Twynau Gwynion (SO 067 107) – Double Sub-Uchaf
reclassified to 490m Sub-Uchaf
There has been a reclassification to the listing
of The Welsh Highlands – Uchafion Cymru,
with the summit height, bwlch height and their locations, the drop and status
of the hill confirmed by 2m DSM LIDAR analysis conducted by Aled Williams.
 |
| LIDAR image of Twynau Gwynion (SO 067 107) |
The criteria for the list that this reclassification applies
to are:
The Welsh Highlands –
Uchafion Cymru
– This is the revised draft title for the Welsh 500m P15s list that takes
in all hills in Wales at or above 500m in height with 15m minimum drop. Accompanying the main list are three sub
lists, these are; 500m Sub-Uchaf, 490m Sub-Uchaf and the Double Sub category,
with this hill being reclassified from Double Sub status to 490m Sub-Uchaf
status. The criteria for Double Sub
status are all Welsh hills at or above 490m in height and below 500m in height
that have 14m or more and below 15m of drop, with the criteria for 490m Sub-Uchaf
status being all Welsh hills at or above 490m in height and below 500m in
height that have 15m minimum drop. The list is co-authored by Myrddyn Phillips and Aled Williams with the Introductionto the list being published on Mapping Mountains in November 2015 and an update relating to the list appearing on Mapping Mountains on the 19th June 2019.
The name the hill is listed
by is Twynau Gwynion and it is adjoined to the Cymoedd Gwent group of hills, which are situated in the eastern part of South Wales (Region C, Sub-Region
C2), and is positioned with the A470 road to its west and the
A465 road to its south, and has the town of Merthyr Tudful towards the south.
When the two 490m sub categories were included as
a part of this list this hill was listed with 16m of drop based on a 498m
summit height that appears as a spot height on the Ordnance Survey 1:50,000
Landranger and 1:25,000 Explorer maps and a 482m bwlch height that appeared as
a spot height on the Ordnance Survey Vector Map Local hosted on the Geograph
website and which is entitled the Interactive Coverage Map.
 |
| 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 hill was subsequently reclassified from 490m
Sub-Uchaf status to Double Sub-Uchaf status based on DTM LIDAR analysis
conducted by Aled Williams resulting in a 496.2m summit height and a 481.3m
bwlch height, with these values giving this hill 14.9m of drop.
The details of this hill have now been re-assessed
using the 2m DSM LIDAR model initially by Aled Williams and subsequently by
Myrddyn Phillips, resulting in a 496.8m summit height and a 481.4m bwlch
height, with these values giving this hill 15.5m of drop, which is sufficient
for it to be re-instated to 490m Sub-Uchaf status.
The full details for the hill are:
Group: Cymoedd Gwent
Name: Twynau Gwynion
OS 1:50,000 map: 160
Summit Height: 496.8m
(LIDAR)
Summit Grid Reference:
SO 06765 10701 (LIDAR)
Bwlch Height: 481.4m (LIDAR)
Bwlch Grid Reference:
SO 06779 10861 (LIDAR)
Drop: 15.5m
(LIDAR)
For the additions, deletions and reclassifications to The Welsh Highlands – Uchafion Cymru
reported on Mapping Mountains please consult the following Change Registers:
Aled Williams and Myrddyn Phillips (February 2020)
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