Friday, 27 May 2016

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

Bryn Llwyd (SN 835 920)  

This is the twenty fifth post under the heading of Summit Relocations, with the Trimble survey that resulted in this summit relocation being conducted on the 5th and 13th May 2016.

The twenty fifth summit relocation initiated from a survey with the Trimble GeoXH 6000 was conducted in the Pumlumon range of hills on two beautiful sunny days, with the hill situated mid-way between the towns of Machynlleth to its north-west and Llanidloes to its south-east.

The name of the hill where the summit has been re-located to is Bryn Llwyd (SN 835 920) and the name of the hill where the summit has been re-located from is Bryn yr Ŵyn (SN 839 925).  Both hills are connected by a bwlch that is only 9.9m below the height of Bryn yr Ŵyn and 11.4m below the height of Bryn Llwyd.

The summits of both hills possess a small uppermost 500m ring contour on Ordnance Survey maps with no adjoined spot height, with the uppermost ring contour for Bryn Llwyd being the larger.

The critical bwlch for the higher of these hills in positioned to the south-west and to my knowledge their drop has only been surveyed on three occasions, once using a basic levelling survey (BLS) method, and the other two using the much more accurate method of the Trimble.  The results from each survey appear below:

Bryn Llwyd:  summit to critical bwlch, 102.5ft (31.2m) drop (BLS), 30.3m drop (Trimble).

Bryn yr Ŵyn:  summit to connecting bwlch with Bryn Llwyd, 42.5ft (13.0m) drop (BLS), 9.9m drop (Trimble).   

I conducted the basic levelling survey in July 2000 and sent the result to Michael Dewey.  Michael assessed the information and decided to include Bryn yr Ŵyn in his list to The 500-Metre Tops of England and Wales.  The margin of uncertainly associated with the BLS method is within the error tolerance of the result, and ever since I have always wanted to re-visit and survey both hills and produce accurate absolute heights for each.  Their heights as surveyed by the Trimble GeoXH 6000 are:

Bryn Llwyd 501.4m (converted to OSGM15) summit at SN 83574 92022 (average of four surveys conducted over two days)

Bryn yr Ŵyn 499.9m (converted to OSGM15) summit at SN 83919 92571 (average of five surveys conducted over two days)

Therefore the position of the relocated summit is at SN 83574 92022 and it consists of grass.  The summit area and the hill itself is relatively easy underfoot and consists of moor grass with a path leading from the critical bwlch to the south-west toward the connecting bwlch with Bryn yr Ŵyn.  This re-located summit position is not given a spot height on current Ordnance Survey maps and it is approximately 660 metres south south-westward from the previously listed summit position.

The full details for the hill are:

Cardinal Hill:  Pumlumon Fawr

Summit Height:  501.4m (converted to OSGM15, and average of four summit surveys)

Name:  Bryn Llwyd

OS 1:50,000 map:  135, 136

Summit Grid Reference (New Position):  SN 83574 92022 
Drop:  30.3m (converted to OSGM15)

The Trimble GeoXH 6000 gathering data at the summit of Bryn Llwyd, with the summit of Bryn yr Ŵyn on the right and in the background of this photograph

For details on the 1st Trimble survey and the 2nd Trimble survey that relocated the summit of this hill.

Myrddyn Phillips and Aled Williams (May 2016)

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