Thursday, 19 January 2017

Mapping Mountains – Hill Reclassifications – 500m Twmpau


Mynydd Llangatwg (SO 202 145) - 500m Twmpau reclassified to 500m Sub-Twmpau

There has been a reclassification to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams.  The hill was previously listed in the 500m height band of Twmpau hills and it has now been reclassified to a 500m Sub-Twmpau, with the criteria for the sub hills being 500m or more and below 600m in height with 20m or more and below 30m of drop.  Details to this height band of the Twmpau were published on the Mapping Mountains site on 15th June 2015.

The hill had been listed with c 31m of drop, based on the 529m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and an estimated bwlch height of c 498m based on bwlch contouring between 490m – 500m.  The summit area of the hill has two ancient cairns on it as well as a triangulation pillar with an adjoined flush bracket height of 529.044m.  

The hill is situated in the easterly part of 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 (C-2).  It is situated between the towns of Bryn-mawr to its south, Gilwern to its east and Crug Hywel (Crickhowell) to its north north-east.

The hill can be accessed from public footpaths originating from its north and from its south; however one of the easiest ways to its summit is from the east where a track heads toward the summit from a minor road that reaches over 350m high.

The name of the hill is Mynydd Llangatwg, however it is also recorded as Mynydd Pencyrn on Ordnance Survey maps and its reclassification to a Sub-Twmpau 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:


Mynydd Llangatwg

Summit Height:  528.4m

Summit Grid Reference:  SO 20283 14511

Bwlch Height:  499.6m

Bwlch Grid Reference:  SO 18452 15398

Drop:  28.8m


Therefore, the 528.4m LIDAR data produced for the summit position at SO 20283 14511 and the 499.6m LIDAR data produced for the bwlch position at SO 18452 15398 is sufficient for this hill to be reclassified from a 500m Twmpau to a 500m Sub-Twmpau with 28.8m of drop.

This now revises the total in the 500m Twmpau which will be updated accordingly.


The full details for the hill are:

Cardinal Hill:  Pen y Fan

Summit Height:  528.4m (LIDAR data)

Name:  Mynydd Llangatwg

OS 1:50,000 map:  161

Summit Grid Reference:  SO 20283 14511 
  
Drop:  28.8m (LIDAR data)



My thanks to Aled Williams for sending the details of this hill to me.


Myrddyn Phillips (January 2017)



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