Mapping Mountains – Significant Height Revisions – The Welsh P15s

Cefn Gwrhyd (SN 737 096) 

There has been a Significant Height Revision to a hill that is 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 Cefn Gwrhyd (SN 737 096)

The criteria for the list that this height revision applies to 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 name the hill is listed by is Cefn Gwrhyd, and it is adjoined to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the A4068 road to its north-east, the A474 road to its west, the A4067 road to its south-east and a minor road to its immediate east, and has the community of Y Gurnos towards the east.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included as with a 291m spot height in the vicinity of its summit and bwlch contouring between 280m – 290m that appear on the contemporary Ordnance Survey 1:25,000 Explorer map, it was judged not to meet the criterion for the main P15 or the accompanying P14 sub list. 

Extract from the Ordnance Survey 1:25,000 Explorer map

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill a 300m summit spot height is given. 

Extract from the interactive mapping hosted on the WalkLakes website

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 Cefn Gwrhyd (SN 737 096)

The summit height produced by LIDAR analysis is 300.5m and when compared to detail on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, 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 or Harvey 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 to the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 300.5m and this was derived from LIDAR analysis.  This is 9.5m higher than the 291m spot height and 5.5m higher than the estimated c 295m summit height if based on interpolation of the uppermost contour on the contemporary Ordnance Survey 1:25,000 Explorer map. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Mynydd Du 

Name:  Cefn Gwrhyd 

OS 1:50,000 map:  160

Summit Height (New Height):  300.5m (LIDAR)                                                           

Summit Grid Reference:  SN 73765 09682 (LIDAR) 

Bwlch Height:  283.1m (LIDAR) 

Bwlch Grid Reference:  SN 73543 09172 (LIDAR) 

Drop:  17.3m (LIDAR) 

 

Myrddyn Phillips (June 2024)