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

Cefn (SH 721 004) – Lesser Dominant reclassified to Dominant

There has been a reclassification to the list of the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from detail on contemporary maps derived from Ordnance Survey data and subsequently confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Cefn (SH 721 004)

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, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Cefn and this was derived from the Tithe map, and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with the A493 road to its north and the A487 road to its south, and has the town of Machynlleth towards the east.

After the original 100m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website were standardised and interpolated heights and drop values also included, this hill was listed with an estimated c 114m summit height, based on interpolation of its uppermost 110m ring contour, and the 59m bwlch spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, with these values giving this hill an estimated c 55m of drop and 48.25% dominance.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 119m summit spot height and when coupled with its 59m bwlch spot height, these values gave this hill 60m of drop and 50.42% dominance.

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.

Therefore, the confirmation of the reclassification of this hill from Lesser Dominant status is due to LIDAR analysis, resulting in a 118.2m summit height and a 59.0m bwlch height, with these values giving this hill 59.2m of drop and 50.08% dominance, which is sufficient for Dominant status.    

The full details for the hill are:

Group:  Tarren y Gesail

Name:  Cefn

OS 1:50,000 map:  135

Summit Height:  118.2m (LIDAR)

Summit Grid Reference:  SH 72126 00402 (LIDAR)

Bwlch Height:  59.0m (LIDAR)

Bwlch Grid Reference:  SH 72569 00795 (LIDAR)

Drop:  59.2m (LIDAR)

Dominance:  50.08% (LIDAR)

Myrddyn Phillips (May 2017)