Mapping Mountains – Hill Reclassifications – 500m Twmpau

Pen y Cerrig Serth (SH 970 429) – 500m Sub-Twmpau addition

There has been an addition of a hill to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips and Aled Williams which took place on the 19^th January 2020.

Pen y Cerrig Serth (SH 970 429)

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

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name the hill is listed by is Pen y Cerrig Serth, and it is adjoined to the Arenig group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A3), and it is positioned with the A5 road to its north-east, the B4501 road to its west, the A4212 road to its south-west and the A494 road to its south-east, and has the town of Y Bala towards the south-west. 

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 522m summit height and a 503m bwlch height, with the former appearing as a spot height on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.  The 19m drop value was ascertained from a basic levelling survey conducted by Myrddyn Phillips on the 1^st May 2004, resulting in a 64ft / 19.5m drop value.

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

However, as the margin of uncertainty applicable to the basic levelling method of surveying over the distance between bwlch and summit and the terrain of this hill is approximately +/- 1.5m, the hill was prioritised for an accurate GNSS survey.

Gathering data at the bwlch of Pen y Cerrig Serth

Gathering data at the summit of Pen y Cerrig Serth

Therefore, the addition of Pen y Cerrig Serth to 500m Sub-Twmpau status is due to a survey with the Trimble GeoXH 6000, resulting in a 520.7m summit height and a 499.9m bwlch height, with these values giving this hill 20.8m of drop which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group:  Arenig

Name:  Pen y Cerrig Serth

OS 1:50,000 map:  125

Summit Height:  520.7m (converted to OSGM15)

Summit Grid Reference:  SH 97054 42933

Bwlch Height:  499.9m (converted to OSGM15)

Bwlch Grid Reference:  SH 96817 42721

Drop:  20.8m

Myrddyn Phillips (June 2020)