Mapping Mountains – Significant Height Revisions – 600m Twmpau and The Welsh Highlands – Uchafion Cymru

Craig Ysgafn (SH 659 443) 

There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau and the The Welsh Highlands – Uchafion Cymru, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Craig Ysgafn (SH 659 443)

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

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips and is published on Mapping Mountains in Google Doc format.

600m Twmpau by Myrddyn Phillips

The Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

The Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

The name the hill is listed by is Craig Ysgafn and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A4085 road to its west, the A487 road to its south and the A496 road to its east, and has the town of Blaenau Ffestiniog towards the east north-east.

When the list of Welsh 500m P15s that later became known as the The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 26m of drop, based on the 689m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 663m bwlch height, based on interpolation of 10m contouring between 660m – 670m. 

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. 

LIDAR summit image of Craig Ysgafn (SH 659 443)

LIDAR analysis gives the summit of this hill as 690.0m positioned at SH 65994 44332, and when compared to its originally listed summit height 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 690.0m and this was derived from LIDAR analysis, this is 1.0m higher than the originally listed summit height of 689m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and importantly this height signifies that an uppermost 690m ring contour is missing from this map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Craig Ysgafn

OS 1:50,000 map:  124

Summit Height (New Height):  690.0m (LIDAR)

Summit Grid Reference:  SH 65994 44332 (LIDAR)  

Bwlch Height:  663.4m (LIDAR)

Bwlch Grid Reference:  SH 66001 44596 (LIDAR)

Drop:  26.6m (LIDAR)

 

Myrddyn Phillips and Aled Williams (May 2026) 

Mapping Mountains – Hill Reclassifications – Welsh Highlands – Uchafion Cymru

 

Craig Ysgafn (SH 659 443) – Welsh Highland Sub addition 

There has been an addition to the listing of the Welsh Highlands – Uchafion Cymru, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Aled Williams. 

Craig Ysgafn (Photo: Aled Williams)

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

Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

Welsh Highlands - Uchafion Cymru by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Craig Ysgafn and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A4085 road to its west, the A487 road to its south and the A496 road to its east, and has the town of Blaenau Ffestiniog towards the north-east. 

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

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with 9.9m of drop, based on a 687.7m summit height and a 677.8m bwlch height, with both heights derived from LIDAR analysis conducted by Aled Williams, with a previous basic levelling survey conducted by Myrddyn Phillips on the 18.04.98 giving 10.0m of drop.

Due to the marginal nature in the drop ascertained from LIDAR analysis, this hill was prioritised for a Trimble GeoXH 6000 survey and it is this result that is being prioritised.

Therefore, the addition of this hill to Welsh Highland Sub status is due to a Trimble GeoXH 6000 survey, resulting in a 688.3m summit height and a 678.0m bwlch height, with these values giving this hill 10.3m of drop, which is sufficient for it to be classified as a Welsh Highland Sub. 

 

The full details for the hill are: 

Group:  Moelwyn Mawr

Name:  Craig Ysgafn

OS 1:50,000 map:  124

Summit Height:  688.3m (converted to OSGM15)

Summit Grid Reference:  SH 65973 44394

Bwlch Height:  678.0m (converted to OSGM15)

Bwlch Grid Reference:  SH 65960 44382   

Drop:  10.3m 

 

For the additions, deletions and reclassifications to the Welsh Highlands – Uchafion Cymru reported on Mapping Mountains please consult the following Change Registers:

 

Welsh Highland P15s

 

Welsh Highland Subs

 

Aled Williams and Myrddyn Phillips (March 2023)

 

 

Mapping Mountains – Trimble Surveys – Moelwyn Mawr

21.01.23  Craig Stwlan (SH 667 446), Craig Ysgafn (SH 659 443) and Pt. 524.3m (SH 661 457) 

I drove up the narrow road to Tanygrisiau with trepidation, but thankfully the gritters had kept it in good condition.  In front of me were snow-covered peaks, cliffs and boulders; while behind me were the distant profiles of other snow-clad mountains commanded by the bulk of Arenig Fawr in full winter splendour.  Having parked the car, the rigid frames of winter boots were reluctantly fitted to cold feet, with crampons and a walking axe strapped to my bag in preparation for any icy ground. 

