Saturday, 25 April 2020

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

Cymin (SO 527 125) – Lesser Dominant addition

There has been an addition to the listing of Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height, their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Cymin (SO 527 125)

The criteria for the list that this addition 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 3rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is listed by is Cymin and it is adjoined to the Fforest y Ddena group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the A4136 road to its north and the A466 road to its south-west, and has the town of Trefynwy (Monmouth) towards the west.

When the original 200m height band of Welsh P30 hills published on Geoff Crowder’s website were standardised and interpolated heights also included, this hill was listed with c 86m of drop and 33.46% dominance, based on an estimated c 257m summit height and an estimated c 171m bwlch height, with these heights estimated from interpolation of 5m contouring that appears on the Ordnance Survey 1:25,000 Outdoor Leisure (latterly Explorer) map.

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 is entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and gives a 256m spot height on the area of this hill’s summit positioned at SO 527 125, and when coupled with the estimated c 171m bwlch height, these values give this hill c 85m of drop and 33.20% dominance, resulting in its deletion from Lesser Dominant status.  The 256m spot height is also shown on Ordnance Survey data that appears on the Magic Maps website.

Extract from the Magic Maps website

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 Cymin

Therefore, the addition of this hill to Lesser Dominant status is due to LIDAR analysis, resulting in a 256.4m summit height and a 170.6m bwlch height, with these values giving this hill 85.8m of drop and 33.47% dominance, which is sufficient for this hill to be reinstated to Lesser Dominant status.

The full details for the hill are:

Group:  Fforest y Ddena

Name:  Cymin

OS 1:50,000 map:  162

Summit Grid Reference:  SO 52771 12511 (LIDAR)

Summit Height:  256.4m (LIDAR)

Bwlch Grid Reference:  SO 53644 12441 (LIDAR)

Drop Summit to Bwlch:  85.8m (LIDAR)

Drop Bwlch to ODN:  170.6m (LIDAR)

Dominance:  33.47% (LIDAR)

Myrddyn Phillips (April 2020)

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