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

Ynys Deullyn (SM 844 340) – 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.

This hill was included as a P30 and a Dominant hill shortly after the Ordnance Survey Interactive Coverage Map became available on the Geograph website, and prior to LIDAR analysis it was listed with 30m of drop based on the 30m summit spot height that appears on this map.

2m DTM LIDAR image of Ynys Deullyn (SM 844 340)

The name of the hill is Ynys Deullyn 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 Carn Llidi 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 northern part of the Pembrokeshire coast, and has the small community of Abercastell towards its west south-west.

If wanting to visit the island 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 some form of scramble.

The confirmation of the addition of Ynys Deullyn 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:

Name:  Ynys Deullyn

Summit Height:  32.6m

Summit Grid Reference:  SM 84479 34095

Bwlch Height:  2.1m

Bwlch Grid Reference:  SM 84513 34087

Drop:  30.5m

Dominance:  93.51%

2m DSM LIDAR image of Ynys Deullyn (SM 844 340)

Therefore, the 32.6m LIDAR analysis for the summit position at SM 84479 34095 and the 2.1m LIDAR analysis for the bwlch position at SM 84513 34087 gives this hill 30.5m of drop and 93.51% dominance, which is sufficient for Dominant status.

The full details for the hill are:

Group:  Carn Llidi

Name:  Ynys Deullyn

Dominance:  93.51% (LIDAR)

OS 1:50,000 map:  157

Summit Grid Reference:  SM 84479 34095 (LIDAR)

Summit Height:  32.6m (LIDAR)

Bwlch Grid Reference:  SM 84513 34087 (LIDAR)

Drop Summit to Bwlch:  30.5m (LIDAR)

Drop Bwlch to ODN:  2.1m (LIDAR)

Myrddyn Phillips (September 2018)