Mapping Mountains – Hill Reclassifications – 200m Twmpau

Chwarel Penrhyn (SH 621 658) – 200m Sub-Twmpau addition

There has been an addition of a hill to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Chwarel Penrhyn (SH 621 658)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Chwarel Penrhyn, and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the B4409 road to its north, a minor road to its west and the A5 road to its east, and has the town of Bethesda towards the north-east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as with no significant contours of note on the contemporary Ordnance Survey 1:25,000 map, it was hard to know whether any hill with qualifying prominence existed and therefore it was considered not to meet the criteria then used for this sub category. 

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

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 addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 220.6m summit height and a 197.0m bwlch height, with these values giving this hill 23.6m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Glyder Fawr 

Name:  Chwarel Penrhyn 

OS 1:50,000 map:  115

Summit Height:  220.6m (LIDAR) 

Summit Grid Reference:  SH 62149 65875 (LIDAR) 

Bwlch Height:  197.0m (LIDAR) 

Bwlch Grid Reference:  SH 61793 65966 (LIDAR) 

Drop:  23.6m (LIDAR) 

 

Myrddyn Phillips (December 2025)