Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Comin y Garth (SN 982 548) – 200m Twmpau reclassified to 200m Sub-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 Comin y Garth (SN 982 548)

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 listed by is Comin y Garth and it is adjoined to the Drygarn Fawr group of hills, which are situated in the central part of the South Wales Region (Region B, Sub-Region B1), and it is positioned with the B4358 road to its north-west, minor roads to its south-west and north-east and the A470 road farther to its east, and has the village of Newbridge-on-Wye towards the 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 included in the Hills to be surveyed sub list that accompanied the main P30 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-assessed and it was listed with an estimated c 32m of drop, based on the 248m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 216m bwlch height, based on interpolation of 10m contouring between 210m – 220m that also appear on the Ordnance Survey 1:25,000 Explorer map. 

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

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 a 218m spot height on the area of the bwlch, and when coupled with the 248m summit spot height these values gave this hill 30m of drop.  The 218m bwlch spot height also appears on the interactive mapping available on the Magic Maps website. 

Extract from the Magic Maps website

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 to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 246.9m summit height and a 217.5m bwlch height, with these values giving this hill 29.5m of drop, which is insufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Comin y Garth 

OS 1:50,000 map:  147

Summit Height:  246.9m (LIDAR) 

Summit Grid Reference:  SN 98265 54889 (LIDAR) 

Bwlch Height:  217.5m (LIDAR) 

Bwlch Grid Reference:  SN 98505 55433 (LIDAR) 

Drop:  29.5m (LIDAR) 

 

Myrddyn Phillips (January 2022)