Mapping Mountains – Hill Reclassifications – 200m Twmpau

Moel Gallt y Cwm (SS 811 909) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been a reclassification to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill initially confirmed by Joe Nuttall who produced a summit analysis programme using LIDAR, and then by LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips.

LIDAR image of Moel Gallt y Cwm (SS 811 909)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Moel Gallt y Cwm, and it is adjoined to the Cymoedd Morgannwg group of hills which are situated in the central part of South Wales (Region C, Sub-Region C2), and it is situated with the B4282 road to its north, the A4107 road to its west, the M4 motorway to its south-west and the A4063 road to its east, and has the town of Port Talbot towards the west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list. 

When 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 an estimated c 24m of drop, based on the 272m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger map and an estimated c 248m bwlch height, based on interpolation of 10m contouring between 240m – 250m 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

Since publication of these P30 lists on Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, whilst others are current and digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This mapping had many spot heights not on other publicly available Ordnance Survey maps, and for this hill it showed a 271m summit spot height and a 247m bwlch spot height, with these values giving this hill 24m of drop.

Extract from the Ordnance Survey 1:50,000 Landranger 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. 

The result produced by LIDAR analysis gives this hill 30.3m of drop.  Therefore, the reclassification of this hill from 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 272.1m summit height and a 241.8m bwlch height, with these values giving this hill 30.3m of drop, which is sufficient for it to be classified as a 200m Twmpau.

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Moel Gallt y Cwm

OS 1:50,000 map:  170

Summit Height:  272.1m (LIDAR)

Summit Grid Reference:  SS 81150 90920 (LIDAR)

Bwlch Height:  241.8m (LIDAR)

Bwlch Grid Reference:  SS 81383 90910 (LIDAR)

Drop:  30.3m (LIDAR)

Myrddyn Phillips (December 2020)