Wednesday, 8 June 2016

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

Ynys Fach (SM 822 327) - Dominant deletion

There has been a deletion 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 main Dominant list and subjected to a life of rather pitiful non dominance.  The hill is in the group known as the Pembrokeshire Islands and is situated on the northern coast of Pembrokeshire, and is positioned between the small communities of Trefin (Trevine) to its east and Porthgain to its west.

The hill is better described as an island, and even more so as a tidal island, as it is connected to the coastline of mainland Wales by a pebbled beach that according to maps is cut off from the mainland at high water.  It can be approached by boat, but an ascent of the island is feasible, albeit with a steep and potentially dangerous descent to the bwlch from the mainland.  This approach starts from directly opposite the island and necessitates finding a route down through scrub undergrowth and bi-passing the rock and cliffs that predominate this part of the coastline.  If attempting this route please be careful.

The Trimble GeoXH 6000 survey that resulted in this island’s deletion was conducted on the 14th May 2016 on a beautiful sunny evening in the company of Adrian Rayner, Ayako and Rob Woodall.  It was Rob’s suggestion to go and have a look at it, and he found the route down to the pebbled beach which constitutes this island’s connecting bwlch.

The name of the island is Ynys Fach and it had been listed with 96.77% Dominance based on a summit height of c 31m estimated from the island having an uppermost 30m ring contour on current Ordnance Survey maps, and an estimated bwlch height of 1m taken to the pebbled beach connecting this island to the mainland.

Two points were surveyed for summit position and these are given below:

Summit 1:  30.8m (30.789m, converted to OSGM15) at SM 82209 32743

Summit 2:  30.7m (30.727m, converted to OSGM15) at SM 82217 32752

The data set for the bwlch was remarkably good when considering that the equipment was placed in what resembled an enclosed and walled canyon, with the resulting height being 1.2m (1.225m, converted to OSGM15) at SM 82180 32693.  These values give Ynys Fach a drop of only 29.6m which is below the minimum of 30m drop required for qualification to the Y Trechol – the Dominant Hills of Wales list.

The full details for the hill are:

Group:  Pembrokeshire Islands

Name:  Ynys Fach

Dominance:  N/A

OS 1:50,000 map:  157

Summit Grid Reference:  SM 82209 32743

Summit Height:  30.8m (converted to OSGM15)

Drop Summit to Bwlch:  29.6m

Drop Bwlch to ODN:  1.2m (converted to OSGM15)  

Ynys Fach (SM 822 327) now deleted from the ranks of Y Trechol - The Dominant Hills of Wales

Myrddyn Phillips (May 2016)

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