Mapping Mountains – Significant Height Revisions – 100m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Cefn (SH 721 004) 

There has been a Significant Height Revision 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 Cefn (SH 721 004)

The criteria for the two lists that this height revision 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. 

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 listed by is Cefn and this was derived from the Tithe map, and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with the A493 road to its north and the A487 road to its south, and has the town of Machynlleth towards the east.

After the original 100m height band of Welsh P30 hills published on Geoff Crowder’s v-g.me website were standardised and interpolated heights and drop values also included, this hill was listed with an estimated c 114m summit height, based on interpolation of its uppermost 110m ring contour, and the 59m bwlch spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, with these values giving this hill an estimated c 55m of drop and 48.25% dominance. 

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

The details for this hill were reassessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Intearctive 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 119m summit spot height and when coupled with its 59m bwlch spot height, these values gave this hill 60m of drop and 50.42% dominance.

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 Cefn (SH 721 004)

The height produced by LIDAR analysis to the summit of this hill is 118.2m positioned at SH 72126 00402, and this in relation to the original interpolated summit height comes 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 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 118.2m and this was derived from LIDAR analysis, this is 4.2m higher than the original estimated summit height which was based on interpolation of the uppermsot 110m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Tarren y Gesail 

Name:  Cefn 

OS 1:50,000 map:  135

Summit Height (New Height):  118.2m (LIDAR) 

Summit Grid Reference:  SH 72126 00402 (LIDAR) 

Bwlch Height:  59.0m (LIDAR) 

Bwlch Grid Reference:  SH 72569 00795 (LIDAR) 

Drop:  59.2m (LIDAR) 

Dominance:  50.08% (LIDAR) 

 

Myrddyn Phillips (October 2024)