Mapping Mountains – Hill Reclassifications – 100m Twmpau

Pt. 156.9m (SN 679 244) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to a hill that is listed in the 100m Twmpau, with the summit height and its location, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Pt. 156.9m (SN 679 244)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 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.

As an appropriate name for the hill is not known either from historic research or from local enquiry it is being listed by the point (Pt. 156.9m) notation, and it is adjoined to the Y Mynydd Du group of hills, which are situated in the north-western part of South Wales (Region C, Sub-Region C1), and it is encircled by minor roads and is positioned with the A40 road and the Afon Tywi (River Towy) to its north-west, and has the village of Llangadog towards the north-east and the town of Llandeilo towards the west south-west.

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 did not meet the criteria then used for the main P30 list; however this sub list has now been standardised and interpolated heights and drop values also included in the main P30 and the accompanying sub list.

When this list was standardised and interpolated heights also included this hill was listed with an estimated c 30m of drop, based on the 157m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 127m bwlch height based on interpolation of 10m contouring.

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

These values were re-assessed when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online, and the listed drop value remained the same.

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. 

The summit height produced by LIDAR analysis is 156.85m and is positioned at SN 67900 24456, and the bwlch height is 125.1m and is positioned at SN 67831 24303, with these values giving this hill 31.7m of drop, which is sufficient for this hill to be classified as a 100m Twmpau.

The full details for the hill are:

Group:  Y Mynydd Du

Name:  Pt. 156.9m

OS 1:50,000 map:  159

Summit Height:  156.85m (LIDAR)

Summit Grid Reference:  SN 67900 24456 (LIDAR)

Bwlch Height:  125.1m (LIDAR)

Bwlch Grid Reference:  SN 67831 24303 (LIDAR)

Drop:  31.7m (LIDAR)

Myrddyn Phillips (March 2019)