Tuesday, 10 January 2017

Mapping Mountains – Hill Reclassifications – Yr Uchafion


Twynau Gwynion (SO 067 106) - 490m Sub-Uchaf reclassified to Double Sub-Uchaf

There has been a reclassification to the listing of Yr Uchafion instigated from analysis of LIDAR data by Aled Williams.  Yr Uchafion is the draft title for a list of the Welsh 500m P15s that takes in all hills in Wales at or above 500m that have a minimum drop of 15m, the list is a joint compilation with Aled Williams.  Details relating to this list were published on the Mapping Mountains site in November 2015.  The status change affects the 490m Sub-Uchaf list with a hill being reclassified to a Double Sub-Uchaf, these are the hills at or above 490m and below 500m in height that have 14m or more and below 15m of drop.

The hill is situated in the Bannau Brycheiniog (Brecon Beacons) group of hills with its Cardinal Hill being Pen y Fan (SO 012 215) and is placed in the Region of South Wales (Region C, Sub-Region C2).  The hill is situated between the small community of Pontsticill to its north-west and the town of Merthyr Tudful to its south.

Current Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer mapping has a 498m summit spot height for this hill, with a previously listed drop value of 16m based on the 482m bwlch spot height that appears on the Ordnance Survey enlarged mapping hosted on the Geograph website.

The hill can be accessed from a public footpath that approaches it from Pontsticill to the north-west or from a track to the south that leads to the disused Twynau Gwynion Quarries which are situated just below the summit of the hill to its west.

The name of the hill is Twynau Gwynion and it is the first 490m Sub-Uchaf to be reclassified to a Double Sub-Uchaf, and its inclusion in the latter category is due to the analysis of LIDAR data by Aled.  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:


Twynau Gwynion

Summit Height:  496.2m

Summit Grid Reference:  SO 06765 10699

Bwlch Height:  481.3m

Bwlch Grid Reference:  SO 06781 10855

Drop:  14.9m


Therefore, the 496.2m LIDAR data produced for the summit position at SO 06765 10699 and the 481.3m LIDAR data produced for the bwlch position at SO 06781 10855 gives this hill 14.9m of drop which is insufficient for it to retain its status as a 490m Sub-Uchaf.

This now revises the total in Yr Uchafion which will be updated accordingly and the hill is now listed as a Double Sub-Uchaf.


The full details for the hill are:

Cardinal Hill:  Pen y Fan

Summit Height:  496.2m (LIDAR data)

Name:  Twynau Gwynion

OS 1:50,000 map:  160

Summit Grid Reference:  SO 06765 10699
   
Drop:  14.9m (LIDAR data)


The reclassifications to the 490m Sub-Uchaf category in Yr Uchafion / The Welsh 500m P15s reported on Mapping Mountains are ills of wales as follows:



490m SUB-UCHAF RECLASSIFICATIONS

Twynau Gwynion (SO 067 106) 490m Sub-Uchaf reclassified to Double Sub-Uchaf with 14.9m of drop.




Myrddyn Phillips and Aled Williams (January 2017)




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