Mapping Mountains – Summit Relocations – 200m Twmpau

Cwm Gwenyn (SN 619 611) 

There has been a Summit Relocation to a hill that is listed in the 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 Cwn Gwenyn (SN 619 611)

The criteria for the list that this summit relocation 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Cwm Gwenyn and this was derived from the Tithe map, and it is adjoined to the Mynydd Bach group of hills, which are situated in the western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its west and east, the B4577 road to its north and the B4342 road to its south, and has the town of Tregaron towards the east.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was listed with a summit height of 209m based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map which is positioned at SN 619 611. 

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 207.4m positioned at SN 61917 61105.  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 Gwenyn (SN 619 611)

The height produced by LIDAR analysis to the natural summit of this hill is 207.6m positioned at SN 61917 61104, and this position in relation to the raised field boundary comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to when the high point is positioned in a different field, to a different feature such as a conifer plantation, within a different map contour, a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered 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 summit height produced by LIDAR analysis is 207.6m and this is to the natural summit of the hill which is positioned at SN 61917 61109, this position is not given a spot height on the contemporary Ordnance Survey 1:25,000 Explorer map, and is approximately 4 metres north from where the high point of the raised field boundary is positioned. 

 

The full details for the hill are:

Group:  Mynydd Bach 

Name:  Cwm Gwenyn 

OS 1:50,000 map:  146

Summit Height:  207.6m (LIDAR) 

Summit Grid Reference (New Position):  SN 61917 61109 (LIDAR) 

Bwlch Height:  172.5m (LIDAR) 

Bwlch Grid Reference:  SN 62494 62171 (LIDAR) 

Drop:  35.1m (LIDAR)

 

Myrddyn Phillips (October 2022)