Bwlch y Fen & Tyddyn
Gwyn (SH 417 783)
There has been a Summit Relocation to a hill that
is listed in the 30-99m Twmpau, with
the summit height, bwlch height and their locations, the drop and status of the
hill derived from detail on contemporary maps produced from Ordnance Survey
data and LIDAR analysis conducted by Myrddyn Phillips.
 |
| LIDAR image of Bwlch y Fen & Tyddyn Gwyn (SH 417 783) |
The criteria for the list 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.
 |
| The 30-99m Twmpau by Myrddyn Phillips |
The name the hill is listed by is
Bwlch y Fen & Tyddyn Gwyn and this was derived from the Tithe map, 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 is positioned with minor roads to
its north, west and east, and the B5109 road to its south, and has the village
of Gwalchmai towards the south-west.
When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not
included in the Hills to be surveyed
sub list, as it was considered not to meet the criteria then used for this sub
category.
After the sub list was standardised, and
interpolated heights and drop values also included the details for this hill
were re-evaluated and it was listed with an estimated c 21m of drop, based on
the 89m summit spot height positioned at SH 41706 78330 that appears on the
Ordnance Survey 1:25,000 Explorer map and an estimated c 68m bwlch height based
on interpolation of 5m contouring between 65m – 70m.
 |
| Extract from the Ordnance Survey 1:25,000 Explorer map |
However, it was not until LIDAR became available
that the details for this 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.
The height produced by LIDAR analysis for the
remaining natural summit is 88.150m and is positioned at SH 41738 78329, with LIDAR
analysis giving a raised field boundary positioned at SH 41714 78319 a height
of 88.169m, and this comes within the parameters of the Summit Relocations used
within this page heading, these parameters are:
The term Summit Relocations applies when the
hill’s high point is found to be positioned; in a different field, within a
different map contour, to a different feature such as in a conifer plantation, to
a different point where a number of potential summit positions are within close
proximity, 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 natural ground or the natural and
intact summit is confirmed compared to a higher point such as a raised field
boundary that is judged to be a relatively recent man-made construct.
 |
| LIDAR summit image of Bwlch y Fen & Tyddyn Gwyn |
Therefore, the summit height produced by LIDAR
analysis is 88.15m and this is positioned at SH 41738 78329, this position is relatively
close to where the 89m spot height appears on the contemporary Ordnance Survey
1:25,000 Explorer map and importantly to the remaining natural summit of this
hill as opposed to the slightly higher raised field boundary.
The full details for the
hill are:
Group: Ynys
Môn
Name: Bwlch y Fen & Tyddyn Gwyn
OS 1:50,000 map: 114,
115
Summit Height: 88.15m (LIDAR)
Summit Grid Reference
(New Position): SH 41738 78329 (LIDAR)
Bwlch Height: c 68m (interpolation)
Bwlch Grid
Reference: SH 41140 80401
(interpolation)
Drop: c 20m (LIDAR summit and interpolated bwlch)
Myrddyn Phillips
(September 2021)
