Cefn Gwrhyd (SN 737 096)
There has been a Significant Height Revision to a hill that is listed in The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips.
LIDAR image of Cefn Gwrhyd (SN 737 096) |
The criteria for the list that this height revision applies
to are:
The Welsh
P15s – Welsh hills with 15m
minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s,
with the criteria for this sub category being all Welsh hills with 14m or more
and below 15m of drop. The list is authored by Myrddyn Phillips, with the
Introduction to the list appearing on Mapping Mountains on the 10th
May 2019.
The Welsh P15s by Myrddyn Phillips |
The name the hill is listed by is Cefn Gwrhyd, and
it is adjoined
to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B,
Sub-Region B5), and it is positioned with the
A4068 road to its north-east, the A474 road to its west, the A4067 road to its
south-east and a minor road to its immediate east, and has the community of Y
Gurnos towards the east.
When the listing that became known as The Welsh P15s was being compiled, this
hill was not included as with a 291m spot height in the vicinity of its summit
and bwlch contouring between 280m – 290m that appear on the contemporary
Ordnance Survey 1:25,000 Explorer map, it was judged not to meet the criterion
for the main P15 or the accompanying P14 sub list.
Extract from the Ordnance Survey 1:25,000 Explorer map |
One of the mapping resources now available online
is the WalkLakes website which hosts an interactive map originated from the
Ordnance Survey Open Data programme.
This map has many spot heights not on other publicly available maps and
for this hill a 300m summit spot height is given.
Extract from the interactive mapping hosted on the WalkLakes website |
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.
LIDAR summit image of Cefn Gwrhyd (SN 737 096) |
The summit height produced by LIDAR analysis is 300.5m
and when compared to detail on the Ordnance Survey 1:50,000 Landranger and
1:25,000 Explorer map, this comes within the parameters of the Significant
Height Revisions used within this page heading, these parameters are:
The term Significant Height Revisions applies to
any listed hill whose interpolated height and Ordnance Survey or Harvey map
summit spot height has a 2m or more discrepancy when compared to the survey
result produced by the Trimble GeoXH 6000 or analysis of data produced via
LIDAR, also included are hills whose summit map data is missing an uppermost
ring contour when compared to the data produced by the Trimble or by LIDAR
analysis.
Therefore, the new listed summit height of this
hill is 300.5m and this was derived from LIDAR analysis. This is 9.5m higher than the 291m spot height
and 5.5m higher than the estimated c 295m summit height if based on
interpolation of the uppermost contour on the contemporary Ordnance Survey 1:25,000
Explorer map.
The full details for the hill are:
Group: Mynydd Du
Name: Cefn Gwrhyd
OS 1:50,000 map: 160
Summit Height (New Height): 300.5m (LIDAR)
Summit Grid Reference: SN 73765 09682 (LIDAR)
Bwlch Height: 283.1m (LIDAR)
Bwlch Grid Reference: SN 73543 09172 (LIDAR)
Drop: 17.3m (LIDAR)
Myrddyn Phillips (June
2024)
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