Saturday, 22 August 2015

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

Twyn Walter (SN 828 175)

This is the eighth post under the heading of Significant Height Revisions, and the following details are retrospective as the Trimble survey that resulted in this height revision was conducted on 10th July 2014.

The eighth major height revision initiated from a survey with the Trimble GeoXH 6000 was conducted in the hills of Y Mynydd Du in south Wales, in the company of Mark Trengove on a lovely summer’s day when we visited the remotest point of mainland Wales as well as the remotest hill of mainland Wales.

The hill whose height has been significantly revised was reclassified from a 500m Twmpau to a Sub-Twmpau after this survey and it can be accessed from Glyntawe on a good path and track that ascends above the Dan-yr-Ogof Showcaves.  Heading west beyond the hill is some of the remotest land in the whole of Wales with the southern land of Y Mynydd Du comprising hills with limestone outcrops and a great wilderness of open grassland, it is highly recommended to visit.

The name of the hill is Twyn Walter and its summit height has been dramatically decreased from its current Ordnance Survey map height of 509m, with the survey conducted with the Trimble GeoXH 6000 giving the height of the hill as 502.1m (converted to OSGM15).  As the height of this hill’s bwlch is estimated as c 478m the hill used to be listed as a 500m Twmpau with c 31m of drop based on the 509m Ordnance Survey summit spot height, as this hill’s summit height has been significantly decreased to the point that it loses its P30 status Twyn Walter is no longer classified as a Twmpau hill and is now listed in the 500m Sub-Twmpau.

Therefore this hill’s new summit height is 6.9m lower than its previously listed height which came from the 509m spot height on current Ordnance Survey 1:25,000 maps and the Ordnance Survey enlarged map on the Geograph website.

The full details for the hill are:

Cardinal Hill:  Fan Brycheiniog

Summit Height (New Height):  502.1m (converted to OSGM15)

Name:  Twyn Walter

OS 1:50,000 map:  160

Summit Grid Reference:  SN 82837 17506
Drop:  c 24m

Gathering data from the summit of Twyn Walter which resulted in this hill's significant height revision

For details on the survey that significantly revised the summit height of this hill please click {here}

Myrddyn Phillips and Aled Williams (July 2014)

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