Tuesday, 20 December 2016

Mapping Mountains – Hill Reclassifications – Y Pedwarau



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










PEDWAR RECLASSIFICATIONS








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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