Mapping Mountains – Significant Height Revisions – Welsh Highlands - Uchafion Cymru and 500m Twmpau

Twyn Walter (SN 828 175)

There has been a Significant Height Revision initiated by a survey conducted with the Trimble GeoXH 6000 to the listing of the Yr Uchafion and the 500m Twmpau (thirty welsh metre prominences and upward), with these details being retrospective as the survey that resulted in this height revision was conducted on 10^th July 2014.

The criteria for these listings appear below:

Yr Uchafion - All Welsh hills at or above 500m in height that have 15m minimum drop.  The list is co-authored by Myrddyn Phillips and Aled Williams

500m Twmpau - All Welsh hills at or above 500m and below 600m in height with 30m minimum drop.  With the 500m Sub-Twmpau being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips.   

The name of the hill is Twyn Walter and it is situated in the Mynydd Du range of hills in south Wales, and the survey was conducted 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 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, a visit is highly recommended.

The summit height of this hill has been dramatically decreased from its current Ordnance Survey 1:25,000 Explorer 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).  Based on the 509m summit map height this hill used to be listed as a 500m Twmpau with over 30m of drop, however its height has now dramatically decreased and its drop value also re-evaluated giving this hill c 26m of drop and therefore it has been reclassified to a 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 Explorer maps and the Ordnance Survey enlarged mapping hosted on the Geograph website.

Since this height revision was announced the height and position of this hill's bwlch has been determined via LIDAR analysis conducted by Aled Williams, the details for which are given below.

The full details for the hill are:

Group:  Mynydd Du

Name:  Twyn Walter

OS 1:50,000 map:  160

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

Summit Grid Reference:  SN 82837 17506

  

Bwlch Height:  475.9m (LIDAR)

Bwlch Grid Reference:  SN 82597 17917 (LIDAR)

Drop:  26.25m (Trimble summit and LIDAR bwlch)

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

Myrddyn Phillips and Aled Williams (August 2015)