Wednesday, 15 March 2017

Mapping Mountains – Hill Reclassifications – 500m Twmpau

Pt. 499.1m (SN 863 760) - 500m Sub-Twmpau reclassified to 400m Sub-Pedwar and 490m Sub-Uchaf

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 Sub-Twmpau hills and it has now been reclassified to a 400m Sub-Pedwar and 490m Sub-Uchaf, with the criteria for the sub hills in the 500m Twmpau listing 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 27.9m of drop and an estimated c 500m summit height based on the flat topped nature of its summit area in relation to the dimension of the small uppermost 500m ring contour that appears on current Ordnance Survey mapping.  

The hill is situated in the Elenydd group of hills with its Cardinal Hill being Draws Drum (SN 790 811) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B2).  The hill is situated with the small community of Llangurig to the north-east and Cwmystwyth to the west south-west and is positioned directly above and to the north of the narrow mountain road that heads north-westward from Rhaeadr Gwy (Rhayader).

As the hill is immediately above a road access to it is relatively easy from where a public footpath is indicated on Ordnance Survey maps leaving the road to the west south-west of the summit and following a stream valley up toward the hill’s west, from here the ascent to the featureless flat topped summit area is on ground that is relatively easy compared to other hills in the vicinity.

The name of the hill is currently being listed by the point (Pt. 499.1m) notation and its reclassification from the ranks of 500m Sub-Twmpau is due to 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:

Pt. 499.1m

Summit Height:  499.1m

Summit Grid Reference:  SN 86343 76073

Bwlch Height:  471.0m

Bwlch Grid Reference:  SN 86323 76358

Drop:  28.1m

Therefore, the 499.1m LIDAR data produced for the summit position at SN 86343 76073 and the 471.0m LIDAR data produced for the bwlch position at SN 86323 76358 gives this hill 28.1m of drop, which is in good accordance with the value produced for this hill when surveyed by John Barnard, Graham Jackson, David Purchase and myself in June 2008 using a level and staff resulting in a drop of 27.9m.  Therefore with a summit height below 500m this hill is not high enough to retain its classification as a 500m Sub-Twmpau.  This now revises the total in the 500m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill:  Draws Drum

Summit Height:  499.1m (LIDAR data)

Name:  Pt. 499.1m

OS 1:50,000 map:  135, 136, 147

Summit Grid Reference:  SN 86343 76073
Drop:  27.9m (line survey)

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

Myrddyn Phillips (March 2017)

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