Mapping Mountains – Summit Relocations – 200m Twmpau

 

Bwlchwernen Fach (SN 606 556) 

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 Bwlchwernen Fach (SN 606 556)

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 now listed by is Bwlchwernen Fach 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 north and west, and the A485 road to its south-east, and has the village of Llanddewibrefi 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 not included in the accompanying 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 hill were re-evaluated and it was listed with an estimated c 24m of drop, based on the 256m summit spot height positioned at SN 60698 55674 that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 232m bwlch height, based on interpolation of 10m contouring between 230m – 240m. 

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 257.4m positioned at SN 60696 55677.  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 Bwlchwernen Fach (SN 606 556)

The height produced by LIDAR analysis to the highest remaining natural ground of this hill is 256.35m positioned at SN 60699 55684, 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 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 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 256.35m and this is positioned at SN 60699 55684, this position is close to where the 256m spot height appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 7 metres north-eastward from the high point of the raised field boundary. 

 

The full details for the hill are: 

Group:  Mynydd Bach 

Name:  Bwlchwernen Fach 

OS 1:50,000 map:  146

Summit Height:  256.35m (LIDAR) 

Summit Grid Reference (New Position):  SN 60699 55684 (LIDAR) 

Bwlch Height:  230.7m (LIDAR) 

Bwlch Grid Reference:  SN 60598 55268 (LIDAR) 

Drop:  25.7m (LIDAR) 

 

Myrddyn Phillips (January 2024)