Friday, 3 June 2016

Mapping Mountains – Significant Height Revisions – Y Pellennig and 30m-99m Twmpau

Emsger (SM 651 226)

This is the twenty ninth post under the heading of Significant Height Revisions, and the Trimble survey that resulted in this height revision was conducted on the 14th May 2016 in good, bright conditions, perched beside a lighthouse on one of the most remote places in the whole of Wales.

The twenty ninth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted on an island off the western Pembrokeshire coast that forms a part of the Bishops and Clerks chain of islands.  These islands form compact groups of islets; all are rocky and are aligned northward to southward with North Bishop, Carreg Rhoson, Daufraich and Emsger being the main named island in each small group. 

The hill is positioned on the island known as Emsger, which is also commonly known as South Bishop.  This island is positioned an approximate 8km (5 miles) west of St David’s Head in Pembrokeshire, and unless being a competent and enthusiastic seal, it can only be approached by boat.

The island is predominantly rock and if not for a series of steps leading up toward its summit its ascent would no doubt prove more problematic.  The upper part of Emsger has a lighthouse on it; this was built in 1839, and was converted to operating by electric in 1959, and demanned and automated in 1983.

The upper part of the island now has the lighthouse and associated buildings and platforms built on it, and therefore what once constituted the natural summit of the island is either buried under the lighthouse / platforms or it was destroyed during construction of the lighthouse.

I visited with Adrian Rayner, Ayako, Rob Woodall, Jon Glew, Sheila Glew, Sarah Kerr and Bob Kerr, and a quick assessment of what is left of natural ground pinpointed the highest point to be situated just to the south-west of the main southerly corner of the lighthouse and its grounds.  This position was adjacent to steps leading down to the island’s helipad.

The only height available of current Ordnance Survey maps is a 37m spot height with the spot positioned on the south-east part of the grounds of the lighthouse.  This is on a part of the construction and is man-made; however natural ground exists close to this point but is significantly lower when compared to where Trimble data was gathered from.

Therefore this island’s new summit height is 33.8m (converted to OSGM15) which is 3.2m lower than its previously listed height of 37m which appears as a spot height on current Ordnance Survey maps.

The full details for the hill are:

Cardinal Island:  Bishops and Clerks

Summit Height:  33.8m (converted to OSGM15)

Name:  Emsger

OS 1:50,000 map:  157

Summit Grid Reference:  SM 65116 22619

Drop:  33.8m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data 0.99m above the highest remaining natural ground at the summit of Emsger, otherwise known as South Bishop (SM 651 226) which resulted in this island's significant height revision

Myrddyn Phillips and Aled Williams (May 2016)

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