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

Carn Ysgubor (SM 699 246) - Lesser Dominant reclassified to Dominant 

Please note; since this Hill Reclassifications post was announced the Ordnance Survey have adopted OSGM15, this remodelling has altered the known accurate height of Welsh hills by approximately 2-4cm, and the conversion to OSGM15 of data produced by the Trimble for Carn Ysgubor has resulted in this hill being reclassified back to the ranks of Lesser Dominant hills.


There has been a reclassification from the listing of Y Trechol – The Dominant Hills of Wales due to a survey with the Trimble GeoXH 6000 and which has resulted in the hill being taken out of The Lesser Welsh Dominant list and included in the Dominant list.  The hill is in the Group known as the Pembrokeshire Islands and is situated on the northern part of Ynys Dewi, which is also commonly known as Ramsey.  This island is situated to the west of St David’s in the south-west of Wales.

The Trimble GeoXH 6000 survey that resulted in this hill’s reclassification was conducted on the 15^th May 2016 on a beautiful morning in still and sunny conditions.

As the hill is situated on an island the only way to approach is via boat and the only company with permission to land on Ynys Dewi is Thousand Island Adventures (tel: 01437-721721).  The hill can be easily accessed from where the boat docks as a gravelled track leads up to The Farmhouse from where a vehicle path heads toward The Bungalow, which is a house on the eastern slopes of the hill.  A narrower path leaves this main route and heads toward the summit of the hill.

The name of the hill is Carn Ysgubor, with its composition presented as Carnysgubor on current Ordnance Survey maps, and it had been listed with 49.50% Dominance based on the 101m summit spot height on Ordnance Survey maps and an estimated bwlch height of c 51m, these values gave this hill c 50m of drop. 

The survey with the Trimble GeoXH 6000 resulted in a 102.4m (102.369m) summit height and a 51.2m (51.182m) bwlch height, giving this hill a Dominance of 50.00% (50.002442%) and therefore it just sneaks in to the ranks of being classified as a Dominant hill and confirms its reclassification from a Lesser Welsh Dominant to the ranks of Y Trechol - The Dominant Hills of Wales.

The full details for the hill are:

Group:  Pembrokeshire Islands

Name:  Carn Ysgubor

Dominance:  50.00%

OS 1:50,000 map:  157

Summit Grid Reference:  SM 69953 24603

Summit Height:  102.4m

Drop Summit to Bwlch:  51.2m

Drop Bwlch to ODN:  51.2m 

Carn Ysgubor (SM 699 246) now reclassified from a Lesser Welsh Dominant to a Dominant hill

For details on the survey that resulted in this hill being reclassified from a Lesser Welsh Dominant to a Dominant hill

Myrddyn Phillips (May 2016)