Mapping Mountains – Summit Relocations – Y Trechol – The Dominant Hills of Wales

 

Cefn Carnau (ST 165 847) 

There has been a Summit Relocation to a hill that is listed in the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill initiated by Joe Nuttall who produced a summit analysis programme using LIDAR, and then by LIDAR analysis initially conducted by Jim Bloomer and subsequently by Myrddyn Phillips. 

LIDAR image of Cefn Carnau (ST 165 847)

The criteria for the list that this summit relocation 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 Cefn Carnau and it is adjoined to the Cymoedd Gwent group of hills, which are situated in the eastern part of South Wales (Region C, Sub-Region C2), and it is positioned with a minor road to its north, the A469 road to its west, the M4 motorway to its south and a minor road to its east, and has the town of Caerffili (Caerphilly) towards the north-west. 

When the original 200m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was listed with a 270m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at ST 16625 84801. 

When the 200m P30 list was published on Geoff Crowder’s v-g.me website the adjacent hill; Comin Caerffili was listed with a 271m summit height based on the spot height that appears adjoined to a triangulation pillar on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.  As Comin Caerffili was given a higher map height than Cefn Carnau it was the former that was listed as a Lesser Dominant hill with 109m of drop and 40.22% dominance, based on the 162m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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

However, it was not until LIDAR became available that the details for these two hills 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. 

The summit height produced by LIDAR analysis for these two hills is given below:

 

271.859m at ST 16511 84707 for Cefn Carnau 

270.760m at ST 15301 85529 for Comin Caerffili

 

With the caveat that the 271.859m summit height for Cefn Carnau is obtained from 2m DTM LIDAR, with 1m DTM LIDAR giving a summit height of 271.791m.  The latter is likely to be an anomaly as 1m LIDAR has a greater accuracy than 2m LIDAR and therefore usually produces a greater height.  Contouring on the 1m DTM LIDAR also suggests an anomaly. 

The heights given above come 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, within a different map contour, to a different feature such as in a conifer plantation, to a different point where a number of potential summit positions are within close proximity, 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, or when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct. 

Therefore, the listed summit height of this hill is 271.9m and its new position is ST 16511 84707, this is approximately 100 metres from the position of the 270m spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map, and is approximately 1450 metres south-eastward from where the previously listed summit is positioned. 

 

The full details for the hill are:

Group:  Cymoedd Gwent

Name:  Cefn Carnau 

OS 1:50,000 map:  171

Summit Height:  271.9m (LIDAR)                                                           

Summit Grid Reference (New Position):  ST 16511 84707 (LIDAR) 

Bwlch Height:  163.8m (LIDAR) 

Bwlch Grid Reference:  ST 14268 85037 (LIDAR) 

Drop:  108.0m (LIDAR) 

Dominance:  39.73% (LIDAR)

 

Myrddyn Phillips (November 2021)