Thursday, 24 September 2015

Mapping Mountains – Significant Height Revisions – 100m Twmpau and Y Trechol - The Dominant Hills of Wales

Bryn Mawr (SJ 251 190)

There has been a Significant Height Revision to a hill listed in the 100m Twmpau and Y Trechol - The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000, with these details being retrospective as the survey that resulted in this height revision was conducted on 8th August 2015.

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

100m Twmpau - These are the Welsh hills at or above 100m and below 200m in height that have a minimum drop of 30m.  The word Twmpau is an acronym for thirty welsh metre prominences and upward

Y Trechol - The Dominant Hills of Wales - These are the Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.

The name of the hill is Bryn Mawr and it is situated in the lower hills of the Carnedd Wen range, and is positioned above the small community of Four Crosses which is situated beside the Afon Efyrnwy (River Vyrnwy) and part of the Montgomeryshire Canal.

The hill was surveyed in the company of Alex Cameron during a multi hill bagging day when the weather was set fine and we visited nine P30 Twmpau hills with seven of them being Trimbled.

Access to Bryn Mawr is good from the west of its summit, as there is sufficient parking close to a gate that accesses a field to another gate which leads on to the hill.  However, the upper section is overgrown and the ancient earthen embankments of its hill fort are now immersed in long grass, nettles and brambles.  The summit is easily identified and the hill is recommended as there is a peaceful ambiance to it.

The hill has a 170m uppermost contour ring on  Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, with the latter having contour intervals at 5m.  Prior to the survey with the Trimble GeoXH 6000 the hill had been listed with an estimated summit height of c 174m based on the dimensions of the uppermost contour in relation to the steepness of the surrounding ground, with the height resulting from the survey with the Trimble GeoXH 6000 being 178.1m (converted to OSGM15).

Therefore this hill’s new summit height is 178.1m (converted to OSGM15) which is 4.1m higher than its previously estimated height of c 174m and 8.1m higher than its uppermost ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Cardinal Hill:  Carnedd Wen

Summit Height (New Height):  178.1m (converted to OSGM15)

Name:  Bryn Mawr

OS 1:50,000 map:  126

Summit Grid Reference:  SJ 25117 19054
Drop:  90m

Dominance:  50.53%

Gathering data from the summit of Bryn Mawr which resulted in this hill's significant height revision

Myrddyn Phillips (September 2015)

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