Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Yr Allt (SN 552 024) – 30-99m Sub-Twmpau addition 

There has been confirmation of an addition to the list of 30-99m 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 Yr Allt (SN 552 024)

The criteria for the list that this addition applies to are:

30-99m Twmpau – Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Yr Allt and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north and west, and the A4138 road to its south, and has the town of Llanelli towards the west south-west.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

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

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 26m of drop, based on the 97m summit spot height and the 71m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 addition of this hill to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 96.8m summit height and a 71.05m bwlch height, with these values giving this hill 25.75m of drop, which is sufficient for it to be classified as a 30-99m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Yr Allt 

OS 1:50,000 map:  159

Summit Height:  96.8m (LIDAR) 

Summit Grid Reference:  SN 55279 02446 (LIDAR)               

Bwlch Height:  71.05m (LIDAR) 

Bwlch Grid Reference:  SN 54918 02542 (LIDAR) 

Drop:  25.75m (LIDAR) 

 

Myrddyn Phillips (July 2024)