Mapping Mountains – Hill Reclassifications – 200m Twmpau

Pen Llys (SN 998 584) – 200m Sub-Twmpau reclassified to 200m Twmpau

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

LIDAR image of Pen Llys (SN 998 584)

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 Llys and it is adjoined to the Elenydd group of hills, which are situated in the central part of the Mid and West Wales Region (Region B, Sub-Region B2), and it is positioned with minor roads to its north-west, east and north-east, the B4358 road to its south-east and the A470 road farther to its north-east, and has the village of Newbridge-on-Wye towards the east. 

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

When 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 30m of drop, based on the 292m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 262m bwlch height, based on interpolation of 10m contouring between 260m – 270m that appear on the Ordnance Survey 1:25,000 Explorer map. 

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 262m spot height on the area of the bwlch, and when coupled with the 292m summit spot height these values gave this hill 30m of drop. 

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. 

The result produced by LIDAR analysis gives this hill a 291.5m summit height, and as the summit has 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 summit of Pen Llys

Therefore, the confirmation of the reclassification of this hill to 200m Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 291.8m summit height and a 261.3m bwlch height, with these values giving this hill 30.6m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Elenydd 

Name:  Pen Llys 

OS 1:50,000 map:  147

Summit Height:  291.8m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SN 99830 58439 (Trimble GeoXH 6000) 

Bwlch Height:  261.3m (LIDAR) 

Bwlch Grid Reference:  SN 99569 58593 (LIDAR) 

Drop:  30.6m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (December 2021)