Mapping Mountains – Hill Reclassifications – 200m Twmpau

Porfa Bwlch (SN 987 491) – 200m Sub-Twmpau deletion

There has been a deletion of a hill from 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 conducted by Myrddyn Phillips.

LIDAR image of Porfa Bwlch (SN 987 491)

The criteria for the list that this deletion 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. 

200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Porfa Bwlch and this was derived from the Tithe map, and it is adjoined to the Mynydd Epynt 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 and east, and the A483 road farther to its north, and has the town of Llanfair-ym-Muallt (Builth Wells) towards the east north-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying 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 2om of drop, based on the 276m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 256m bwlch height, based on interpolation of 10m contouring between 250m – 260m. 

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 deletion of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 273.8m summit height and a 253.8m bwlch height, with these values giving this hill 19.95m of drop, which is insufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  Porfa Bwlch 

OS 1:50,000 map:  147

Summit Height:  273.8m (LIDAR) 

Summit Grid Reference:  SN 98775 49136 & SN 98777 49139 (LIDAR) 

Bwlch Height:  253.8m (LIDAR) 

Bwlch Grid Reference:  SN 98928 49112 (LIDAR) 

Drop:  19.95m (LIDAR) 

 

Myrddyn Phillips (May 2024)