Wednesday 15 February 2023

Mapping Mountains – Trimble Surveys – Y Llethr


02.01.23  Pt. 687.5m (SH 666 238) and Crib y Rhiw (SH 663 248, only bwlch surveyed) 

After the indulgencies of the festive period, I was looking forward to blowing away the cobwebs with a healthy burst of exercise.  The high pressure forecasted for the second day of the New Year provided the ideal opportunity and a round of Mynydd Cors y Gedol was chosen as the venue.  The high point of the day would be Diffwys at an elevation of 750.4m; however, the focus would be the survey of its north-east top, a hill that I had wished to survey since 2006, since it stood a chance of making P15 status.  Accompanying me on this trip would be my latest surveying assistant, my other half, Rebecca. 

Rebecca at Pont Scethin (Photo: Aled Williams)

The walk started at the ancient house of Cors y Gedol, where parking is allowed at the end of the minor road for a very reasonable price.  A gradual plod along the continuing track eventually took us to Pont Scethin, a beautiful stone-arched bridge that was once on a drovers’ route.  The track on either side of the bridge was badly eroded by the gouging influence of scrambler bikes.  These vehicles and their impact on the land are sadly becoming a commoner sight on the Welsh mountains and more should be done to prevent them.  As I got off my soapbox, three bikers revved to the top of the ridge we were about to ascend.  They noisily made their way down the hillside, roaring past us as they raced onwards to Llyn Bodlyn, avoiding the worst of the deep mud that their contemporaries has previously created, thus creating further damage! 

Llyn Bodlyn (Photo: Aled Williams)

As we ascended the hillside, Llyn Irddyn came into view in its hidden recess with the sweep of Bae Tremadog providing the backdrop.  The morning mist had risen by this time, leaving us with clear views ahead as we crested the broad ridge to Diffwys.  The grassy dome of Y Llethr out peaked the series of hills visible to our left, with the dark and rocky Rhinog Fawr playing second best in terms of height.  To our right was Cadair Idris, its bulk completely enveloped in cloud.  A stiff climb shortly took us to the summit of the appropriately named Diffwys, the ‘precipice’, where a surprisingly dramatic view of Cwm Mynach and Y Garn greeted us.  We carefully traversed the slippery edge of the defile until we reached the trig point. 

Rebecca with Rhinog Fawr and Y Llethr as background (Photo: Aled Williams)

Descending from the summit, we prepared ourselves for the first survey.  The critical bwlch for the north-east top was easily located beside a stile on the northern side of the wall.  The Trimble was placed on top of the wall for optimal signal reception and Rebecca assisted me in measuring the offset height between the Trimble’s receptor and the ground below. 

Gathering data at the 688m map heighted summit (Photo: Aled Williams)

The first survey completed, we then headed to the summit of the rocky north-east top, which is currently listed as a P10 Sub-Top in The Welsh Highlands list that I co-author with Myrddyn Phillips.  The summit area consisted of a cairn and a number of small outcrops of rock, one of which we judged to be the high point.  The Trimble was placed atop the rocky pinnacle and measurement duly commenced.  Myrddyn had requested that the bwlch of Crib y Rhiw be surveyed if the opportunity presented itself and so as data collected, a quick check of the time was made to assess whether this could be fitted in to the day.  The answer was that it could, but only if we hurried! 

Looking back at the 688m map heighted summit (Photo: Aled Williams)

We ascended Crib y Rhiw quickly.  The views down the steep eastern slope brought back memories from my first visit that had taken place sixteen years ago - it felt good to revisit.  The bwlch of the hill was easily located a short distance south-west of the impressive dry-stone wall that adorns the ridge.  The survey went without a hitch.  Having completed our surveying objectives, we began the long journey back to Cors y Gedol and as we hit our tenth mile of the day, the sun shone low above the sea in front of us as it too began its descent. 

The end of a long day on the hill (Photo: Aled Williams)

Aled Williams (January 2023)

 

Survey Result: 

 

Pt. 687.5m

Summit Height:  687.5m (converted to OSGM15)

Summit Grid Reference:  SH 66665 23885

Bwlch Height:  675.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 66468 23837

Drop:  12.4m

Dominance:  1.80%


 

Crib y Rhiw

Summit Height:  681.3m (converted to OSGM15) (from previous Trimble GeoXH 6000 survey) (significant height revision)

Summit Grid Reference:  SH 66368 24868 (from previous Trimble GeoXH 6000 survey)

Bwlch Height:  652.1m (converted to OSGM15)

Bwlch Grid Reference:  SH 66080 25302

Drop:  29.2m

Dominance:  4.28%



For details on the summit survey of Crib y Rhiw 


For further details please consult the Trimble Survey Spreadsheet

 

 

 

  

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