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

Ynys Ddu (SM 886 388) – Dominant addition

There has been confirmation of an addition to the listing of Y Trechol – The Dominant Hills of Wales due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

Y Trechol – The Dominant Hills of Wales - Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

This hill did not appear in the main P30 list or the accompanying Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website, as it was thought not to have 30m of drop and therefore was not considered for Dominant status.

Ynys Ddu (SM 886 388).  Photo: Jon Glew

It was Mick Moore who brought the attention of this hill’s P30 status to the hill bagging community and prior to LIDAR analysis the hill was listed with an estimated summit height of c 32m based on a small 30m uppermost contour ring on Ordnance Survey 1:25,000 Explorer maps.

The name of the hill is Ynys Ddu and as its name implies it is an island, or more strictly speaking a tidal island that is attached to mainland Wales at low tide.  The hill is adjoined to the Mynydd Preseli group of hills, which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), with the island being positioned on the western part of Pen-caer (Stumble Head), and has the town of Abergwaun (Fishguard) towards the east south-east.

The summit of Ynys Ddu.  Photo: Jon Glew

If wanting to visit the hill permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so caution is advised as the easiest approach will probably necessitate a scramble.

The confirmation of the addition of Ynys Ddu to Dominant status is due to LIDAR analysis conducted by Myrddyn Phillips.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

The 2m DSM LIDAR analysis gives the hill the following details:

Ynys Ddu

Summit Height:  31.9m

Summit Grid Reference:  SM 88673 38869

Bwlch Height:  0.5m

Bwlch Grid Reference:  SM 88721 38853

Drop:  31.3m

Dominance:  98.28% 

LIDAR image of Ynys Ddu

Therefore, the 31.9m LIDAR analysis for the summit position at SM 88673 38869 and the 0.5m LIDAR analysis for the bwlch position at SM 88721 38853 gives this hill 31.3m of drop and 98.28% Dominance, which is sufficient for Dominant status.

The full details for the hill are:

Group:  Mynydd Preseli

Name:  Ynys Ddu

Dominance:  98.28%

OS 1:50,000 map:  157

Summit Grid Reference:  SM 88673 38869 (LIDAR)

Summit Height:  31.9m (LIDAR)

Drop Summit to Bwlch:  31.3m (LIDAR)

Drop Bwlch to ODN:  0.5m (LIDAR)

My thanks to Jon Glew for permission to reproduce his photos of Ynys Ddu

Myrddyn Phillips (July 2018)