Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Bryn Llwyn (SN 672 998) – 100m Sub-Twmpau reclassified to 100m 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 Bryn Llwyn (SN 672 998)

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.  The list is authored by Myrddyn Phillips, 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 Bryn Llwyn, and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north and the A493 road to its south-east, and has the village of Cwrt towards the east.

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 Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list.

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 27m of drop, based on an estimated c 173m summit height and an estimated c 146m bwlch height, with both heights based on interpolation of 10m contouring 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. 

Extract bwlch image from Google Maps

LIDAR bwlch image of Bryn Llwyn

Therefore, the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 175.4m summit height and a 145.4m bwlch height, with these values giving this hill 30.0m of drop, which is sufficient for it to be classified as a 100m Twmpau.  With the caveat that LIDAR analysis gives a higher 146.0m bwlch position on the valley to valley traverse at SN 66943 99797 which is a part of a raised track that is considered a relatively recent man-made construct and therefore discounted from the drop value of this hill, with this hill qualifying for P30 status due to the highest remaining natural ground protocol being used. 

 

The full details for the hill are: 

Group:  Tarren y Gesail 

Name:  Bryn Llwyn 

OS 1:50,000 map:  135

Summit Height:  175.4m (LIDAR) 

Summit Grid Reference:  SN 67202 99820 (LIDAR) 

Bwlch Height:  145.4m (LIDAR) 

Bwlch Grid Reference:  SN 66942 99795 (LIDAR) 

Drop:  30.0m (LIDAR) 

 

Myrddyn Phillips (October 2025)