Mapping Mountains – Hill Reclassifications – 500m Twmpau

 

Carreg yr Ogof (SN 777 214) – 500m Sub-Twmpau addition

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on recent and contemporary maps produced from Ordnance Survey data. 

The criteria for the list that this addition 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 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg yr Ogof and it is adjoined to the Y Mynydd Du group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and it is positioned with the A40 road to its north, the A4069 road to its west, the A4068 road to its south south-west and the A4067 road to its east, and has the small community of Llanddeusant towards the north.

When the list that later became known as the 500m Twmpau was first compiled this hill was not included as details on contemporary Ordnance Survey maps of the day give a 585m summit height adjoined to a triangulation pillar and an interpolated bwlch height of c 567m based on contours between 560m – 570m, with these values giving this hill an estimated c 18m of drop. 

Extract from the Ordnance Survey 1:25,00 Explorer map

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and gave a 566m spot height on the bwlch area of this hill, this when coupled with its 585m summit map height gave this hill 19m of drop. 

The details for this hill were then re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and it used to have contours at 5m intervals which proved 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.  This mapping had bwlch contouring between 565m – 570m. 

However, online photographs of the summit area of this hill show ground a few metres from the base of the triangulation pillar as significantly higher.  The trig pillar has a flush bracket height of 585.520m and is positioned at SN 77778 21455.  Accepting the accuracy of this height the ground at the base of the trig pillar would be approximately 585.2m in height and the summit of the hill has been estimated to be approximately 0.8m higher. 

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to detail derived from recent and contemporary maps produced from Ordnance Survey data, resulting in an estimated c 586m summit height and a 566m bwlch height, with these values giving this hill an estimated c 20m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

 

The full details for the hill are:

Group:  Y Mynydd Du

Name:  Carreg yr Ogof

OS 1:50,000 map:  160

Summit Height:  c 586m (interpolation)

Summit Grid Reference:  SN 77780 21461 (interpolation)

Bwlch Height:  566m (spot height)

Bwlch Grid Reference:  SN 77792 21045 (spot height)

Drop:  c 20m (interpolated summit and spot height bwlch)

 

Myrddyn Phillips (January 2021)