Mapping Mountains – Significant Height Revisions – The Welsh P15s

Bryn Llwyd (SH 396 644) 

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 Bryn Llwyd (SH 396 644)

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 Bryn Llwyd and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is encircled by the Newborough Forest, and has the coast to its south-west and the A4080 road to its north-east, and has the village of Niwbwrch (Newborough) towards the east north-east. 

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main P15 list with an estimated c 15m of drop, based on an estimated c 46m summit height and an estimated c 31m bwlch height, with both heights based on interpolation of 5m contouring that appear on the Ordnance Survey 1:25,000 Explorer 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 Bryn Llwyd

The summit height produced by LIDAR analysis is 43.4m and 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, this hill’s new listed summit height is 43.4m and this was derived from LIDAR analysis, this is 2.6m lower than the previous listed summit height of c 46m which was based on interpolation of the uppermost 45m 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:  Ynys Môn 

Name:  Bryn Llwyd 

OS 1:50,000 map:  114

Summit Height (New Height):  43.4m (LIDAR)                                                           

Summit Grid Reference:  SH 39665 64415 (LIDAR) 

Bwlch Height:  28.6m (LIDAR) 

Bwlch Grid Reference:  SH 39795 64510 (LIDAR) 

Drop:  14.9m (LIDAR) 

 

Myrddyn Phillips (May 2021)