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

Carn Gelli (SM 982 375) 

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 Carn Gelli (SM 982 375)

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 that have 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 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 Carn Gelli, and it is adjoined to the Mynydd Preseli group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its west and the A487 road to its south, and has the town of Abergwaun (Fishguard) towards the west.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed under the name of Carn Bica (latterly listed as Cnwc y Meirch) with a summit height of 143m, based on a spot height positioned at SM 98226 37698 and which appears on the Ordnance Survey 1:25,000 Explorer map.  With a separate summit named Carn Gelli positioned to the south and which has an uppermost 140m contour but no spot height on contemporary Ordnance Survey maps. 

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 summit image of Carn Gelli (SM 982 375) with Cnwc y Meirch (SM 982 376) to the north

The summit height produced by LIDAR analysis for Carn Gelli is 143.6m positioned at SM 98233 37531, whilst the height produced by LIDAR analysis for Cnwc y Meirch is 143.3m positioned at SM 98231 37693, 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 judged to be 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 143.6m and this is positioned at SM 98233 37531, 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 positioned to a different rock tor approximately 160 metres southward from where the originally listed summit is positioned. 

 

The full details for the hill are: 

Group:  Mynydd Preseli 

Name:  Carn Gelli 

OS 1:50,000 map:  157

Summit Height:  143.6m (LIDAR) 

Summit Grid Reference (New Position):  SM 98233 37531 (LIDAR) 

Bwlch Height:  87.7m (LIDAR) 

Bwlch Grid Reference:  SM 99189 36904 (LIDAR) 

Drop:  55.8m (LIDAR) 

Dominance:  38.90% (LIDAR) 

 

Myrddyn Phillips (March 2023)