Tuesday, 16 August 2016

Mapping Mountains – Significant Height Revisions – Yr Uchafion and 500m Twmpau

Mynydd Ton (SS 949 940)

This is the thirty fourth post under the heading of Significant Height Revisions, and the Trimble survey that resulted in this height revision was conducted on the 23rd July 2016 in good, clear conditions.

The thirty fourth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the hills of the Y Cymoedd in south Wales on a warm and sunny afternoon.  The Y Cymoedd are an extensive group of hills and comprise the area of the south Wales valleys.

The hill is situated between Cwm Ogwr Fawr to its west and the Rhondda Fawr valley to its east and is positioned above the towns of Treorci (Treorchy) and Treherbert to its north and Nant-y-moel, Price Town and Ogmore Vale to its south-west, and the hill can be easily accessed from a track that leaves the A 4061 near to its high point and continues south-eastward to the north-western edge of a large conifer plantation, a path continues gaining the access point to a wide forest break which is relatively near the summit of the hill.

The name of the hill is Mynydd Ton and prior to the survey with the Trimble GeoXH 6000 the height of the hill was listed as 539m at SS 94796 93958 which is beside an ancient cairn.  This height was based on a 1,769.3ft surface height given on the Ordnance Survey Six-Inch map published in 1885.  The 539m height was a relatively new addition to this hill’s listed height as its summit position was originally listed as ground beside its map heighted 535m triangulation pillar situated at SS 94931 94058.

The highest ground at the base of and beside the ancient cairn was surveyed with the Trimble GeoXH 6000 as being 534.2m (converted to OSGM15) high.  This data set was good with Estimated Accuracies of 5-15cm: 99.41%, a Standard Deviation of 0.0m and a margin of uncertainty for the height placement of the Trimble of 0.1m.

On my previous visit to this hill in February 2003 I judged ground approximately one minute away from the trig pillar to be slightly higher, and as the flush bracket adjoined to the trig pillar is given a height of 535.335m in the OS Trig Database it means that ground at its base is approximately 535m high.  This figure is dependent upon the accuracy of the flush bracket height which is given as 3rd order.   Therefore the ground at the base of or near to the triangulation pillar is likely to be higher than that beside the ancient cairn; this is dependent upon the accuracy of the flush bracket height and the Trimble survey.

The 534.2m (converted to OSGM15) height produced by the Trimble is 4.8m lower than the previously listed height of 539m, which was based on the 1,769.3ft surface height on the 1885 Ordnance Survey Six-Inch map and therefore 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 surveyed Trimble height, 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 for detailing these revisions.

The summit of this hill is easily distinguishable and consists of ground at or near to the triangulation pillar, and therefore this hill’s new summit height is based on the 535m map height adjoined to the trig pillar, as the 534.2m (converted to OSGM15) result produced by the Trimble GeoXH 6000 is 4.8m lower than the hill’s previously listed height of 539m which appears as a 1,769.3ft imperial height on the Ordnance Survey Six-Inch map.

The full details for the hill are:

Cardinal Hill:  Werfa

Summit Height (New Height):  535m

Name:  Mynydd Ton

OS 1:50,000 map:  170

Summit Grid Reference:  SS 94931 94058 
Drop:  92m

The Trimble GeoXH 6000 gathering data beside the ancient cairn at the old summit position of Mynydd Ton which resulted in this hill's significant height revision

Myrddyn Phillips and Aled Williams (August 2016)

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