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

Gewni (SM 796 236) – 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.

Gewni (SM 796 236)

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 35m of drop based on the 35m summit spot height that appears on this map.

The name of the hill is Gewni and it is 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 to the west of where the Afon Solfach (River Solva) enters the sea, and has the village of Solfach (Solva) towards its north-east.

If wanting to visit the hill it is 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 Gewni 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 1m DTM LIDAR analysis gives the hill the following details:

Name:  Gewni

Summit Height:  35.7m

Summit Grid Reference:  SM 79696 23606

Bwlch Height:  0.3m

Bwlch Grid Reference:  SM 79689 23698

Drop:  35.3m

Dominance:  99.11%

LIDAR image of Gewni

Therefore, the 35.65m LIDAR analysis for the summit position at SM 79696 23606 and the 0.3m LIDAR analysis for the bwlch position at SM 79689 23698 gives this hill 35.3m of drop and 99.11% dominance, which is sufficient for Dominant status.

The full details for the hill are:

Group:  Carn Llidi

Name:  Gewni

Dominance:  99.11% (LIDAR)

OS 1:50,000 map:  157

Summit Grid Reference:  SM 79696 23606 (LIDAR)

Summit Height:  35.7m (LIDAR)

Bwlch Grid Reference:  SM 79689 23698 (LIDAR)

Drop Summit to Bwlch:  35.3m (LIDAR)

Drop Bwlch to ODN:  0.3m (LIDAR)

Myrddyn Phillips (August 2018)