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

Deg Erw Llandafin (ST 003 731) – Lesser Dominant deletion

There has been a deletion 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 Deg Erw Llandafin (ST 003 731)

The criteria for the list that this deletion 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.

The name the hill is now listed by is Deg Erw Llandafin and this was derived from the Tithe map, and it is adjoined to the Bro Morgannwg group of hills, which are situated in the southern part of South Wales (Region C, Sub-Region C2), and it is positioned with the A4222 and A48 roads to its north with minor roads to its west, south and east, and has the town of Y Bont-faen (Cowbridge) to the north-west.

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 Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used in the main P30 list. 

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 a 67m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map which is positioned outside of the uppermost 65m ring contour and an estimated bwlch height of c 37m based on interpolation of 5m contouring between 35m – 40m, with these values giving this hill an estimated c 30m of drop and 44.78% dominance.

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 has additional spot heights not available on any other form of Ordnance Survey publicly available map and its larger dimension enabled easier study.  It was at this stage that the 67m spot height was considered erroneous and the drop of the hill was listed as an estimated c 29m.

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. 

The summit height produced by LIDAR analysis is 66.4m and is positioned at ST 00313 73177 and ST 00315 73179, and the bwlch height is 37.2m and is positioned at ST 00721 73222, with these values giving this hill 29.2m of drop which is insufficient for it to be considered for Lesser Dominant status.

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Deg Erw Llandafin

OS 1:50,000 map:  170

Summit Grid Reference:  ST 00313 73177 & ST 00315 73179 (LIDAR)

Summit Height:  66.4m (LIDAR)

Bwlch Grid Reference:  ST 00721 73222 (LIDAR)

Drop Summit to Bwlch:  29.2m (LIDAR)

Drop Bwlch to ODN:  37.2m (LIDAR)

Dominance:  N/A, less than 30m of drop

Myrddyn Phillips (October 2019)