Mapping Mountains – Summit Relocations – 100m Twmpau

 

Pt. 162.1m (SN 508 034) 

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. 162.1n (SN 508 034)

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. 162.1m) 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 and the B4309 road to its west and a minor road and the A476 road to its east, 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 161m summit height, based on the spot height positioned on a minor road that appears at SN 50822 03406 on the contemporary Ordnance Survey 1:50,000 Landranger and 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 23m of drop, based on the 161m summit spot height and the 138m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 102.6m positioned at SN 50798 03441.  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. 162.1m (SN 508 034)

The height produced by LIDAR analysis to the natural summit of this hill is 162.1m and is positioned at SN 50860 03434 and SN 50868 03434, 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 162.1m and is positioned at SN 50860 03434 and SN 50868 03434, 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 62 metres eastward from the high point of the raised field boundary. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Pt. 162.1m 

OS 1:50,000 map:  159

Summit Height:  162.1m (LIDAR) 

Summit Grid Reference (New Position):  SN 50860 03434 & SN 50868 03434 (LIDAR) 

Bwlch Height:  138.2m (LIDAR) 

Bwlch Grid Reference:  SN 50307 03907 (LIDAR) 

Drop:  23.9m (LIDAR) 

 

Myrddyn Phillips (September 2024)