Tuesday, 28 November 2017

Mapping Mountains – Summit Relocations – Yr Uchafion and 700m Twmpau

Twyn Llech (SO 255 353)

There has been a Summit Relocation confirmed by a survey with the Trimble GeoXH 6000 to the listings of the Yr Uchafion and 700m Twmpau, with the survey that confirmed this summit relocation being conducted on the 28th September 2017 in good, clear conditions with just a breath of breeze.

The two lists that this summit relocation affects are:

Yr Uchafion – All Welsh hills at and above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams, with the Introduction to the list appearing on Mapping Mountains on the 4th November 2015.

700m Twmpau – All Welsh hills at and above 700m and below 800m in height that have 30m minimum drop, with the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.  The list is authored by Myrddyn Phillips.

The name of the hill is Twyn Llech and the survey was conducted in the Mynyddoedd Duon (Black Mountains) range, which is an extensive group of hills in the south-eastern part of South Wales, and the hill is situated above and between the upper part of the Olchon Valley to its south-east and the Dyffryn Ewias (Vale of Ewyas) to its south.  The hill is also situated between the small communities of Craswall towards its east and Capel-y-ffin towards its south.

Twyn Llech (SO 255 353)

The hill can be accessed from a car park just below the high point of Bwlch yr Efengyl (Gospel Pass) via the summit of Hay Bluff, or it can be approached from the south-east via paths leading to its southerly ridge from either the Olchon Valley or the Dyffryn Ewias (Vale of Ewyas).

When this hill was first included in the listings that later became known as the Yr Uchafion and 700m Twmpau it was given a 703m summit height and a grid reference of SO 255 350, this was based on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps which give a summit spot height of 703m positioned at SO 25586 35010.  However, as many visual inspections of the land taking in the summit area of this hill had concluded that land to the north of where this spot height appears on the ground was in fact higher and this position matched that given on the Harvey 1:40,000 Offa’s Dyke South map, it was this latter map’s 706m summit spot height that was then used in the latest Yr Uchafion and 700m Twmpau listings.

The Ordnance Survey 703m summit position

The Harvey maps 706m summit position

Two points were surveyed for summit position using the Trimble GeoXH 6000, one on the southerly point where the Ordnance Survey give their 703m spot height and one on the northerly point where Harvey maps give their 706m spot height.  The summit area of this hill has two rises which complement these spot height positions and which have a slight dip in between.

The results produced by the Trimble GeoXH 6000 appear below:

Northerly high point:  703.639m (converted to OSGM15) at SO 25520 35383

Southerly high point:  702.721m (converted to OSGM15) at SO 25572 35032

Therefore, the position of the relocated summit has been confirmed and this is at SO 25520 35383 and it consists of tussock grass close to the main and now well maintained paved path that crosses this hill on a south – north direction.  This re-located summit position is not given a spot height on current Ordnance Survey maps, although it is given a 706m spot height on Harvey maps, and it is approximately 350 metres north from the original listed summit position. 

The full details for the hill are:

Cardinal Hill:  Twyn Llech

Summit Height:  703.6m (converted to OSGM15)

Name:  Twyn Llech

OS 1:50,000 map:  161

Summit Grid Reference (New Position confirmed):  SO 25520 35383  

Drop:  154.5m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the original listed summit position with the now confirmed relocated higher summit in the background of this photo

Myrddyn Phillips and Aled Williams (November 2017)

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