Mapping Mountains – Significant Height Revisions – The Welsh P15s

Wylfa (SH 353 936) 

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 confirmed by LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Wylfa (SH 353 936)

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 Wylfa and this was derived from the Tithe map 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 positioned with the coast to its north, west and east and a minor road and the A5025 road farther to its south-east, and has the village of Cemaes towards the east. 

When the listing that became known as The Welsh P15s was being compiled, this hill was listed with an estimated c 18m of drop, based on an estimated c 36m summit height and an estimated c 18m 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 Wylfa (SH 353 936)

The summit height produced by LIDAR analysis is 38.2m 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 38.2m and this was derived from LIDAR analysis, this is 2.2m higher than the previous listed summit height of c 36m which was estimated from interpolation of 5m contouring that appear 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:  Wylfa 

OS 1:50,000 map:  114

Summit Height (New Height):  38.2m (LIDAR)                                                           

Summit Grid Reference:  SH 35343 93609 (LIDAR) 

Bwlch Height:  20.45m (LIDAR) 

Bwlch Grid Reference:  SH 35438 93650 (LIDAR) 

Drop:  17.8m (LIDAR) 

 

Myrddyn Phillips (June 2021)