Friday, 4 August 2017

Mapping Mountains – Significant Height Revisions – Y Trichant

Teisen Priodas (SN 820 108)

There has been a Significant Height Revision to a hill that is now listed in the Y Trichant due to analysis of LIDAR data by Kevin McGovern and also independently by Aled Williams and George Gradwell, which was subsequently confirmed by a survey with the Trimble GeoXH 6000 that was conducted on the 13th June 2017. 

Y Trichant is the title for the hills in the 300m height band of the Twmpau (thirty welsh metre prominences and upward) and takes in all Welsh hills at or above 300m and below 400m in height that have a minimum 30m of drop, with the introduction to the re-naming of this list appearing on Mapping Mountains on the 13th May 2017.

The ridge that this hill is a part of is named Mynydd y Drum and it consists of three listed hills, one being a sub hill which is deemed natural, and two being P30s that are the consequence of mine workings, with one of these P30s having been fully landscaped.  It is this fully landscaped P30 whose details are given in this article.  

When visiting this hill I made place-name enquiries with two of the remaining six mine workers and was told that the name the hill is known by is Teisen Priodas, which translated into English means the wedding cake, an apt name as the hill has three tiers and is relatively flat on top.

Teisen Priodas 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), with the hill positioned between the small communities of Abercraf (Abercrave) to the north and Y Coelbren towards the east.

As the hill is a part of the Nant Helen Opencast Mine permission to visit should be sought, however Ordnance Survey maps still show public footpaths crossing the opencast workings with the nearest one placed just to the west of the summit and crossing land associated with the mine from a south to a north direction, this public footpath is probably on a part of land that has been landscaped.

Teisen Priodas consists of waste spoil that is now landscaped, and as it is customary for Ordnance Survey maps to show mine workings without any form of contour line the hill had not been classified prior to the analysis of LIDAR data, although the area of this hill’s bwlch does possess contours.  The mine is situated on the north-eastern part of the Mynydd y Drum ridge, with the south-western part of this broad ridge still intact having not been mined and possessing a 200m Sub-Twmpau that is listed by the name of Mynydd y Drum.

The highest ring contour shown on Ordnance Survey 1:25,000 Explorer maps for the whole  broad ridge of Mynydd y Drum is 290m which appears on the south-western part of this ridge where the 296.2m summit of the Sub-Twmpau is situated at SN 80718 09753, and also at the eastern periphery of the mine at SN 833 112.  

The survey with the Trimble GeoXH 6000 produced a summit height for Teisen Priodas of 337.9m (converted to OSGM15) which comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revision applies to any listed hill whose Ordnance Survey summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000, or a previous estimated summit height based on interpolation of contours.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 map is being prioritised in favour of the 1:50,000 map for detailing these revisions.

The summit area of this hill is now landscaped and relatively flat and expansive, with the high point toward the eastern part of the summit plateau which consists of long grasses.

Therefore this hill’s new summit height is 337.9m (converted to OSGM15), as no summit contours exist for this hill above the 290m given to the far eastern part of the mine workings on the Ordnance Survey 1:25,000 Explorer map, this is a dramatic 47.9m increase in height compared to its uppermost ring contour. 

The full details for the hill are:

Cardinal Hill:  Fan Gyhirych 

Summit Height (New Height):  337.9m (converted to OSGM15)

Name:  Teisen Priodas

OS 1:50,000 map:  160

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

The Trimble GeoXH 6000 gathering data at the summit of Teisen Priodas (SN 820 108) which confirmed this hill's significant height revision

Myrddyn Phillips (August 2017)

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