Mapping Mountains – Significant Height Revisions – The Welsh P15s

Graig Pen y Bont (SO 349 055) 

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 Graig Pen y Bont (SO 349 055)

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 now listed by is Graig Pen y Bont, and this was derived from the Tithe map, and it is adjoined to the Ysgyryd Fawr group of hills, which are situated in the south-eastern part of South Wales (Region B, Sub-Region B4), and it is positioned with minor roads to its north and east, and the B4598 road to its west, and has the town of Brynbuga (Usk) towards the south-east.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included as with an uppermost 60m contour and bwlch contouring between 50m – 60m that appear on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, it was judged not to meet the criterion for the main P15 or the accompanying P14 sub list, with subsequent interpolation giving it an estimated c 68m summit height. 

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 Graig Pen y Bont (SO 349 055)

The summit height produced by LIDAR analysis is 70.3m 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, 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 70.3m and this was derived from LIDAR analysis.  This is 10.3m higher than the uppermost 60m contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and 2.3m higher than the subsequent interpolated height of c 68m. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Ysgyryd Fawr 

Name:  Graig Pen y Bont 

OS 1:50,000 map:  161

Summit Height (New Height):  70.3m (LIDAR)                                                           

Summit Grid Reference:  SO 34901 05538 (LIDAR)                                                  

Bwlch Height:  55.9m (LIDAR) 

Bwlch Grid Reference:  SO 34994 05530 (LIDAR) 

Drop:  14.4m (LIDAR) 

 

Myrddyn Phillips (May 2025)