Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

 

Pen y Foel (SH 425 844) – Lesser Dominant reclassified to Dominant

There has been a reclassification to the listing of 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 a Trimble GeoXH 6000 summit survey and analysis of partial LIDAR bwlch coverage conducted by Myrddyn Phillips. 

Pen y Foel (SH 425 844)

The criteria for the list that this reclassification 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 Pen y Foel and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it has the B5111 road to its north-west and south, and has the village of Llannerch-y-medd towards the south-west. 

When 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 59m of drop and 48.36% dominance, based on an estimated c 122m summit height and an estimated c 63m bwlch height, with each based on interpolation of 5m contouring that appears on the Ordnance Survey 1:25,000 Explorer map. 

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 on other publicly available Ordnance Survey maps and gave a 123m summit spot height and a 62m spot height on the area of the bwlch for this hill, which increased its drop value to 61m and its dominance to 49.59%. 

The summit of this hill was subsequently surveyed with the Trimble GeoXH 6000 resulting in a 122.5m height positioned at SH 42575 84415.  This resulted in its dominance amended to 49.40%. 

The Trimble GeoXH 6000 gathering data at the summit of Pen y Foel

However, it was not until LIDAR became available that the details for this hill could again be re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

Extract from the Ordnance Survey 1:25,000 Explorer map showing the bwlch of Pen y Foel

LIDAR does not cover the actual bwlch position of this hill, but there is sufficient coverage of the area of this hill’s bwlch for a better interpolated height compared to the position and height of the 62m spot height previously used for the drop and dominance values of this hill. 

LIDAR bwlch image of Pen y Foel

LIDAR bwlch image of Pen y Foel

Therefore, the reclassification of this hill from Lesser Dominant status is due to a Trimble GeoXH 6000 summit survey and analysis of partial LIDAR bwlch coverage, resulting in a 122.5m summit height and an estimated c 61m bwlch height, with these values giving this hill an estimated c 62m of drop and 50.21% dominance, which is sufficient for it to be classified as a Dominant hill. 

 

The full details for the hill are:

Group:  Ynys Môn

Name:  Pen y Foel 

OS 1:50,000 map:  114, 115

Summit Grid Reference:  SH 42575 84415 (Trimble GeoXH 6000)

Summit Height:  122.5m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference:  SH 44053 82788 (interpolation)

Drop Summit to Bwlch:  c 62m (Trimble GeoXH 6000 summit and interpolation of partial LIDAR bwlch coverage)

Drop Bwlch to ODN:  c 61m (interpolation of partial LIDAR coverage)

Dominance:  50.21% (Trimble GeoXH 6000 summit and interpolation of partial LIDAR bwlch coverage)

 

Myrddyn Phillips (March 2021)