Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Carreg Rhoson (SM 672 256) – 30-99m Twmpau reclassified to Double Sub-Twmpau 

There has been a reclassification to the list of 30-99m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on the DataMapWales. 

Carreg Rhoson (SM 672 256) 

The criteria for the list that this reclassification 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, 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 a part of the islet chain known as Carreg Rhoson, and it is positioned to the west of the Pembrokeshire coast, with the small city of Tyddewi (St David’s) 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 main P30 list as the small uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map had been missed.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 30m summit height, based on interpolation of its uppermost 30m ring contour.  As this hill is the high point of an island it was also listed with an estimated c 30m drop. 

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

However, it was not until the DataMapWales became available online that the details of this hill could be checked against a product viewed as being more accurate than the contouring on the contemporary Ordnance Survey 1:25,000 Explorer map.  This mapping has a 28m spot height for this hill and this is similar to the 27.42m summit height produced by the surface analysis programme developed by Joe Nuttall (JNSA) that uses the OS Terrain product where LIDAR coverage is not available. 

Extract from the DataMapWales

Therefore, the reclassification of this hill from 30-99m Twmpau status is due to detail on the DataMapWales, resulting in a 28m summit height, and as this hill is the high point of an island the drop value is also 28m, which is sufficient for Double Sub-Twmpau status. 

 

The full details for the hill are:

Group:  Garn Fawr

Name:  Carreg Rhoson

OS 1:50,000 map:  157

Summit Height:  28m (spot height)

Summit Grid Reference:  SM 67228 25658 (JNSA)

Bwlch Height:  N/A (island)

Bwlch Grid Reference:  N/A (island)

Drop:  28m (spot height)

 

Myrddyn Phillips (July 2024)