Mapping Mountains – Hill Reclassifications – 500m Twmpau

Drum Ddu (SH 931 167) – 500m Sub-Twmpau deletion

There has been a deletion of a hill in the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by detail on contemporary maps produced from Ordnance Survey data

The summit of Drum Ddu (SH 931 167)

The criteria for the list that this deletion applies to are:

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m in height with 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 the hill is listed by is Drum Ddu, and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the A470 road to its west, the A458 road to its south and the B4393 road to its north-east, and has the town of Dinas Mawddwy towards the west south-west. 

When the list that later became known as the 500m Twmpau was first compiled this hill was listed with c 25m of drop based on the 538m summit spot height and an estimated bwlch height of c 513m based on bwlch contouring between 510m – 520m that appear on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

However, as Drum Ddu has an adjacent hill named Tapiau y Gell (SH 927 168 [refer to Y Pellennig – The Remotest Hills of Wales by Aled Williams and Myrddyn Phillips]) of similar height that has no spot height on either the Ordnance Survey 1:50,000 Landranger or the 1:25,000 Explorer map, it was noted that if Tapiau y Gell is higher, it would also be the qualifying hill for the 500m Sub-Twmpau.

ills of WalesHillThe details for this hill were re-evaluated when the WalkLakes interactive map became available online.  This map originated from the Ordnance Survey Open Data programme and has many spot heights not on other publicly available Ordnance Survey maps and gives a 539m spot height on the summit area of Tapiau y Gell.

Extract from the WalkLakes website

Therefore, the deletion of Drum Ddu from 500m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data with this hill given a 538m summit spot height as opposed to the 539m summit spot height given to Tapiau y Gell.  Therefore, the position of the bylchau for these hills are swapped resulting in Drum Ddu now being listed with a 538m summit height and c 523m bwlch height, with these values giving this hill c 15m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group:  Y Berwyn

Name:  Drum Ddu

OS 1:50,000 map:  125

Summit Height:  538m (spot height)

Summit Grid Reference:  SH 93180 16796 (hand-held GPS via DoBIH)

Bwlch Height:  c 523m (interpolation)

Bwlch Grid Reference:  SH 93019 16980 (interpolation)

Drop:  c 15m (spot height summit and interpolated bwlch)

Myrddyn Phillips (June 2020)