Monday, 27 March 2017

Mapping Mountains – Hill Reclassifications – Y Trichant

Teisen Priodas (SN 820 108) - Trichant addition

There has been an addition to the listing of the Y Trichant, these are the hills within the 300m height band of the Twmpau (thirty welsh metre prominences and upward) due to analysis of LIDAR data by Kevin McGovern and also independently by Aled Williams, and subsequently confirmed via a survey with the Trimble GeoXH 6000.  With the high point of the hill previously listed in the 200m height band when the Welsh P30 hills below 500m in height were published on Geoff Crowder’s website.

The hill had been listed with a 297m summit height positioned at SN 807 098 based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map numbers 12 and 165, whilst the Ordnance Survey 1:50,000 Landranger map number 160 gives a 298m summit spot height at the same position.  This latter spot height is derived from the 978.1ft (298.1ft) height on the Ordnance Survey old Six-Inch map.

When this hill was originally listed in the Welsh P30 lists on the website it had an accompanying note which stated ‘3 points of 300m on 1985 map may probably no longer exist (quarrying)’.  The hand written Master Lists stated ‘Due to opencast workings the three points of 300c at GR 830 112 on 1985 1:50,000 map probably no longer exist’.  These three points are in the vicinity of where a 984ft (299.9m) height appears on the Ordnance Survey old Six Inch map and in all likelihood signified where the summit of this hill was once positioned.  Since this time the Nant Helen Openacast Mine workings which take in the northern and eastern part of the ridge known as Mynydd y Drum have destroyed the old 984ft (299.9m) summit, but in its place are two new P30 summits comprising the spoil from the mine workings, with this Hill Reclassifications post concentrating on the higher of these two artificial summits, whilst the 296.2m (converted to OSGM15) natural summit at SN 80718 09753 and listed as Mynydd y Drum still remains. 
Prior to analysis of LIDAR data the ridge known as Mynydd y Drum was listed with one summit, the opencast mine has now created a further two summits which are artificial and comprise the remains of waste spoil, the summit which this Hill Reclassifications post details is now higher than the previously listed summit, resulting in new bylchau heights and positions and also drop values for these three hills, all of these will be detailed in this and two further Hill Reclassifications posts, with the name Teisen Priodas being found from local enquiry and the other two hills being Mynydd y Drum (SN 807 097) and MRF Tip (SN 828 114).

The hill is adjoined to the Fforest Fawr group with its Cardinal Hill being Fan Gyhirych (SN 880 191) and is placed in the Region of South Wales (Region C, Sub-Region C2), and it is positioned between the small communities of Abercraf (Abercrave) to the north and Y Coelbren towards the east.

The summit of the hill is situated within the Nant Helen Opencast Mine so permission to visit the high point should be sought, however it has open access land to its south-west and a public footpath marked on Ordnance Survey maps that heads in a north- south direction and at its closest point is just to the west of the summit of this hill.

The name of the hill is Teisen Priodas and its reclassification to a Trichant is due to the analysis of LIDAR data by Kevin McGovern and also independently by Aled Williams.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Kevin’s analysis of LIDAR data gives the hill the following details:

Teisen Priodas

Summit Height:  338.2m (converted to OSGM15)

Summit Grid Reference:  SN 82014 10832

Bwlch Height:  244.8m (converted to OSGM15)

Bwlch Grid Reference:  SN 84566 11384

Drop:  93.4m

Therefore, the 338.2m LIDAR data produced for the summit position at SN 82014 10832 and the 244.8m LIDAR data produced for the bwlch position at SN 84566 11384 is sufficient for this hill to be classified as a Trichant with 93.4m of drop and for it to be listed with a higher summit than the 296.2m (converted to OSGM15) summit of Mynydd y Drum at SN 80718 09753, with the details from the Trimble GeoXH 6000 survey being 337.9m (converted to OSGM15) summit at SN 82013 10833 and 244.6m (converted to OSGM15) bwlch at SN 84569 11383, giving this hill 93.3m of drop.

This now revises the total in the Y Trichant and the Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill:  Fan Gyhirych

Summit Height:  337.9m (converted to OSGM15, Trimble data)

Name:  Teisen Priodas

OS 1:50,000 map:  160

Summit Grid Reference:  SN 82013 10833 
Drop:  93.3m (converted to OSGM15, Trimble data)

Teisen Priodas (SN 820 108) a new addition to the Y Trichant list

My thanks to Mark Jackson for bringing the details of this hill to my attention and to Kevin McGovern and Aled Williams for their analysis of LIDAR data.

Myrddyn Phillips (March 2017)

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