Twyn Disgwylfa (SS 995 959) - Pedwar reclassified to 400m Sub-Pedwar
There has been a change to the listing of the Y Pedwarau with a
Pedwar being reclassified to the ranks of 400m Sub-Pedwar by analysis of LIDAR data
by Aled Williams. The Y Pedwarau are all
hills in Wales at or above 400m and below 500m in height that have a minimum
drop of 30m.
The hill is situated in the group of hills known
as the Y Cymoedd with its Cardinal Hill being Craig y Llyn (SS 995 959) and is
placed in the Region of South Wales (C-2).
The hill is positioned between the small town of Ferndale in Cwm Rhondda
(Rhondda Valley) to its north north-east and the village of Ystrad in Cwm Rhondda
Fawr (Rhondda Fawr Valley) to its south-west, and it can be easily accessed from
the upper reaches of Penrhys to the south-east using a track until open
hillside takes you to the summit.
The name of the hill is Twyn Disgwylfa and it was
listed as a Pedwar based on the 429m summit spot height and the 399m spot
height on the area of the bwlch on the Ordnance Survey enlarged mapping hosted on the
Geograph website which also has an unusually spelt typo for the name of this
hill; Twyn Diogisylfa, these values gave the hill the minimum qualifying drop
value of 30m. Whereas the Ordnance
Survey 1:25,000 Explorer map has a 429m summit spot height and a 397m spot
height on the area of the bwlch.
As Aled was analysing other hills in the vicinity
against LIDAR data, he did likewise with Twyn Disgwylfa. LIDAR (Light Detection & Ranging) is highly
accurate height data that is now freely available for much of England and
Wales.
Aled’s analysis of LIDAR data gives the hill the
following details:
Twyn
Disgwylfa
Summit
Height: 428.1m
Summit
Grid Reference: SS 99572 95976
Bwlch
Height: 399.3m
Bwlch
Grid Reference: SS 99239 96220
Drop: 28.8m
Therefore, the 428.1m LIDAR data produced for the
summit position at SS 99572 95976 and
the 399.3m LIDAR data produced for the bwlch position at SS
99239 96220 is sufficient for this hill
to be reclassified to a 400m Sub-Pedwar with 28.8m of drop, and therefore Twyn
Disgwylfa is taken out of the listing of the Y Pedwarau and the list will be
updated accordingly.
The full details for the hill are:
Cardinal Hill: Craig y
Llyn
Summit Height: 428.1m
(LIDAR data)
Name: Twyn Disgwylfa
OS 1:50,000 map: 170
Summit Grid Reference: SS 99572 95976
Drop: 28.8m (LIDAR
data)
This now brings the overall total for the Y Pedwarau to 445 hills
with six additions, and eight reclassifications to 400m Sub-Pedwar status since
publication of the list by Europeaklist in May 2013. The hill will be taken out of the Pedwar list
and added to the 400m Sub-Pedwar list in forthcoming publications. The list of Pedwar hills is available from
the Haroldstreet website (January 2014) with all subsequent changes available
via the Mapping Mountains site.
The list of additions and reclassifications from the Pedwar
list since the 1st edition of the Y Pedwarau was published by
Europeaklist are as follows:
PEDWAR ADDITIONS
Pt. 499.5m (converted to OSGM15) (SH 665 310) 400m Sub-Pedwar reclassified to Pedwar with 30.3m drop
PEDWAR RECLASSIFICATIONS
Pt. 422.3m (converted to OSGM15) (SO 073 822) Pedwar reclassified to 400m Sub-Pedwar with 29.0m drop
Pt. 407.7m (converted to OSGM15) (SO 180 692) Pedwar reclassified to 400m Sub-Pedwar with 28.3m drop
Twyn Disgwylfa (LIDAR data) (SS 995 959) Pedwar reclassified
to 400m Sub-Pedwar with 28.8m drop
Myrddyn Phillips and Aled Williams (December 2016)
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