Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Carreg Rhoson (SM 665 254) – Double Sub-Twmpau deletion 

There has been a deletion to the list of 30-99m Twmpau, with the summit height and its location, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 665 254) - second right of the highest island

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms part of an islet chain positioned to the west of mainland Wales with a minor road on the mainland to its east, and has the city of Tyddewi (St David’s) on the mainland also towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this island were re-evaluated and it was listed with an estimated c 20m of drop, based on interpolation of the uppermost 20m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the mapping resources now available online is on the Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping only has an uppermost 15m ring contour and therefore compliments the LIDAR result.

Extract from the Magic Maps website

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. 

LIDAR image of Carreg Rhoson (SM 665 254)

Therefore, the deletion of this hill from Double Sub-Twmpau status is due to LIDAR analysis, resulting in a 19.1m summit height and as this hill is an island, this value also gives it 19.1m of drop, which is insufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Garn Fawr 

Name:  Carreg Rhoson 

OS 1:50,000 map:  157

Summit Height:  19.1m (LIDAR) 

Summit Grid Reference:  SM 66538 25421 (LIDAR)               

Bwlch Height:  N/A (sea level) 

Bwlch Grid Reference:  N/A (sea level) 

Drop:  19.1m (LIDAR) 

 

Myrddyn Phillips (March 2026)