Mapping Mountains – Hill Reclassifications – 500m Twmpau

Twyn Walter (SN 828 175) - 500m Twmpau reclassified to 500m Sub-Twmpau

There has been a reclassification of a 500m Twmpau hill by surveying with the Trimble.   The hill is situated in the south-eastern lands of Y Mynydd Du in South Wales, and is positioned north-west of Glyntawe and the A4067 as it speeds its way between Pontsenni (Sennybridge) to the north and Ystradgynlais to the south.

These details are retrospective as the Trimble GeoXH 6000 survey that resulted in the reclassification of this hill from 500m Twmpau to 500m Sub-Twmpau was conducted on 10^th July 2014, in the company of Mark Trengove when we visited the remotest spot in mainland Wales, as well as the remotest hill in mainland Wales.

Access to the hill from the south-east of its summit is relatively easy as a broad path ascends from near to the Campsite entrance that is adjacent to Dan yr Ogof Showcaves.  As height is gained this path resembles a vehicle track and eventually heads north-west into the remote grasslands of Mynydd Du, on its way it passes within approximately 260 metres of the summit.

     

The hill is named Twyn Walter (written as Twynwalter on current Ordnance Survey maps) and its summit is positioned at SN 828 175.  The hill was listed with c 31m of drop based on a 509m summit spot height and an estimated bwlch height of c 478m.  The area of the bwlch has a number of sink holes on it which complicates pinpointing where the critical bwlch is positioned.  However, contouring suggests that there are two distinct positional possibilities for where this critical bwlch lies, these are at SN 82525 17584 and SN 82611 17986.

The survey with the Trimble GeoXH 6000 resulted in a 502.1m (converted to OSGM15) summit height, unfortunately daylight was quickly disappearing and this was the last survey of a long day on the hill and this dictated that the two options for the position of the bwlch were not surveyed.  However, the dramatic decrease in summit height from the 509m map height to the 502.1m (converted to OSGM15) surveyed height and the re-evaluated and estimated bwlch height from contour interpolation of c 476m gives this hill a drop value of only c 26m.

This now brings the overall total for the 500m Twmpau hills to 226, please click {here} to access the list and {here} for its detailed Introduction.

Since this reclassification was announced the height and position of this 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:  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 at the summit of Twyn Walter, which resulted in this hill being reclassified from a 500m Twmpau to a 500m Sub-Twmpau

For details on the survey that reclassified this hill from 500m Twmpau to 500m Sub-Twmpau please click {here}

Myrddyn Phillips (July 2014)