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 22nd 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 3rd 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)
Myrddyn Phillips (May 2019)
No comments:
Post a Comment