Mapping Mountains – Summit Relocations – 200m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Coed Garth Gwynion (SN 733 986) 

There has been confirmation of a Summit Relocation to a hill that is listed in the 200m Twmpau and 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 LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Coed Garth Gwynion (SN 733 986)

The criteria for the two listings that this summit relocation applies to are:

200m Twmpau - Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

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 now listed by is Coed Garth Gwynion, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with the A487 road to its north-west and a minor road to its east, and has the town of Machynlleth towards the north-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a 228m summit height, based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and which is positioned at SN 73306 98691. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 131m of drop, based on the 228m summit spot height and an estimated c 97m bwlch height, based on interpolation of 10m contouring between 90m – 100m.  The summit position was subsequently relocated to match the ten figure grid reference given on the Hill Bagging 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 summit image of Coed Garth Gwynion (SN 733 986)

The height produced by LIDAR analysis to the highest ground on this hill is 229.7m and this is positioned at SN 73301 98604, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the summit height produced by LIDAR analysis is 229.7m and this is positioned at SN 73301 98604, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 80 metres southward from the originally listed summit position and importantly it is placed to a different feature; an enclosed field compared to an enclosed wood. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Coed Garth Gwynion 

OS 1:50,000 map:  135

Summit Height:  229.7m (LIDAR) 

Summit Grid Reference (New Position):  SN 73301 98604 (LIDAR) 

Bwlch Height:  96.7m (LIDAR) 

Bwlch Grid Reference:  SN 73700 98378 (LIDAR) 

Drop:  133.0m (LIDAR) 

Dominance:  57.91% (LIDAR) 

 

Myrddyn Phillips (November 2024)