Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Mynydd Machen (ST 223 900) – Dominant deletion

There has been a deletion to a hill that is listed in the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Mynydd Machen (ST 223 900)

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

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

The name of the hill is Mynydd Machen and it is adjoined to the Cymoedd Gwent group of hills, which are situated in the eastern part of South Wales (Region C, Sub-Region C2), and it is positioned with the A467 road to its north and the A468 road to its south, and has the town of Rhisga (Risca) towards its north-east.

When the original Welsh 300m height band of P30 hills published on Geoff Crowder’s v-g.me website were standardised with interpolated heights and drop values this hill was listed with an estimated c 193m of drop, based on the 362m summit spot height adjoined to a triangulation pillar on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated bwlch height of c 169m based on interpolation of 10m contouring between 160m – 170m.  Therefore, when the original Dominants list was compiled this hill was listed with 53.31% dominance.

Extract from the Ordnance Survey 1:25,000 Explorer map showing detail for Mynydd Machen

Extract from the Ordnance Survey 1:25,000 Explorer map showing detail for Mynydd y Grug

The details for this hill were re-assessed when it was noted that Mynydd y Grug (ST 177 906) which is an adjacent and connecting hill is given a 375m summit spot height on the Ordnance Survey Vector Map, however as this spot height applied to land that has been artificially raised from mine spoil a lengthy discussion developed on whether such a summit should be included in preference to a natural summit such as Mynydd Machen.  As artificially raised summits through mine spoil are included in the Dominants list if considered solid and stable, which that of Mynydd y Grug is, it meant that as Mynydd y Grug is now considered higher than Mynydd Machen their respective bylchau are swapped and their new drop values resulted in the deletion of Mynydd Machen from Dominant status.    

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 image of Mynydd y Grug (ST 177 906)

LIDAR image for what was once the critical bwlch of Mynydd Machen and what is now the critical bwlch of Mynyd y Grug (bwlch positioned at ST 167 951)

The confirmation of this hill’s deletion from Dominant status is due to LIDAR analysis, resulting in a 363.2m summit height and a 250.6m bwlch height, with these values giving this hill 112.7m of drop and 31.01% dominance.  With LIDAR analysis of Mynydd y Grug resulting in a 374.2m summit height and a 169.85m bwlch height, with these values giving this hill 204.3m of drop and 54.61% dominance.

The full details for the hill are:

Group:  Cymoedd Gwent

Name:  Mynydd Machen

OS 1:50,000 map:  171

Summit Grid Reference:  ST 22381 90013 (LIDAR)

Summit Height:  363.2m (LIDAR)

Bwlch Grid Reference:  ST 19887 90335 (LIDAR)

Drop Summit to Bwlch:  112.7m (LIDAR)

 

Drop Bwlch to ODN:  250.6m (LIDAR)

Dominance:  31.01% (LIDAR)

Myrddyn Phillips (July 2019)