Mapping Mountains – Significant Height Revisions – Y Pedwarau

Twyn y Waun (SO 082 070) 

There has been a Significant Height Revision to a summit of a hill that is now listed as a Dual Summit in the Y Pedwarau, and which was initiated by a survey with the Trimble GeoXH 6000, with the survey that confirmed this height revision being conducted on the 1^st December 2017.

The criteria for the list this height revision affects are:

Y Pedwarau – All Welsh hills at and above 400m and below 500m in height that have 30m minimum drop.  Whilst the criteria for 400m Sub-Pedwar status are all Welsh hills at and above 400m and below 500m in height that have 20m or more and below 30m of drop.

A hill classified as a Dual Summit is defined as one which has an extant natural summit coupled with that of an artificial summit, which can be described as being stable in character.  Twyn y Waun meets these criteria as it has an intact 450.6m natural summit at SO 08442 07378 and a 476.0m (converted to OSGM15) higher man-made summit at SO 08209 07041, and this article relates to the latter man-made summit.

The name of the hill is Twyn y Waun and it is situated in the Y Cymoedd – dwyreiniol range of hills which are in the central part of South Wales (Region C, Sub-Region C2), and the hill is positioned between the town of Merthyr Tudful (Merthyr Tydfil) to its west and Rhymni (Rhymney) to its east.

As this man-made hill is a part of a working mine permission to visit should be sought, however its summit is on the border of designated open access land shown on the Ordnance Survey 1:25,000 Explorer map, and for those wishing to visit a public footpath leaves the convenience of a near minor road to the east of the hill.

Prior to the survey with the Trimble GeoXH 6000 the man-made summit was not classified although it had been noted; most recently by Rob Woodall who reported its significant height difference when compared to the natural summit of Twyn y Waun.  The man-made summit possesses no ring contour on Ordnance Survey maps with conflicting highest ring contours for the natural Dual Summit of Twyn y Waun of 430m on the 1:25,000 Explorer map and 450m on the 1:50,000 Landranger map and the Interactive Coverage Map hosted on the Geograph website.  The resulting survey with the Trimble GeoXH 6000 produced a summit height of 476.0m (converted to OSGM15) which is a dramatic increase in height compared to map data and therefore comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000, 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 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, the new height for this man-made Dual Summit is 476.0m (converted to OSGM15), and as no contours exist for the upper part of this Dual Summit this is a dramatic 26m increase in height compared to the uppermost 450m contour on the Ordnance Survey 1:50,000 Landranger map and a 46m increase in height compared to the uppermost 430m contour on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Cardinal Hill:  Twyn y Waun

Summit Height (New Height):  476.0m (converted to OSGM15) for man-made Dual Summit

Name:  Twyn y Waun

OS 1:50,000 map:  160

Summit Grid Reference:  SO 08209 07041

Drop:  96.0m (Trimble GeoXH 6000 summit and LIDAR bwlch) for man-made Dual Summit

The Trimble GeoXH 6000 gathering data at the man-made Dual Summit of Twyn y Waun (SO 08209 07041) which resulted in this summit's significant height revision

For details on the survey that significantly revised the height of this Dual Summit

Myrddyn Phillips and Aled Williams (January 2018)