Mapping Mountains – Hill Reclassifications – 100m Twmpau

Pen y Foel (SN 000 381) – 100m Twmpau reclassified to 100m Sub-Twmpau

There has been a reclassification to the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Pen y Foel (SN 000 381) 

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Pen y Foel, and it is adjoined to the Mynydd Preseli group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its west, and the A487 road to its south-east, and has the town of Abergwaun (Fishguard) towards the west south-west.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list, as the contemporary Ordnance Survey 1:25,000 Explorer map gives this hill an uppermost 110m contour and bwlch contouring between 75m – 80m, resulting in this hill having a minimum of 30m of drop.

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 31m of drop, based on an estimated c 110m summit spot height and an estimated c 79m bwlch height, with both heights based on interpolation of 5m contouring that appears 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 109m summit spot height which resulted in the drop value of this hill being amended to an estimated c 30m.

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.

Therefore, the reclassification of this hill to 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 108.5m summit height and an 80.1m bwlch height, with these values giving this hill 28.4m of drop, which is insufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Preseli 

Name:  Pen y Foel 

OS 1:50,000 map:  145, 157

Summit Height:  108.5m (LIDAR) 

Summit Grid Reference:  SN 00073 38103 (LIDAR) 

Bwlch Height:  80.1m (LIDAR) 

Bwlch Grid Reference:  SN 00179 37983 (LIDAR) 

Drop:  28.4m (LIDAR) 

 

Myrddyn Phillips (March 2023)