Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Pen yr Allt (SH 842 025) – 200m Sub-Twmpau addition

There has been confirmation of an addition to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Pen yr Allt (SH 842 025)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Pen yr Allt and it is adjoined to the Pumlumon group of hills which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is encircled by minor roads with the A470 road farther to its north-east, and has the village of Comins-coch towards the north-east. 

When the original 200m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimated c 25m of drop, based on the 267m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 242m bwlch height based on interpolation of 10m contouring between 240m – 250m. 

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

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR summit image of Pen yr Allt (SH 842 025)

Currently LIDAR only covers the summit area of this hill and as the summit and bwlch have now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the bwlch of Pen yr Allt

The Trimble GeoXH 6000 gathering data at the summit of Pen yr Allt

Therefore, the confirmation of the addition of this hill to 200m Sub-Twmpau status is due to LIDAR summit analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 266.6m summit height and a 241.9m bwlch height, with these values giving this hill 24.7m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Pen yr Allt 

OS 1:50,000 map:  135, 136

Summit Height:  266.6m (converted to OSGM15) 

Summit Grid Reference:  SH 84232 02576 

Bwlch Height:  241.9m (converted to OSGM15) 

Bwlch Grid Reference:  SH 84202 02341 

Drop:  24.7m 

 

Myrddyn Phillips (July 2021)