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

Llwynderw Hill (SJ 196 036) – Lesser Dominant deletion

There has been a deletion to the listing of 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 and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

LIDAR image of Llwynderw Hill (SJ 196 036)

The criteria for the list that this deletion 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 Llwynderw Hill and this was derived from local enquiry, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with minor roads to its north, west and south and the A483 road to its south-east, and has the town of Y Trallwng (Welshpool) towards the north-east. 

After the original 200m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website were standardised and interpolated heights also included, this hill was listed with an estimated c 80m of drop and 33.61% dominance, based on the 238m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 158m bwlch height, based on interpolation of 5m contouring between 155m – 160m that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

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 analysis confirmed the natural summit position of this hill and decreased its dominance to 33.87%, and as the summit of this hill has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised in preference to the LIDAR height and position, resulting in a summit height of 237.9m positioned at SJ 19600 03662. 

The Trimble GeoXH 6000 gathering data at the summit of Llwynderw Hill

Therefore, the deletion of this hill from Lesser Dominant status is due to LIDAR bwlch analysis and a Trimble GeoXH 6000 summit survey, resulting in a 237.9m summit height and a 159.7m bwlch height, with these values giving this hill 78.3m of drop and 32.89% dominance, which is insufficient for it to be classified as a Lesser Dominant hill. 

 

The full details for the hill are:

Group:  Carnedd Wen

Name:  Llwynderw Hill 

OS 1:50,000 map:  136

Summit Grid Reference:  SJ 19600 03662 (Trimble GeoXH 6000)

Summit Height:  237.9m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference:  SJ 18827 03989 (LIDAR)

Drop Summit to Bwlch:  78.3m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Drop Bwlch to ODN:  159.7m (LIDAR)

Dominance:  32.89% (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (June 2021)