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

Old Castle Down (SS 898 758) – 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 summit image of Old Castle Down (SS 898 758)

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 of the hill is Old Castle Down 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 B4265 road to its west, a minor road to its south and the Afon Alun to its east, and has the small community of Saint-y-brid (St Bride’s Major) towards the south 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 also included, this hill was listed with 34m of drop and 33.33% dominance, based on the 102m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and the 68m bwlch spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website for the summit

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website for the bwlch

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 bwlch image of Old Castle Down

The summit height produced by LIDAR analysis is 102.0m and is positioned at SS 89898 75814, and the bwlch height is 68.9m and is positioned at SS 90383 73586, with these values giving this hill 33.1m of drop and 32.44% dominance which is insufficient for its continued inclusion as a Lesser Dominant hill.

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Old Castle Down

OS 1:50,000 map:  170

Summit Grid Reference:  SS 89898 75814 (LIDAR)

Summit Height:  102.0m (LIDAR)

Bwlch Grid Reference:  SS 90383 73586 (LIDAR)

Drop Summit to Bwlch:  33.1m (LIDAR)

Drop Bwlch to ODN:  68.9m (LIDAR)

Dominance:  32.44% (LIDAR)

Myrddyn Phillips (September 2019)