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

Rhos Ymryson (SN 460 500) – Dominant reclassified to Lesser Dominant

There has been a reclassification to the listing of Y Trechol – The Dominant Hills of Wales initiated by re-evaluation of the summit height of the hill and subsequent LIDAR bwlch analysis conducted by Myrddyn Phillips, resulting in this hill being reclassified to the Lesser Dominant list. 

The criteria for this listing are:

Y Trechol – The Dominant Hills of Wales - Welsh P30s whose prominence equal or exceed half that of their absolute height, with the criteria for Lesser Dominant status being those additional Welsh P30s whose prominence is 33% or more and below 50% of their absolute height.

Prior to this re-evaluation it was listed as a Dominant hill based on the 327m summit spot height  and the 163m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map, these values gave this hill 164m of drop and 50.15% Dominance.  Importantly the 327m summit spot height is given to the top of a covered reservoir; these constructions are not included in the criterion for summit height within this list, with ground at the immediate base of such constructions considered natural ground and therefore the point at which the height of the summit should be taken.  The ten figure grid reference reported for the highest remaining natural ground of this hill is at SN 46038 50017, this is extremely close to where a 1062ft (323.7m) levelled height appears on the Ordnance Survey 1906 Six-Inch map which matches the 324m rounded up height on the Ordnance Survey 1:50,000 Landranger map, and it is this height that is being taken as that for the highest remaining natural ground of this hill.

However, it was not until LIDAR became available that the bwlch 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 name of the hill is Rhos Ymryson and it is the highest point in its own group of hills which are situated in Mid and West Wales (Region B, Sub-Region B3).  The hill overlooks the B 4338 road which is to its north, with the small community of Talgarreg to its west north-west and Mydroilyn towards its north.

As the summit of the hill is not on designated open access land permission to visit should be sought, for those wishing to do so an access track to the covered reservoir leaves the convenience of the B 4338 road to the north of the summit with a public footpath leaving this same road to the west of the access track.

The reclassification of Rhos Ymryson from Dominant to Lesser Dominant hill is due to the re-evaluation of its summit height and as the 327m spot height that appears on the Ordnance Survey 1:25,000 Explorer map is given to the top of a covered reservoir, it is the old levelled height on the Ordnance Survey Six-Inch map which matches the rounded up 324m height on the Ordnance Survey 1:50,000 Landranger map that is being taken as the summit height of this hill, and when coupled with the 162.5m bwlch height derived from LIDAR analysis, these values give this hill 161m of drop and 49.83% dominance, which is insufficient for it to be classified as a Dominant hill.

The full details for the hill are:

Group:  Rhos Ymryson

Name:  Rhos Ymryson

Dominance:  49.83%

OS 1:50,000 map:  146

Summit Grid Reference:  SN 46038 50017

Summit Height:  324m

Drop Summit to Bwlch:  161m

Drop Bwlch to ODN:  162.5m 

Myrddyn Phillips (June 2018)