Thursday, 18 August 2016

Mapping Mountains – Summit Relocations – Yr Uchafion and 500m Twmpau

Mynydd Ton (SS 949 940)

There has been a Summit Relocation initiated by a survey with the Trimble GeoXH 6000  to the listings of Yr Uchafion and the 500m Twmpau, with the survey that resulted in this summit relocation being conducted on the 23rd July 2016 in good, clear and sheltered conditions.

The two lists that this summit relocation affects are:

Yr Uchafion, this is the working title for a list that takes in all hills in Wales at and above 500m in height that have a minimum 15m of drop.  This list is co-authored by Myrddyn Phillips and Aled Williams.

500m Twmpau (thirty welsh metre prominences and upward), with the criteria being all Welsh hills at or above 500m and below 600m in height that have a minimum 30m of drop.  This list is authored by Myrddyn Phillips.

The survey was conducted in the range of hills known as the Y Cymoedd, these are the hills associated with 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.

Access to the hill is relatively easy as a track 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 south-eastward adjacent to the forest boundary gaining the access point to a wide forest break which is relatively near the summit of this 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 figure 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.  The processed data gave Estimated Accuracies of 5-15cm: 99.41%, with the Standard Deviation being 0.0m, and the margin of uncertainty given for the height placement of the Trimble was 0.1m.  All of the above signify that the data set was good and accurate to within the parameters of the equipment.

I had previously visited this hill in February 2003 and judged ground approximately one minute walk away from the trig pillar to be slightly higher than ground at the base of the trig pillar, 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.

Therefore the position of the relocated summit is at SS 94931 94058 and consists of ground at the base of, or near to the triangulation pillar.  This re-located summit position is at the hill’s originally listed summit and is given a 535m height on current Ordnance Survey maps and it is approximately 175 metres north-eastward from the previously listed summit position.

The full details for the hill are:

Cardinal Hill:  Werfa

Summit Height:  535m

Name:  Mynydd Ton

OS 1:50,000 map:  170

Summit Grid Reference (New Position):  SS 94931 94058 
Drop:  92m

The Trimble GeoXH 6000 gathering data beside the ancient cairn on Mynydd Ton, with the relocated position of the summit approximately 175 metres to the north-east and to the right of the forest break in the centre background of this photograph

Myrddyn Phillips and Aled Williams (August 2016)

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