Mapping Mountains – Summit Relocations – Y Trichant – The 300m Hills of Wales

Cefn Twrch (SN 899 317)

There has been a Summit Relocation to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR.

LIDAR image of Cefn Twrch (SN 899 317)

This was initiated by Joe Nuttall who produced a summit analysis programme that used LIDAR with an alternative height map (DEM) allowing identification of summits and cols and thereby drops.  The resulting spreadsheet that Joe produced contains over 29,600 hills.

This spreadsheet is being evaluated by DoBIH Editors and others, and for this particular hill it was Chris Crocker who initially assessed its height and that of its adjacent peak via LIDAR analysis.

Myrddyn Phillips then evaluated this hill’s details via LIDAR analysis and confirmed its height and drop and hence the confirmation of its summit relocation.

The criteria for the list this summit relocation affects are:

Y Trichant- The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13th May 2017.

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Cefn Twrch, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the south-eastern part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with minor roads to its east, north and west and the A40 road to its south, and has the hamlet of Pentre-bach towards the north-east.

When the original 300m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the accompanying main P30 list, and it was listed as a 378m map heighted twin summit with the prioritised summit positioned at SN 898 321 and its twin map heighted summit positioned at SN 904 307.   

When 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 32m of drop based on the twin 378m summit spot heights that appear on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 346m bwlch height based on interpolation of 10m contouring between 340m – 350m.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not shown on other publicly available Ordnance Survey maps and for this hill it showed a 342m bwlch spot height and when coupled with the 378m summit spot height it gave this hill 36m of drop. 

However, it was not until LIDAR became available that the details for these two summits 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 LIDAR result for each summit appears below:

Cefn Twrch (previous prioritised twin summit)    379.4m at SN 89923 31751

Cefn Twrch (previous non-prioritised twin summit)    377.8m at SN 90489 30697

The prioritised summit is confirmed the higher, although its position has moved from where the 378m spot height appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps, and its drop confirmed sufficient for it to be classified as a Trichant, with the drop of the old twin summit confirmed as 11.6m which is insufficient for it to be classified as a Sub-Trichant.

The summit height produced by LIDAR analysis is 379.4m positioned at SN 89923 31751 and this revised summit position comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the hill’s high point is in a different field, or where a number of potential summit positions are within close proximity and the highest point is not where previously given, or when it is positioned to a different feature such as in a conifer plantation, or when the high point of the hill is placed within a different map contour, or 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 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.

LIDAR summit image for Cefn Twrch

Therefore, the listed summit height of this hill is 379.4m and its new position is SN 89923 31751, this position is not given a spot height on contemporary Ordnance Survey maps and is approximately 440 metres southward from where the previously listed summit is positioned.

The full details for the hill are:

Group:  Mynydd Epynt

Name:  Cefn Twrch

OS 1:50,000 map:  160

Summit Height:  379.4m (LIDAR)

Summit Grid Reference (New Position):  SN 89923 31751 (LIDAR)

Bwlch Height:  347.2m (LIDAR)

Bwlch Grid Reference:  SN 89591 32567 (LIDAR)

Drop:  32.2m (LIDAR)

Myrddyn Phillips (September 2020)