Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Pt. 83.1m (SN 539 026) 

There has been a Significant Height Revision to a hill that was listed in The Welsh P15s, 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. 83.1m (SN 539 026) 

The criteria for the list that this hill used to be included in are:

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The hill is being listed by the point (Pt. 83.1m) 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 the A476 road to its north-west and minor roads to its immediate south and east, and has the town of Llanelli towards the south-west.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the P14 sub list with an estimated c 14m of drop, based on an estimated c 86m summit height derived from interpolation of the uppermost 85m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map, and the 72m 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 summit image of Pt. 83.1m (SN 539 026)

The summit height produced by LIDAR analysis is 83.1m and when compared to the originally listed summit height this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR.  Also included are hills whose summit map data is missing an uppermost ring contour when compared with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 83.1m and this was derived from LIDAR analysis.  This is 2.9m lower than the estimated c 86m summit height previously lsited and which was derived from interpolation of the uppermost 85m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Pt. 83.1m 

OS 1:50,000 map:  159

Summit Height (New Height):  83.1m (LIDAR)                                                           

Summit Grid Reference:  SN 53910 02627 (LIDAR)                                                  

Bwlch Height:  70.6m (LIDAR) 

Bwlch Grid Reference:  SN 54258 03092 (LIDAR) 

Drop:  12.6m (LIDAR) 

 

Myrddyn Phillips (January 2025)