Mapping Mountains – Summit Relocations – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Plas Gwyn (SH 524 817)

There has been a Summit Relocation to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, its location, the drop and status of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, with the latter taking place on the 22^nd October 2018.

Plas Gwyn (SH 524 817)

The criteria for the two listings that this summit relocation applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

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, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015.

The name of the hill is Plas Gwyn and this was derived from local enquiry, and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it has the A5025 road to its west and the coast to its north and east, and overlooks Traeth-coch (Red Wharf Bay) to its south-east, and has the village of Benllech towards its north-west.

This hill was first listed in the original Welsh 30-99m P30 list published on Geoff Crowder’s v-g.me website with a 78m summit height positioned at SH 526 817 and which is based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website with the cursor indicating the position of the newly relocated summit

Prior to the Trimble GeoXH 6000 summit survey the details for this hill were analysed with LIDAR.  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 the summit of Plas Gwyn

The height produced by LIDAR analysis and the Trimble GeoXH 6000 summit survey is 76.1m which is positioned at SH 52494 81743, this is not a dramatic difference in position compared to some relocations, but it does come within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these relocations.

The Trimble GeoXH 6000 gathering data at the new summit position of Plas Gwyn

The Trimble GeoXH 6000 gathering data at the old summit position of Plas Gwyn

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 76.1m and is positioned at SH 52494 81743, this position is not given a spot height on contemporary Ordnance Survey maps and is approximately 110 metres west from where the 78m spot height appears on the Ordnance Survey 1:25,000 Explorer map and which was the position of the previously listed summit and surveyed as being 75.7m in height.

The full details for the hill are:

Group:  Ynys Môn

Name:  Plas Gwyn

OS 1:50,000 map:  114, 115

Summit Height:  76.1m (converted to OSGM15)

Summit Grid Reference (new position):  SH 52494 81743

Bwlch Height:  41.9m (LIDAR)

Bwlch Grid Reference:  SH 52209 81404 (LIDAR)

 

Drop:  34.2m (Trimble summit and LIDAR bwlch)

Dominance:  44.91% (Trimble summit and LIDAR bwlch)

For details on the survey that confirmed the summit relocation of this hill

Myrddyn Phillips (May 2019)