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

Carreg Rhoson (SM 672 256) – Dominant deletion

There has been a deletion from the list of the Y Trechol – The Dominant Hills of Wales, with the summit height and its location, the drop, dominance and status of the hill initially derived from detail on the DataMapWales and its status subsequently confirmed from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 672 256) is positioned on the right of this photograph

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, and the list is now available inits entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms a part of the islet chain known as Carreg Rhoson, and it is positioned to the west of the Pembrokeshire coast, with the small city of Tyddewi (St David’s) towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the main P30 list as the small uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map had been missed. 

Extract from the Ordnance Survey 1:25,000 Explorer map

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m summit height, based on interpolation of its uppermost 30m ring contour.  As this hill is the high point of an island it was also listed with an estimated c 30m drop, which was sufficient for it to be classified as a Dominant hill with 100.00% dominance.

However, it was not until the DataMapWales became available online that the details of this hill could be checked against a product viewed as being more accurate than the contouring on the contemporary Ordnance Survey 1:25,000 map.  This mapping has a 28m spot height for this hill and this is similar to the 27.42m summit height produced by the surface analysis programme developed by Joe Nuttall (JNSA) that uses the OS Terrain product where LIDAR coverage is not available. 

Extract from the DataMapWales

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 image of Carreg Rhoson (SM 672 256)

Therefore, the confirmation of the deletion of this hill from Dominant status is due to LIDAR analysis, resulting in a 29.8m summit height, and as this summit is the high point of an island the drop value is also 29.8m, which is insufficient for Dominant status. 

 

The full details for the hill are:

Group:  Garn Fawr

Name:  Carreg Rhoson

OS 1:50,000 map:  157

Summit Height:  29.8m (LIDAR)

Summit Grid Reference:  SM 67227 25662 (LIDAR)

Bwlch Height:  N/A (island)

Bwlch Grid Reference:  N/A (island)

Drop:  29.8m (LIDAR)

 

Myrddyn Phillips (June 2024)