Mapping Mountains – Hill Reclassifications – 100m Twmpau

Allt Pen y Lan (SN 707 323) – 100m Sub-Twmpau addition

There has been confirmation of an addition 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 Allt Pen y Lan (SN 707 323)

The criteria for the list that this addition 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. 

100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Allt Pen y Lan, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north-west, the A40 road to its south and the A482 road to its east, and has the village of Llanwrda towards the south-east.

When the original 100m height band of Welsh P30 hills were 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 137m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 112m bwlch height, based on interpolation of 10m contouring between 110m – 120m. 

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.

Therefore, the confirmation of the addition of this hill to 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 136.5m summit height and a 112.0m bwlch height, with these values giving this hill 24.5m of drop, which is sufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Allt Pen y Lan 

OS 1:50,000 map:  146, 160

Summit Height:  136.5m (LIDAR) 

Summit Grid Reference:  SN 70714 32376 & SN 70715 32374 & SN 70717 32374 (LIDAR) 

Bwlch Height:  112.0m (LIDAR) 

Bwlch Grid Reference:  SN 70316 32246 (LIDAR) 

Drop:  24.5m (LIDAR) 

 

Myrddyn Phillips (April 2024)