Mapping Mountains – Trimble Surveys – Arenig Fawr

28.07.24  Pen Tyrau (SH 837 382) 

The early morning start provided negligible respite from the summer heat.  I stood forlornly upon the bank of Llyn Arenig Fawr as I eyed up the steep sections of the arm that would take me to Bwlch Blaen Nant.  The anxiety lapsed as I took in the beauty of the scene; abstract reflections of a rugged mountain framed by a cloudless sky, the calm intercepted momentarily by the shrieking calls of peregrine falcons.

The morning view from Llyn Arenig Fawr.  Photo: Aled Williams 

Ahead lay Pen Tyrau, which was the surveying objective of the day, having recently been deleted as a P10 Sub-Top within The Welsh Highlands list.  Analysis of LIDAR data indicated that the prominence of the hill only reached 9.7m, which is 30cm short of the qualifying threshold of 10m.  Armed with Myrddyn’s Trimble GeoXH 6000, my intention was to confirm the hill’s status by obtaining an accurate value for its drop.

Looking toward the summit area of Pen Tyrau.  Photo: Aled Williams 

Despite its modest credentials as a separate hill, the top caps an extensive crag-rimmed plateau that forms the north-eastern projection of Arenig Fawr.  This promontory is known locally as Pen Tyrau, a name that has suffered slippage on OS maps over the years.  Its elevated position is a wild place consisting of moorland dappled with small pools and dotted with rough outcrops and small glacial erratics. Having traversed through the bogs of Blaen Nant, I soon found myself standing beside the summit cairn where data was to be collected.

The view from the summit of Pen Tyrau.  Photo: Aled Williams

Surveying was not made easy by the clouds of midges that were swarming the plateau.  Having completed the first survey, I begrudgingly decided to take another set of measurements from a nearby rock that the Abney level had deemed to be close in height to the cairned high point.  As I waited for the Trimble to complete its measuring, I looked out to Arenig Fach and noted the similarities between the character and topography of that hill to that of Pen Tyrau.

The Trimble GeoXH 6000 gathering data at the summit of Pen Tyrau.  Photo: Aled Williams

I headed to the hill’s bwlch for the final survey of the day.  A characteristic of such places is the tricks they play on the eyes, with ups looking like downs and vice versa depending on where one stands.  This bwlch proved to be no exception and I unashamedly used the Trimble as a hand-held GPS to bring me to the point LIDAR had determined to be both the trough in the hill-to-hill line and the crest in the valley-to-valley traverse.

Gathering data at the bwlch of Pen Tyrau.  Photo: Aled Williams

The end of the surveying brought a sudden end to the veritable feast that the midges were enjoying.  I escaped via my inward route to Bwlch Blaen Nant and stopped atop the crags of Y Castell to admire the view down into the waters of Llyn Arenig Fawr.  Behind me towered the upper ridge of Arenig Fawr, which the midday sun would not entice me to visit.

Arenig Fawr from the descent of Pen Tyrau.  Photo: Aled Williams

Aled Williams (July 2024)

 

Survey Result: 

 

Pen Tyrau

Summit Height:  697.4m (converted to OSGM15)

Summit Grid Reference:  SH 83737 38250

Bwlch Height:  687.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 83601 38142

Drop:  10.0m (Welsh Highland Sub addition)

Dominance:  1.44%

 

For further details please consult the Trimble Survey Spreadsheet