Arenig Fawr (Photo: Aled Williams)

The plan was to visit the series of rocky tops to the east of Llyn Stwlan, before contouring around the lake to Bwlch Stwlan, whereupon a traverse of Craig Ysgafn would take me to the summit of Moelwyn Mawr.  Three surveys were fitted into the walk and the absence of a companion indicated that I would have my hands full when conducting these. 

View ahead from Clogwyn yr Oen (Photo: Aled Williams)

Having scrambled up to the summit of Clogwyn yr Oen, the view ahead looked beautiful but daunting.  The powder snow was deep in places and made progress difficult as I broke trail towards Ceseiliau Moelwyn.  The Welsh Highlands Sub-Top of Clogwyn y Bustach was visited, but not surveyed given that its status within the list is secure.  Ahead stood Craig Stwlan, which I had noted as a marginal P20, as such, I knew Myrddyn Phillips would appreciate having the hill surveyed, given that it was a candidate for his 500m Sub-Twmpau list.

Clogwyn y Bustach (Photo: Aled Williams)

The summit of Craig Stwlan was first to be surveyed.  This went quickly and efficiently, no doubt benefitting from the experience of recent surveys.  The views were stunning and I could discern the path that I would take to reach Bwlch Stwlan.  The bwlch of Craig Stwlan took a bit longer to locate.  This is often dictated by the flat topography of low points, but the winter conditions doubled the challenge of the task.  Once data had been collected, I ploughed onwards through the snow, thankfully joining a set of tracks that greatly eased progress. 

Trimble atop Craig Stwlan (Photo: Aled Williams)

I then joined the path known locally as Llwybr Gaseg.  This was spectacular in the conditions, perched above the freezing waters of Llyn Stwlan and the snow plastered slopes and gullies of Moelwyn Mawr and Craig Ysgafn above.  Care was needed in places, but no ice was met.  Upon reaching Bwlch Stwlan, a splendid view of Traeth Mawr and Porthmadog opened out. 

Along Llwybr Gaseg (Photo: Aled Williams)

Bwlch Stwlan (Photo: Aled Williams)

Craig Ysgafn looked imposing from Bwlch Stwlan, but the scarcity of ice made it an easier endeavour.  The south top was visited, another Sub-Top in the Welsh Highlands list, followed by the highest summit of the ridge.  However, the focus was the northern top, which classified a marginal P10 stood a chance of being promoted to the sub-list of The Welsh Highlands list that I co-author with Myrddyn Phillips. 

Craig Ysgafn (Photo: Aled Williams)

Summit of Craig Ysgafn looking toward Moelwyn Mawr (Photo: Aled Williams)

The bwlch of the north top was found to be a snug notch in the ridge, with an airy view down a gully that led straight down to the depths of Llyn Stwlan.  The survey was completed quickly and a short climb led to the summit of the north top.  A number of rocky protuberances competed for the highest point and so the Abney level was used to identify the correct outcrop.  Three men bypassed the summit as data collected, looking a little perplexed at the beeping noises emanating from the Trimble! 

Bwlch of the north top of Craig Ysgafn (Photo: Aled Williams)

Surveying the bwlch of the north top of Craig Ysgafn (Photo: Aled Williams)

Having collected data I made my way to the base of the upper slope of Moelwyn Mawr.  A determined plod progressively took me up to the steep slope, where a surprising amount of deep snow had settled. 

