Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Allt Mes (SN 795 397) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been confirmation of 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 Allt Mes (SN 795 397)

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 Allt Mes, 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 minor roads to its north-east, north-west and south-west, and the A483 road to its south-east, and has the town of Llanymddyfri (Llandovery) towards the south south-west. 

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-evaluated and it was listed with an estimated c 33m of drop, based on the 217m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 184m bwlch height, based on interpolation of 10m contouring between 180m – 190m that appear 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 reclassification of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 219.4m summit height and a 182.8m bwlch height, with these values giving this hill 36.6m of drop, which is sufficient for it to be classified as a 200m Twmpau. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Allt Mes 

OS 1:50,000 map:  146, 160

Summit Height:  219.4m (LIDAR) 

Summit Grid Reference:  SN 79575 39715 (LIDAR) 

Bwlch Height:  182.8m (LIDAR) 

Bwlch Grid Reference:  SN 79199 39937 (LIDAR) 

Drop:  36.6m (LIDAR) 

 

Myrddyn Phillips (January 2022)