Mapping Mountains – Hill Reclassifications – 500m Twmpau

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

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips and Aled Williams and LIDAR bwlch analysis conducted by Aled Williams.

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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains. 

500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Pen y Cerrig Serth, and it is adjoined to the Carnedd y Filiast group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A2), 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.  With the 19m drop value also ascertained from a basic levelling survey conducted by Myrddyn Phillips on the 1^st May 2004, resulting in a 64ft / 19.5m of drop.

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 a GNSS survey.

Gathering data at the bwlch area of Pen y Cerrig Serth

Gathering data at the summit of Pen y Cerrig Serth

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 summit survey and subsequent LIDAR bwlch analysis, resulting in a 520.7m summit height and a 500.0m bwlch height, with these values giving this hill 20.7m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group:  Carnedd y Filiast

Name:  Pen y Cerrig Serth

OS 1:50,000 map:  125

Summit Height:  520.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 97054 42933 (Trimble GeoXH 6000)

Bwlch Height:  500.0m (LIDAR)

Bwlch Grid Reference:  SH 96806 42746 (LIDAR)

Drop:  20.7m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (June 2020)