Climbing Moelwyn Mawr (Photo: Aled Williams)

Summit ridge of Moelwyn Mawr (Photo: Aled Williams)

The summit ridge of Moelwyn Mawr looked airy and alpine-like, with Yr Wyddfa and its neighbouring Eryri peaks looking majestic to the north.  Below was Llyn Croesor and Bwlch Rhosydd, between which was a hillock that I had recently identified as a potential Sub-Top for the Welsh Highlands list and as such, was the third surveying objective of the day. 

Summit of Moelwyn Mawr (Photo: Aled Williams)

Yr Wyddfa (Photo: Aled Williams)

The steep descent from the summit of Moelwyn Mawr was undertaken slowly and carefully.  I had been warned from a Blaenau Ffestiniog local whom I had met on the way up Moelwyn Mawr that the snow above Rhosydd was deep.  This warning was confirmed by the appearance of a partially buried stile and fence!  However, I was yet again fortunate to follow in the footsteps of others and so quick progress was made to the shores of Llyn Croesor. 

Deep snow (Photo: Aled Williams)

The final surveying objective (Photo: Aled Williams)

On the other side of the lake stood the craggy hillock that was to be surveyed, a short but enjoyable climb led to the summit, whereupon the Trimble was placed on the highest point of outcropping rock.  The scenery was breath-taking and I felt fortunate to be out in such a wild place.  However, the serenity would soon be broken by hard surveying work! 

Surveying the summit (Photo: Aled Williams)

The bwlch of the hill was unexpectedly complex.  I quickly realised that the gap had been plugged by a dam that formerly held the north side of Llyn Croesor when operating at reservoir capacity.  Furthermore, an outflow leat consisting of earth and large slabs of rock had been built on the northern side of the dam. 

The bwlch (Photo: Aled Williams)

The dam wall (Photo: Aled Williams)

Ironically, the ice axe strapped to my bag was finally unclipped and brought into use, with the adze helping to displace snow so that a better assessment of the terraformed bwlch could be made.  Two sets of data were gathered with the Trimble; one at the bottom of the dam wall and one above the leat at the spot where remaining natural ground was identified.  However, post survey, further analysis work revealed that the natural bwlch may well be extant to the north of the wall within the leat. 

The leat (Photo: Aled Williams)

Looking back at the final hill (Photo: Aled Williams)

Once the surveying equipment was packed away, a set of snow tracks sign-posted the way to Bwlch Rhosydd and ultimately the way back to civilisation.  Before I set off, a quick glance behind my shoulder revealed beautiful cloud formations above Moelwyn Mawr, brought in from the east and heralding a change in the weather.  It had been a great day out and one no doubt to be remembered for years to come.

Clouds above Moelwyn Mawr (Photo: Aled Williams)

Walkers ascending Moelwyn Mawr (Photo: Aled Williams)

 

Moelwyn Mawr clouds (Photo: Aled Williams)

Aled Williams (January 2023)

 

Survey Result: 

 

Craig Stwlan

Summit Height:  568.4m (converted to OSGM15)

Summit Grid Reference:  SH 66736 44654

Bwlch Height:  548.3m (converted to OSGM15)

Bwlch Grid Reference:  SH 66693 44717

Drop:  20.1m (500m Sub-Twmpau addition)

Dominance:  3.53%

Craig Ysgafn

Summit Height:  688.3m (converted to OSGM15)

Summit Grid Reference:  SH 65973 44394

Bwlch Height:  678.0m (converted to OSGM15)

Bwlch Grid Reference:  SH 65960 44382

Drop:  10.3m (Welsh Highland Sub addition)

Dominance:  1.49%

Pt. 524.3m

Summit Height:  524.3m (converted to OSGM15)

Summit Grid Reference:  SH 66117 45783

Bwlch Height:  513.5m (converted to OSGM15)

Bwlch Grid Reference:  SH 66131 45774

Drop:  10.7m (Welsh Highland Sub addition)

Dominance:  2.05%

 

For further details please consult the Trimble Survey Spreadsheet