Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Bigyn (SS 512 998) – 30-99m Sub-Twmpau addition 

There has been 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 Bigyn (SS 512 998)

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 listed by is Bigyn 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 the A484 road to its north and the B4304 road to its south, and has the town of Llanelli surrounding it.

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.

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 16m of drop, based on the 43m 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 27m bwlch height, based on interpolation of 5m contouring between 25m – 30m. 

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 addition of this hill to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in an 45.8m summit height and a 20.7m bwlch height, with these values giving this hill 25.1m 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:  Bigyn 

OS 1:50,000 map:  159

Summit Height:  45.8m (LIDAR) 

Summit Grid Reference:  SS 51218 99827 (LIDAR)               

Bwlch Height:  20.7m (LIDAR) 

Bwlch Grid Reference:  SN 51552 00728 (LIDAR) 

Drop:  25.1m (LIDAR) 

 

Myrddyn Phillips (September 2024)