by the time Thomson published his guide-book ‘Climbing in the Ogwen District’ in 1910, he had discovered its actual name through local enquiry to be Craig Cwrwgl.  124 years later, the plan for the day was to climb up the pillar in one-pitch via its easiest route that first ascends to the bwlch along the westward-draining gully and then across the upper rock-bridge to the feature referred to by Thomson as the ‘Rift’, which is one of the several deep fissures that divide the upper half of the pillar into separate towers.  The climb would be led by the safe-hands of experienced local climber Gwyn Jones, as kindly arranged by Eric Jones, who accompanied us to the foot of the pillar.

Climbing to the bwlch.  Photo: Aled Williams

Gwyn led confidently up the gully and made it look easy until a slippery slab slowed his progress three-quarters of the way up.  After a bit of gardening he soon overcame the slab and quickly disappeared into the alcove underneath the upper rock-bridge.  He re-emerged and swiftly shifted up and across the wedged boulders, and within another 5 minutes he was standing on top of the pillar.  The gauntlet had been laid!  After some sound advice from Eric I followed Gwyn’s route up the gully.  All went relatively well until I arrived at the slippery slab, which was smooth, muddy and slimy, dotted with woodrush plants that would slide off the rock upon any attempt to use them as climbing aids!  After a good few minutes of undignified dithering I stretched up and found the key hold, and with a sturdy pull upwards I was soon above the slab and into the relative safety of the bwlch.  The next set of moves involved gaining the upper rock-bridge, which was done by climbing a short chimney on the Elidir Fawr side of the bwlch.  An easy walk over the suspended boulders brought me to the bottom of Thomson’s ‘Rift’.

Thomson's 'Rift' and the view from the upper rock bridge.  Photo: Aled Williams

As I squeezed into the fissure, Gwyn’s voice echoed down from above advising me to freestyle with the use of any body parts to inch my way up it.  Despite a couple of awkward places, the chimney proved enjoyable; no doubt helped by the reduced exposure felt within its claustrophobic structure.  As described by Thomson in 1910, emergence from the fissure onto the summit rocks of the pillar did indeed prove to be ‘somewhat of a coup de théâtre’!

The summit of Craig Cwrwgl.  Photo: Aled Williams

We were not alone on the summit, as two other climbers were in the process of finishing their climb on the east side of the pillar.  The views were stunning and unusual, with a distant Mynydd Hiraethog framed in by the rugged precipices of Pen yr Ole Wen and Foel Goch.  Perched on a luxurious mat of grass and crowberry, it was easy to forget the encircling precipices when looking out at such a vista.  I peered over the edge and waved down to Eric to inform him that we had made it.

Eric from the summit.  Photo: Aled Williams

It was now time to survey the summit.  The high point was easy to locate and having asked Gwyn and the other two climbers as to their opinion, we all agreed on the point to be surveyed.  As the other climbers prepared to abseil down, I hopped over a deep and wide crack between the summit towers and set the Trimble up for measurement.  Once sufficient data was collected, we waited for the climbers to complete their abseil before Gwyn prepared the gear for our likewise descent to the bwlch.

Surveying the summit with the Trimble GeoXH 6000.  Photo: Aled Williams

A short abseil brought me back down to the upper rock-bridge.  I set the Trimble up on the boulder closest to Elidir Fawr, immediately above the low-point of the upper rock-bridge, taking a 106cm offset with a tape measure.

Returning to the bwlch.  Photo: Aled Williams

By the time a full dataset had been collected by the Trimble, Gwyn was also down and was free to help me with the surveying.  I descended to the alcove underneath the rock-bridge to take the measurement offset from the Trimble position to the low point of the original bwlch, which came to 405cm.  Gwyn then set up another abseil point and brought us down safely and quickly to the base of the pillar.  By early evening we were back at the vehicles after a great day out on the hill.  With great gratitude I thanked Gwyn and Eric for getting me up Craig Cwrwgl and for their unwavering patience with the time-consuming surveying.

