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 LIDAR analysis conducted by Aled Williams and subsequenty confirmed via a survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips on the 30th June 2018.  

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.

Prior to LIDAR analysis the hill was 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 Interactive Coverage Map hosted on the Geograph website. 

The name of the hill is Fan y Big and it 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 reclassification of Fan y Big to 700m Sub-Twmpau status is due to LIDAR analysis conducted 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 LIDAR analysis 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, with the details from the Trimble GeoXH 6000 survey being 716.7m (converted to OSGM15 and average of two surveys) summit at SO 03652 20658 and 688.2m (converted to OSGM15 and average of two surveys) bwlch at SO 03773 19876, with these values giving this hill 28.5m of drop. 

The Trimble GeoXH 6000 gathering data at the summit of Fan y Big

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:  716.7m (converted to OSGM15)

Name:  Fan y Big

OS 1:50,000 map:  160

Summit Grid Reference:  SO 03652 20658 
Drop:  28.5m (converted to OSGM15)

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

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