Mapping Mountains – Hill Reclassifications – 500m Twmpau

Garn Fach (SO 041 815) – 500m Sub-Twmpau addition

There has been an addition to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams.  The Twmpau is the title for the list of Welsh P30s and takes in all Welsh hills that have a minimum 30m of drop, with this addition affecting the 500m height band of hills within the Twmpau.

Accompanying the main list of Twmpau hills is a sub category, with the criteria for this sub list in the 500m height band being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop.  It is this sub category that this addition affects and details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

Prior to analysis of LIDAR data the hill had been listed with 19m of drop based on the 503m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and the 484m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

The name of the hill is Garn Fach and it is situated in the Pegwn Mawr group of hills with its Cardinal Hill being Pegwn Mawr (SO 023 812) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B1).  The hill is situated between the small community of Llanbadarn Fynydd towards its south-east and the town of Llanidloes towards its west north-west.

If wanting to visit the hill it is situated on open access land which is covered in a conifer plantation, however on my only visit I followed a forest track leading in from the east which gave access to a forest ride that led towards the summit.

The addition of Garn Fach to 500m Sub-Twmpau status 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:

Garn Fach

Summit Height:  504.3m

Summit Grid Reference:  SO 04184 81512

Bwlch Height:  484.3m

Bwlch Grid Reference:  SO 03783 81771

Drop:  20.0m

Therefore, the 504.3m LIDAR data produced for the summit position at SO 04184 81512 and the 484.3m LIDAR data produced for the bwlch position at SO 03783 81771 gives this hill 20.0m of drop which is sufficient for its inclusion as a 500m Sub-Twmpau.

The full details for the hill are:

Cardinal Hill:  Pegwn Mawr

Summit Height:  504.3m (LIDAR data)

Name:  Garn Fach

OS 1:50,000 map:  136

Summit Grid Reference:  SO 04184 81512

Drop:  20.0m (LIDAR data)

The overall total for the 500m Twmpau remains at 225 hills with the overall total for the 500m Sub-Twmpau increasing by one to 97 hills.  The list of the 500m Twmpau was published on 15^th June 2015 with all subsequent changes having been detailed on Mapping Mountains. 

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

Myrddyn Phillips (September 2017)