Mapping Mountains – Hill Reclassifications – 500m Twmpau

Carreg y Big (SH 927 201) – 500m Sub-Twmpau deletion

There has been a deletion 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 produced by Joe Nuttall in his surface analysis progamme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Carreg y Big (SH 927 201)

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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains. 

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg y Big and it is adjoined to the Esgeiriau Gwynion group of hills, which are situated in the southern part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north and west, and the B4393 road to its east north-east, and has the village of Llanwddyn towards the east.

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the accompanying sub list with an estimated c 21m of drop, based on the 589m summit height that appears on the contemporary Ordnance Survey 1:25,000 Outdoor Leisure map, and an estimated c 568m bwlch height, based on interpolation of 10m contouring between 560m – 570m. 

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 569m bwlch spot height, and therefore the drop value was revised to 20m.

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the deletion of this hill from 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 589.4m summit height and a 569.8m bwlch height, with these values giving this hill 19.6m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

 

The full details for the hill are:

Group:  Esgeiriau Gwynion

Name:  Carreg y Big

OS 1:50,000 map:  125

Summit Height:  589.4m (LIDAR) 

Summit Grid Reference:  SH 92712 20119 (LIDAR) 

Bwlch Height:  569.8m (LIDAR) 

Bwlch Grid Reference:  SH 92769 19949 & SH 92769 19953 (LIDAR) 

Drop:  19.6m (LIDAR)

 

Myrddyn Phillips (August 2023)