Mapping Mountains – Summit Relocations – 100m Twmpau and Y Trechol – The Dominant Hills of Wales

Penlan (SN 595 863) 

There has been a Summit Relocation to a hill that is listed in the 100m 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 Penlan (SN 595 863)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 100m Twnpau 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 listed by is Penlan and this was derived from the Tithe map, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the coast to its west, a minor road to its south and the B4572 road to its east, and has the village of Llangorwen towards the south. 

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a 137m summit height based on the spot height positioned at SN 59517 86313 that appears on the Ordnance Survey 1:50,000 Landranger and 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 gives the highest ground on this hill as 137.7m positioned at SN 59518 86326 and SN 59526 86319.  However, this is to the top of a raised field boundary and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

The height produced by LIDAR analysis to the natural summit of this hill is 137.3m and is positioned at SN 59520 86313 and SN 59520 86317, 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 either on Ordnance Survey maps or interactive mapping, 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 the de-twinning of a summit, 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.

LIDAR summit image of Penlan (SN 595 863)

Therefore, the summit height produced by LIDAR analysis is 137.3m and this is positioned at SN 59520 86313 and SN 59520 86317, and this is approximately 13 metres south south-eastward from where the highest point of the raised field boundary is positioned. 

 

The full details for the hill are:

Group:  Banc Llechwedd Mawr

Name:  Penlan

OS 1:50,000 map:  135

Summit Height:  137.3m (LIDAR) 

Summit Grid Reference (New Position):  SN 59520 86313 & SN 59520 86317 (LIDAR) 

Bwlch Height:  83.7m (LIDAR) 

Bwlch Grid Reference:  SN 59658 86698 (LIDAR) 

Drop:  53.6m (LIDAR) 

Dominance:  39.02% (LIDAR) 

 

Myrddyn Phillips (June 2021)