Mapping Mountains – Significant Height Revisions – Y Pedwarau

Twyn y Waun (SO 085 065) 

There has been a Significant Height Revision to a summit of a hill that is now listed 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.

The name of the hill is Twyn y Waun and this hill is the result of mine spoil from the Ffos-y-fran opencast mine, as a result it is considered a man-made hill.  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 it 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, for those wishing to do so a public footpath leaves the convenience of a near minor road to the north north-east of the hill.

Prior to the survey with the Trimble GeoXH 6000 this man-made hill was not classified although it had been noted; most recently by Rob Woodall who reported its approximate height and prominence.  This hill possesses no ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps with the continuation of contours from the unmined land to the east of the hill being between 400m – 410m high compared to where the summit of this man-made hill is now situated. 

The resulting survey with the Trimble GeoXH 6000 produced a summit height of 460.5m (converted to OSGM15) and a bwlch height of 419.5m (converted to OSGM15), with these values giving this hill 41.0m of drop, with the summit height being a dramatic increase in height compared to the continuation of map contours from the unmined land to the east and therefore it 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 hill 460.5m (converted to OSGM15), and as no contours exist for this hill on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps this is a dramatic 50.5m increase in height compared to the continuation of the 410m contour from the unmined land to the east of this hill.

The full details for the hill are:

Cardinal Hill:  Twyn y Waun

Summit Height (New Height):  460.5m (converted to OSGM15)

Name:  Twyn y Waun

OS 1:50,000 map:  160

Summit Grid Reference:  SO 08549 06591

Drop:  41.0m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Twyn y Waun (SO 085 065) which resulted in this hill's significant height revision

For details on the survey that significantly revised the height of this hill

Myrddyn Phillips and Aled Williams (January 2018)