Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Pen Brwsh (SM 880 396) – 30-99m Twmpau addition 

There has been confirmation of an addition to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Pen Brwsh (SM 880 396)

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

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Pen Brwsh, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1).  As this is a tidal island it is surrounded by sea at high tide, and it is positioned with a minor road to its east and the A487 road farther to its south-east, and has the town of Abergwaun (Fishguard) towards the east south-east.

When the original 30-99m 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-evaluated and it was listed with an estimated c 42m of drop, based on an estimated c 42m summit height based on interpolation of its uppermost 40m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

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 30-99m Twmpau status is due to LIDAR analysis, resulting in a 41.8m summit height and a 0.2m bwlch height, with these values giving this hill 41.6m of drop, which is sufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Garn Fawr 

Name:  Pen Brwsh 

OS 1:50,000 map:  157

Summit Height:  41.8m (LIDAR) 

Summit Grid Reference:  SM 88067 39675 (LIDAR)               

Bwlch Height:  0.2m (LIDAR) 

Bwlch Grid Reference:  SM 88117 39673 (LIDAR) 

Drop:  41.6m (LIDAR) 

 

Myrddyn Phillips (June 2023)