Mapping Mountains – Hill Reclassifications – 600m Twmpau

Foel Penolau (SH 661 347 and SH 661 348) – 600m Sub-Twmpau reclassified to 600m Twmpau

There has been a reclassification to a hill that is listed in the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a survey with the Trimble GeoXH 6000 which was conducted over two days by Myrddyn Phillips and Aled Williams on the 17^th November 2018 and by Myrddyn Phillips on the 26^th November 2018.

Foel Penolau (SH 661 347 and SH 661 348)

The criteria for the list that this hill reclassification applies to are:

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name of the hill is Foel Penolau and it is adjoined to the Rhinogydd group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A496 road to the west and the A470 road and the village of Trawsfynydd towards the east.

When the list that later became known as the 600m Twmpau was first compiled this hill was listed with c 26m of drop based on the 614m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and an estimated bwlch height of c 588m based on interpolation of 10m contouring between 580m – 590m.

Extract from the Ordnance Survey 1:25,000 Explorer map

The details for this hill were re-evaluated when the Ordnance Survey Vector Map Local became available online; this map is hosted on the Geograph website and is entitled the Interactive Coverage Map.  As well as showing twin 614m map heighted summits it also gave a 588m spot height in the area of this hill’s bwlch, but as lower ground was known to exist on the hill to hill traverse compared to where this spot height is positioned the drop value was amended to an estimated c 27m.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

This hill’s details were again re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and these showed that the 588m spot height that appears on the Ordnance Survey Vector Map Local is not positioned at the critical bwlch of this hill.  These contours placed the position of the critical bwlch in the boulder field below this hill’s eastern face and resulted in an estimated c 30m of drop, based on an estimated c 584m bwlch height based on interpolation of contours between 580m – 585m and the 614m spot height given to the two twin map heighted summits.

Extract from the OS Maps website

The confirmation of the reclassification of Foel Penolau to 600m Twmpau status is due to a survey with the Trimble GeoXH 6000 resulting in a 614.4m summit height (with 1mm difference between the northern and southern summits the Trimble was unable to split these tops) and a 582.5m bwlch height, with these values giving this hill 31.9m of drop which is sufficient for it to be classified as a 600m Twmpau.

The Trimble set-up position at the northerly summit of Foel Penolau

 

The Trimble set-up position at the critical bwlch of Foel Penolau

The full details for the hill are:

Group:  Rhinogydd

Name:  Foel Penolau

OS 1:50,000 map:  124

Summit Height:  614.4m (converted to OSGM15)

Summit Grid Reference:  SH 66184 34832 and SH 66145 34774

Bwlch Height:  582.5m (converted to OSGM15)

Bwlch Grid Reference:  SH 66099 34735

 

Drop:  31.9m

For details on the survey of Foel Penolau

Myrddyn Phillips (May 2019)