Mapping Mountains – Hill Reclassifications – 600m Twmpau

Drum yr Eira (SN 851 589) – 600m Sub-Twmpau addition

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data.

The criteria for the list that this addition 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 the hill is listed by is Drum yr Eira and it is adjoined to the Elenydd group of hills, which are situated in the northern part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the B4343 road to its north-west, the A483 road to its south-east and the A470 road to its east, and has the town of Rhaeadr Gwy (Rhayader) towards the north-east.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included but it was listed with an estimated c 19m of drop, based on a c 601m summit height and a c 582m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

Since the original compilation of this list there are now a number of interactive maps originated from Ordnance Survey data that are available online.  One of these was the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This map had many spot heights not on other publicly available Ordnance Survey maps and showed a 582m spot height at the area of this hill’s bwlch; substantiating the previously listed interpolated bwlch height.

Another resource now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available Ordnance Survey maps and for this hill it  shows a 602m spot height on the summit area of this hill.

Extract from the WalkLakes website

The details for this hill were also re-examined against the Ordnance Survey non-contour Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.  This is another map that has many spot heights that do not appear on other publicly available Ordnance Survey maps and it also gives a 602m summit spot height.

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

The positions of these two spot heights were then compared to the contouring on the OS Maps website.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 602m summit height and a 582m bwlch height, with these values giving this hill 20m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau.

 

The full details for the hill are:

Group:  Elenydd

Name:  Drum yr Eira

OS 1:50,000 map:  147

Summit Height:  602m (spot height)

Summit Grid Reference:  SN 85120 58920 (spot height)

Bwlch Height:  582m (spot height)

Bwlch Grid Reference:  SN 85606 58495 (spot height)

 

Drop:  20m (spot height summit and bwlch)

Myrddyn Phillips (August 2020)