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

Mynydd y Glyn (ST 031 896) – Lesser Dominant reclassified to Dominant

There has been a reclassification to the listing of Y Trechol – The Dominant Hills of Wales, with the bwlch height and its location confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR bwlch image of Mynydd y Glyn

The criteria for the list that this reclassification 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 y Glyn 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 positioned with the Afon Rhondda Fawr (Rhondda River) and the A4058 road to its north and the A4233 road to its west, and has the town of Pontypridd towards its 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 182m of drop, based on the 377m summit spot height and an estimated bwlch height of c 195m based on interpolation of 10m contouring between 190m – 200m that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

Summit extract from the Ordnance Survey 1:25,000 Explorer map

Bwlch extract from the Ordnance Survey 1:25,000 Explorer map

When the original Dominants list was compiled this hill was listed as a Lesser Dominant with 48.28% dominance based on the 377m summit height and the estimated c 182m drop value, as detailed above.

The drop value for this hill was next re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map, and this resulted in the estimated drop value of c 182m remaining 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 reclassification of Mynydd y Glyn from Lesser Dominant to Dominant status is due to LIDAR bwlch analysis, with LIDAR not covering the summit of this hill, resulting in a 187.7m bwlch height, which when coupled with the 377m summit height gives this hill 189m of drop and 50.20% dominance.

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Mynydd y Glyn

OS 1:50,000 map:  170

Summit Grid Reference:  ST 03193 89647

Summit Height:  377m

Bwlch Grid Reference:  ST 00330 90578 (LIDAR)

Drop Summit to Bwlch:  189m (LIDAR bwlch) 

Drop Bwlch to ODN:  187.7m (LIDAR)

Dominance:  50.20% (LIDAR bwlch)

Myrddyn Phillips (May 2019)