Clytiau Poethion (SH 763 717)
There has been a Summit Relocation to a hill that
is listed in the 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 10th
October 2018.
 |
| LIDAR image of Maes Glas Mawr and Clytiau Poethion |
The criteria for the listing that this summit relocation
applies to are:
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 bounded land
where the summit of this hill is situated is Clytiau Poethion, and this was
derived from the Tithe map and it is the name that this hill is now listed by,
and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of
North Wales (Region A, Sub-Region A1), and
the hill is encircled by minor roads with the B5106 and the Afon Conwy (River
Conwy) to its east, and has the town of Conwy to its
north.
When the original 100m height band of Welsh P30
hills were published on Geoff Crowder’s v-g.me website the adjacent northerly
hill known as Maes Glas Mawr (SH 76876 72414) was included in the main P30 list
as its summit was thought to be higher than that of Clytiau Poethion (SH 76337
71799), based on a 107m summit spot height that appears on the Ordnance Survey
1:50,000 Landranger map as opposed to an upper 100m contour ring given to
Clytiau Poethion.
 |
| Extract from the Ordnance Survey 1:50,000 Landranger map |
Therefore, when the Dominant list was first
compiled it was the hill now listed as Maes Glas Mawr that was included in the
Lesser Dominant list with 36.45% dominance.
However, it was not until LIDAR became available
that the details for each hill could be accurately re-assessed. The LIDAR (Light Detection & Ranging)
technique produced highly accurate height data that is now freely available for
much of England and Wales.
LIDAR analysis gives Clytiau Poethion as higher
than Maes Glas Mawr and this has subsequently been confirmed by a Trimble GeoXH
6000 survey, therefore the bylchau for each hill is swapped as are their
classifications, resulting in a Trimble GeoXH 6000 summit height for Clytiau
Poethion of 109.1m positioned at SH 76337 71799, as opposed to the 108.5m
summit height for Maes Glas Mawr positioned at SH 76876 72414. This confirms the qualifying hill’s summit
position compared to where previously listed, and it comes 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.
 |
| LIDAR bwlch image for Clytiau Poethion |
Therefore, the summit
height produced by the Trimble GeoXH 6000 survey is 109.1m and is positioned at
SH 76337 71799, this position is not given a spot height on contemporary Ordnance
Survey maps and is approximately 800 metres south-westward from where the
previously listed summit is positioned.
The full details for the hill are:
Group: Carneddau
Name: Clytiau Poethion
OS 1:50,000 map: 115
Summit Height: 109.1m (converted
to OSGM15)
Summit Grid Reference (new position): SH 76337 71799
Bwlch Height: 68.7m (LIDAR)
Bwlch Grid Reference:
SH 77496 72493 (LIDAR)
Drop: 40.35m (Trimble
summit and LIDAR bwlch)
Dominance: 37.00% (Trimble
summit and LIDAR bwlch)
Myrddyn Phillips (April 2019)
No comments:
Post a Comment