Mapping Mountains – Significant Height Revisions – 600m Twmpau, The Welsh Highlands – Uchafion Cymru and Y Pellennig – The Remotest Hills of Wales

Cefn Dylif (SJ 089 369) 

There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau, The Welsh Highlands – Uchafion Cymru and Y Pellennig – The Remotest Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams with assistance from Mark Trengove. 

The criteria for the three listings that this height revision applies to are: 

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. 

The 600m Twmpau by Myrddyn Phillips

The Welsh Highlands – Uchafion Cymru – This is the revised draft title for the Welsh 500m P15s list that takes in all hills in Wales at or above 500m in height with 15m minimum drop.  Accompanying the main list are three sub lists, these are; 500m Sub-Uchaf, 490m Sub-Uchaf and the Double Sub category.  The list is co-authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19^th June 2019.

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

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 name the hill is listed by is Cefn Dylif and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the A5 road to its north, the B4401 road to its west and the B4391 road to its south-west, and has the village of Llandrillo towards the west. 

When the original list of Welsh 500m P15s that later became known as Yr Uchafion and latterly as The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 36m of drop based on the 621m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 585m bwlch height based on interpolation of 10m contouring between 580m – 590m.  The drop value for this hill was later amended to an estimated c 38m, based on an estimated c 583m bwlch height based on interpolation of 5m contouring between 580m – 585m that appeared on the OS Maps website. 

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

The summit of this hill was now been surveyed with the Trimble GeoXH 6000, resulting in a height of 618.3m.  This is not a dramatic height revision when compared to some survey results, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated summit height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  

The Trimble GeoXH 6000 gathering data at the summit of Cefn Dylif

Therefore, this hill’s new listed summit height is 618.3m and this was derived from a Trimble GeoXH 6000 survey, this is 2.7m lower than the previously listed height of 621m which was based on the spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Y Berwyn

Name:  Cefn Dylif

OS 1:50,000 map:  125

Summit Height (New Height):  618.3m (converted to OSGM15)

Summit Grid Reference:  SJ 08943 36942

Bwlch Height:  581.3m (converted to OSGM15)

Bwlch Grid Reference:  SJ 09068 36590 

Drop:  37.0m

 

Myrddyn Phillips and Aled Williams (June 2021)