Mapping Mountains – Summit Relocations – 100m Twmpau

Cwm y Rhidyll (SN 625 249) 

There has been a Summit Relocation to a hill that is listed in the 100m 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 Cwm y Rhidyll (SN 625 249)

The criteria for the list that this summit relocation applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Cwm y Rhidyll and this was derived from the Tithe map, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north-east, north-west and south, and the A40 road farther to its south and the B4302 road to its east, and has the town of Llandeilo towards the south.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 157m summit height, based on the spot height positioned at SN 62592 24875 which appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

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.

LIDAR analysis gives the highest ground on this hill as 157.1m positioned at SN 62593 24935.  However, this is a part of a raised field boundary and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Cwm y Rhidyll (SN 625 249)

The height produced by LIDAR analysis to the natural summit of this hill is 156.6m and is positioned at SN 62543 24916, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is judged to be a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the height produced by LIDAR analysis to the natural summit of this hill is 156.6m and is positioned at SN 62543 24916, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 50 metres north-westward from where the 157m spot height is positioned and approximately 50 metres west south-westward from where the high point of the raised field boundary is positioned.

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Cwm y Rhidyll 

OS 1:50,000 map:  159

Summit Height:  156.6m (LIDAR) 

Summit Grid Reference (New Position):  SN 62543 24916 (LIDAR) 

Bwlch Height:  113.7m (LIDAR) 

Bwlch Grid Reference:  SN 62913 25209 (LIDAR) 

Drop:  42.9m (LIDAR) 

 

Myrddyn Phillips (February 2024)