Mapping Mountains – Summit Relocations – Y Pellennig – The Remotest Hills of Wales, The Welsh Highlands – Uchafion Cymru and 600m Twmpau

Carnedd Llechwedd Llyfn (SH 857 444) 

There has been a Summit Relocation to a hill that is listed in the Y Pellennig – The Remotest Hills of Wales, The Welsh Highlands – Uchafion Cymru and the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Carnedd Llechwedd Llyfn (SH 857 444)

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

Y Pellennig –The Remotest Hills of Wales - Welsh hills whose summit is at least 2.5km from the nearest paved public road and the hill has a minimum 15m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available as a downloadable e-booklet or print-booklet version on Mapping Mountains Publications with the up-to-date master list available on Mapping Mountains to download in Google Doc format. 

Y Pellennig - The Remotest Hills of Wales by Myrddyn Phillips and Aled Williams

The Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

The Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

600m Twmpau - Welsh hills at or above 600m and below 700m in height with 30m minimum drop, with an accompanying sub list entitled the 600m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 600m and below 700m 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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains. 

600m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carnedd Llechwedd Llyfn and it is adjoined to the Carnedd y Filiast group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A1), and it is positioned with minor roads to its north-west and north-east, the A5 road farther to its north, the B4407 road farther to its north-west and the A4212 road to its south, and has the town of Y Bala towards the south-east.

When the original list that later became known as the The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 25m of drop, based on the 643m summit spot height positioned at SH 85778 44459 that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and an estimated c 618m bwlch height, based on interpolation of 10m contouring between 600m – 610m. 

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

Since the original compilation of these lists there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a twin 643m summit spot height positioned at SH 85789 44665 (latterly shown on the DataMapWales) and a 618m bwlch spot height, with these values giving this hill 25m of drop and resulting in the hill being listed as a twin top. 

Extract from the DataMapWales

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 Carnedd Llechwedd Llyfn (SH 857 444)

The height produced by LIDAR analysis to the summit of this hill is 644.1m and is positioned at SH 85783 44460, whilst LIDAR gives the highest ground adjacent to where the 643m spot height appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map as 643.6m positioned at SH 85785 44677 and SH 85782 44676 and SH 85785 44674 and SH 85779 44667, 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 listing of a new twin summit or 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.

Therefore, the new listed summit height of this hill is 644.1m and this is positioned at SH 85783 44460 and this was derived from LIDAR analysis, this position approximately matches that of the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and has resulted in the de-twinning of this hill.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Carnedd y Filiast

Name:  Carnedd Llechwedd Llyfn

OS 1:50,000 map:  124, 125

Summit Height:  644.1m (LIDAR) 

Summit Grid Reference (New Position):  SH 85783 44460 (LIDAR) 

Bwlch Height:  618.7m (LIDAR) 

Bwlch Grid Reference:  SH 86005 44925 (LIDAR) 

Drop:  25.4m (LIDAR) 

Remoteness:  2.860km

 

Myrddyn Phillips and Aled Williams (September 2026)