Thursday, 13 April 2017

Mapping Mountains – Hill Reclassifications – 700m Twmpau

Fan y Big (SO 036 206) - 700m Twmpau reclassified to 700m 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 listed in the 700m height band of P30 Twmpau hills and it has now been reclassified to a 700m Sub-Twmpau, with the criteria for the sub hills being 700m or more and below 800m 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 April 2015.

The hill had been listed with 30m of drop, based on the 719m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and the 689m bwlch spot height that appears on the Ordnance Survey enlarged mapping hosted on the Geograph website. 

The hill is situated in the central part of the main ridge that forms 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).  It is situated between the towns of Aberhonddu (Brecon) to its north and Merthyr Tudful to its south.

The hill can be accessed from public footpaths originating from its north and from its south; or it can be combined as a longer expedition taking in the main Bannau Brycheiniog ridge either from its west or from its east.

The name of the hill is Fan y Big and its reclassification to a 700m Sub-Twmpau is due to the analysis of LIDAR data by Aled Williams.  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:

Fan y Big

Summit Height:  716.3m

Summit Grid Reference:  SO 03651 20662

Bwlch Height:  688.1m

Bwlch Grid Reference:  SO 03773 19876

Drop:  28.2m

Therefore, the 716.3m LIDAR data produced for the summit position at SO 03651 20662 and the 688.1m LIDAR data produced for the bwlch position at SO 03773 19876 is sufficient for this hill to be reclassified from a 700m Twmpau to a 700m Sub-Twmpau with 28.2m of drop.

This now revises the total in the 700m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill:  Pen y Fan

Summit Height:  716.3m (LIDAR data)

Name:  Fan y Big

OS 1:50,000 map:  160

Summit Grid Reference:  SO 03651 20662 
Drop:  28.2m (LIDAR data)

Fan y Big (SO 036 206) now reclassified to a 700m Sub-Twmpau

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

Myrddyn Phillips (March 2017)

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