How the bwlch was surveyed: the red ‘X’ marks the spot where elevation was measured with the Trimble GeoXH 6000, the yellow line roughly represents the 106cm offset from the Trimble GeoXH 6000 to the hill-to-hill bwlch (i.e. the rock-bridge) and the white line roughly represents the 405cm offset from the Trimble GeoXH 6000 to the valley-to-valley bwlch (i.e. the original bwlch)

 

Aled Williams (August 2024) 

 


Survey Result:

 

Craig Cwrwgl

Summit Height:  761.2m (converted to OSGM15)

Summit Grid Reference:  SH 61544 61586

Bwlch Height:  746.7m (converted to OSGM15)

Bwlch Grid Reference:  SH 61546 61584

Drop:  14.5m

Dominance:  1.90%

 

Note: if taking the rock-bridge as the bwlch, the drop comes to 11.5m

 

 

For further details please consult the Trimble Survey Spreadsheet

 

Wednesday 16 October 2024

Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Graig y Barcut (SN 546 027) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Graig y Barcut (SN 546 027)

The criteria for the list that this reclassification applies to are:

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Graig y Barcut and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north, west and east, and the A4138 road to its south, and has the town of Pontarddulais towards the east.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list.

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 32m of drop, based on the 104m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 72m bwlch height, based on interpolation of 5m contouring between 70m – 75m. 

Extract from the Ordnance Survey 1:25,000 Explorer map

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 72m bwlch spot height, and when coupled with the 104m summit spot height, these values gave this hill 32m of drop.

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the confirmation of the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 104.0m summit height and a 71.8m bwlch height, with these values giving this hill 32.2m of drop, which is sufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Graig y Barcut 

OS 1:50,000 map:  159

Summit Height:  104.0m (LIDAR) 

Summit Grid Reference:  SN 54698 02770 (LIDAR) 

Bwlch Height:  71.8m (LIDAR) 

Bwlch Grid Reference:  SN 54693 03117 (LIDAR) 

Drop:  32.2m (LIDAR) 

 

Myrddyn Phillips (October 2024)

 

 

Tuesday 15 October 2024

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Penllwyngwyn Mawr (SN 549 007) 

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips.               

LIDAR image of Penllwyngwyn Mawr (SN 549 007)

The criteria for the two listings that this height revision applies to are:

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Penllwyngwyn Mawr and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the B4297 road to its immediate north and to its south, and has the town of Llanelli towards the west.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a 72m summit height, based on the spot height positioned at SN 55146 00863 that appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

Extract from the Ordnance Survey 1:25,000 Explorer map

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 47m of drop, based on an estimated c 75m summit height positioned at SH 54904 00700 and an estimated c 28m bwlch height, based on interpolation of 5m contouring between 25m – 30m.  With the caveat that the uppermost 75m ring contour had been missed when the details for this hill were first compiled and published.

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR summit image of Penllwyngwyn Mawr (SN 549 007)

The summit height produced by LIDAR analysis is 76.8m and is positioned at SN 54907 00706, and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 76.8m and this was derived from LIDAR analysis, this is 4.8m higher than the originally listed summit height of 72m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Sylen 

Name:  Penllwyngwyn Mawr 

OS 1:50,000 map:  164, 178

Summit Height (New Height):  76.8m (LIDAR) 

Summit Grid Reference:  SN 54907 00706 (LIDAR)               

Bwlch Height:  25.0m (LIDAR) 

Bwlch Grid Reference:  SN 53996 01771 (LIDAR) 

Drop:  51.8m (LIDAR) 

Dominance:  67.44% (LIDAR)

 

Myrddyn Phillips (October 2024)

Monday 14 October 2024

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Ffridd (SN 708 986) – 30-99m Sub-Twmpau addition 

There has been confirmation of an addition to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Ffridd (SN 708 986)

The criteria for the list that this addition applies to are:

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Ffridd, and this was derived from the Tithe map, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with the A487 road to its west and a minor road to its east, and has the town of Machynlleth towards the north-east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 21m of drop, based on the 98m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 77m bwlch height, based on interpolation of 10m contouring between 70m – 80m. 

Extract from the Ordnance Survey 1:25,000 Explorer map

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the confirmation of the addition of this hill to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 97.6m summit height and a 74.5m bwlch height, with these values giving this hill 23.1m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Ffridd 

OS 1:50,000 map:  135

Summit Height:  97.6m (LIDAR) 

Summit Grid Reference:  SN 70875 98692 (LIDAR)               

Bwlch Height:  74.5m (LIDAR) 

Bwlch Grid Reference:  SN 71072 98917 (LIDAR) 

Drop:  23.1m (LIDAR) 

 

Myrddyn Phillips (October 2024)