Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Glantrenfach (SN 524 426) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been a reclassification to 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 Glantrenfach (SN 524 426)

The criteria for the list that this reclassification 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. 

The 200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Glantrenfach and this was derived from the Tithe map, and it is adjoined to the Mynydd Pencarreg 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 immediate west, the A485 road to its north-west and the B4337 road to its north-east, and has the town of Llanybydder towards the north.

When the original 200m 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 category.

When 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 28m of drop, based on the 201m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 173m bwlch height, based on interpolation of 10m contouring between 170m – 180m. 

Extract from the Ordnance Survey 1:25,000 Explorer map

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

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 5m contouring, enabling the interpolated bwlch height to be better judged, resulting it this being amended to c 172m, with this value giving this hill an estimated c 29m of drop.

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 from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 201.5m summit height and a 170.4m bwlch height, with these values giving this hill 31.1m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Pencarreg 

Name:  Glantrenfach 

OS 1:50,000 map:  146

Summit Height:  201.5m (LIDAR) 

Summit Grid Reference:  SN 52425 42608 (LIDAR) 

Bwlch Height:  170.4m (LIDAR) 

Bwlch Grid Reference:  SN 52798 42397 (LIDAR) 

Drop:  31.1m (LIDAR) 

 

Myrddyn Phillips (July 2022)