Mapping Mountains – Summit Relocations – 100m Twmpau

 

Pt. 120.4m (SN 507 022) 

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 Pt. 120.4m (SN 507 022)

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 hill is being listed by the point (Pt. 120.4m) notation and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with a minor road to its east and north, the B4309 road to its west and the A476 road to its south, and has the town of Llanelli towards the south.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list with a none interpolated 120c summit height, based on the uppermost contour that appears at SN 50750 02281 on the contemporary 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-assessed and it was listed with an estimated c 24m of drop, based on an estimated c 121m summit height and an estimated c 97m bwlch height, with both heights based on interpolation of 5m contouring. 

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 121.7m positioned at SN 50761 02286.  However, this is to the top 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 Pt. 120.4m (SN 507 022)

The height produced by LIDAR analysis to the natural summit of this hill is 120.4m and is positioned at SN 50743 02266 and SN 50741 02271 and SN 50744 02273 and SN 50753 02271, 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 120.4m and is positioned at SN 50743 02266 and SN 50741 02271 and SN 50744 02273 and SN 50753 02271, this is relatively close to where the interpoated summit position was given, and is approximately 20 metres south-westward from the high point of the raised field boundary. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Pt. 120.4m 

OS 1:50,000 map:  159

Summit Height:  120.4m (LIDAR)               

Summit Grid Reference (New Position):  SN 50743 02266 & SN 50741 02271 & SN 50744 02273 & SN 50753 02271 (LIDAR) 

Bwlch Height:  98.1m (LIDAR) 

Bwlch Grid Reference:  SN 51034 02544 (LIDAR) 

Drop:  22.3m (LIDAR) 

 

Myrddyn Phillips (October 2024)