Mapping Mountains – Hill Reclassifications – 100m Twmpau

Y Lan (SN 283 229) – 100m Twmpau reclassified to 100m Sub-Twmpau

There has been a reclassification to the list of 100m 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 Y Lan (SN 283 229)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Y Lan 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 minor roads to its north, west and south and the B4299 road to its east, and has the small town of Sanclêr (St Clears) towards the south.

When the original 100m 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.

After 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 30m of drop, based on the 188m summit spot height and the 158m bwlch spot height that appear on the Ordnance Survey 1:50,000 Landranger and 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 reclassification of this hill to 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 188.0m summit height and a 158.0m bwlch height, with these values giving this hill 29.97m of drop, which is insufficient for it to be classified as a 100m Twmpau. 

 

The full details for the hill are: 

Group:  Mynydd Pencarreg 

Name:  Y Lan 

OS 1:50,000 map:  145, 158

Summit Height:  188.0m (LIDAR) 

Summit Grid Reference:  SN 28371 22966 (LIDAR) 

Bwlch Height:  158.0m (LIDAR) 

Bwlch Grid Reference:  SN 28481 23410 & SN 28481 23413 (LIDAR) 

Drop:  29.97m (LIDAR) 

 

Myrddyn Phillips (July 2022)