Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Coed Darcy (SS 711 955) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau

There has been a reclassification to the listing of 30-99m Twmpau, with the summit height, its location, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Coed Darcy (SS 711 955)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 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.

The name the hill is now listed by is Coed Darcy and it is adjoined to the Fforest Fawr group of hills, which are situated in the northern part of South Wales (Region C, Sub-Region C2), and it is positioned with the B4290 and M4 roads to its east, and has the town of Castell-neth (Neath) towards the north-east.

When the original 30—99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill appeared in the accompanying 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 with drop values and interpolated heights also included in the main P30 and the accompanying sub list.

Extract from the Ordnance Survey 1:25,000 Explorer map from 1998

After this list was standardised and interpolated heights also included this hill was listed with 31m of drop, based on the 83m summit spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map and a bwlch height of 52m based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

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. 

LIDAR summit image of Coed Darcy

The summit height produced by LIDAR analysis is 80.4m and is positioned at SS 71147 95565, and the bwlch height is 53.0m and is positioned at SS 71330 95324, with these values giving this hill 27.4m of drop which is insufficient for continued 30-99m Twmpau status.

The full details for the hill are:

Group:  Fforest Fawr

Name:  Coed Darcy

OS 1:50,000 map:  170

Summit Height:  80.4m (LIDAR)

Summit Grid Reference:  SS 71147 95565 (LIDAR)

Bwlch Height:  53.0m (LIDAR)

Bwlch Grid Reference:  SS 71330 95324 (LIDAR)

Drop:  27.4m (LIDAR)

Myrddyn Phillips (April 2019)