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

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

Please note; Carn Ysgubor was originally listed as a Lesser Dominant based on map detail, it was then reclassified to a Dominant based on a survey with the Trimble GeoXH 6000, since this survey 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 to the listing of Y Trechol – The Dominant Hills of Wales which has resulted in this hill being reinstated to The Lesser Dominant Hills of Wales list due to a survey with the Trimble GeoXH 6000 and the subsequent conversion of these data to OSGM15. 

The details relating to this hill’s reclassification and reinstatement as a Lesser Welsh Dominant are retrospective as the survey with the Trimble GeoXH 6000 took place on the 15^th May 2016 and the conversion of data to OSGM15 was instigated on Mapping Mountains shortly after 30^th August 2016.

The criteria for Dominant status are those Welsh P30s whose prominence equal or exceed 50% of their absolute height, whilst the criteria for Lesser Dominant status are those additional Welsh P30s whose prominence is 33% or more and below 50% of their absolute height.

The hill was originally listed as a Lesser Dominant with 49.50% Dominance based on the 101m summit spot height that appears on Ordnance Survey 1:25,000 Explorer maps and an estimated bwlch height of c 51m, with these values giving the hill c 50m of drop. 

The hill was subsequently reclassified to a Dominant with 50.00% Dominance based on a survey with the Trimble GeoXH 6000 resulting in a 102.4m (102.369m) summit height and a 51.2m (51.182m) bwlch height, with these values giving this hill 51.2m (51.187m) of drop and 50.00244% of Dominance.

The hill is listed in the Pembrokeshire Islands group and is placed in the Region of Mid and West Wales (Region B, Sub-Region B4) with its Cardinal Island being Ynys Dewi, the hill is positioned on the northern part of Ynys Dewi, this island is also commonly known as Ramsey, and is situated to the west of St David’s in the south-west of Wales.

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 has been reclassified and reinstated to The Lesser Dominant Hills of Wales list as the conversion using OSGM15 has resulted in an increase of 0.026m in both its summit and bwlch heights as surveyed using the Trimble GeoXH 6000, with the summit height amended from 102.369m to 102.395m and the bwlch height amended from 51.182m to 51.208m.  The increase in heights has also affected the Dominance of the hill from 50.00% to 49.99% and therefore Carn Ysgubor is reclassified from a Dominant to a Lesser Dominant hill.  

Carn Ysgubor is only the third hill that I know of whose status has been altered due to the Ordnance Survey remodelling known as OSGM15.  This remodelling has increased the accurately known height of hills in Wales by approximately 2-4cm, and the other two hills whose status has been altered through the conversion to OSGM15 are Calf Top (SD 664 856) and Pen y Gaer (SO 139 976).

The full details for the hill are:

Group:  Pembrokeshire Islands

Name:  Carn Ysgubor

Dominance:  49.99%

OS 1:50,000 map:  157

Summit Grid Reference:  SM 69953 24603

Summit Height:  102.4m (converted to OSGM15)

Drop Summit to Bwlch:  51.2m

Drop Bwlch to ODN:  51.2m (converted to OSGM15) 

Carn Ysgubor (SM 699 246) has now been reclassified and reinstated to the Lesser Dominant list due to a survey with the Trimble GeoXH 6000 and the conversion of data to OSGM15

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

Myrddyn Phillips (July 2017)