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

 

Pen Lan Fawr (SN 626 222) – Lesser Dominant addition

There has been an addition 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 confirmed by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Pen Lan Fawr (SN 626 222)

The criteria for the list that this addition 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 Lan Fawr and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north, the Afon Tywi to its south and the A483 road to its east, and has the town of Llandeilo towards the north-east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used in the main P30 category.

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 29m of drop, based on the 95m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 66m bwlch height, based on interpolation of 10m contouring between 60m – 70m. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website 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 that was 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 65m bwlch spot height, and when coupled with the 95m summit spot height these values gave this hill 30m of drop, with this drop value not giving the hill sufficient dominance to be included as a Lesser Dominant hill.

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.

Therefore, the addition of this hill to Lesser Dominant status is due to LIDAR analysis, resulting in a 98.3m summit height and a 64.9m bwlch height, with these values giving this hill 33.4m of drop and 33.97% dominance, which is sufficient for it to be classified as a Lesser Dominant hill. 

 

The full details for the hill are:

Group:  Mynydd Mallaen

Name:  Pen Lan Fawr

OS 1:50,000 map:  159

Summit Grid Reference:  SN 62626 22229 (LIDAR)

Summit Height:  98.3m (LIDAR)

Bwlch Grid Reference:  SN 61761 22561 (LIDAR)

Drop Summit to Bwlch:  33.4m (LIDAR)

Drop Bwlch to ODN:  64.9m (LIDAR)

Dominance:  33.97% (LIDAR)

 

Myrddyn Phillips (April 2022)