Mapping Mountains – Summit Relocations – 100m Twmpau

 

Crug Cwm (SN 612 257) 

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 detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Crug Cwm (SN 612 257)

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 Craig Cwm 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 encircled by minor roads, with the A40 road farther to its south and the B4302 road farther to its east, and has the town of Llandeilo towards the south south-east.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a non-interpolated c 190m summit height positioned at SN 61434 26099, based on the largest of four uppermost 190m ring contours which appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

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

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 34m of drop, based on an estimated c 193m summit height positioned at SN 61434 26099, with the next largest 190m ring contour estimated as c 192m positioned at SN 61257 25745, and an estimated c 159m bwlch height, based on interpolation of 10m contouring between 150m – 160m.

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 summit image of Crug Cwm (SN 612 257)

LIDAR analysis gives the highest ground on this hill as 192.8m positioned at SN 61260 25753, as opposed to 191.4m to the previously listed summit positioned at SN 61430 26107 and SN 61432 26105, 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 summit of this hill is 192.8m and is positioned at SN 61260 25753, 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 350 metres south-westward from where the previously listed summit is positioned. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen 

Name:  Crug Cwm 

OS 1:50,000 map:  146

Summit Height:  192.8m (LIDAR) 

Summit Grid Reference (New Position):  SN 61260 25753 (LIDAR) 

Bwlch Height:  159.0m (LIDAR) 

Bwlch Grid Reference:  SN 63103 27207 (LIDAR) 

Drop:  33.8m (LIDAR) 

 

Myrddyn Phillips (February 2024)