Mapping Mountains – Significant Height Revisions – The Welsh P15s

Plas Cemlyn (SH 327 936) 

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 Plas Cemlyn (SH 327 936)

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 Plas Cemlyn 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 almost entirely surrounded by the coast with minor roads to its south and south-east, and has the village of Cemaes towards the east. 

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

When the listing that became known as The Welsh P15s was being compiled, this hill was listed with an estimated c 15m of drop, based on the 18m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, with this spot height now appearing on the interactive mapping on the Magic Maps website and an estimated c 3m bwlch height based on interpolation of 5m contouring between 0 – 5m. 

Extract from the Magic Maps 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. 

The summit height produced by LIDAR analysis is 20.3m 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. 

LIDAR summit image of Plas Cemlyn

Therefore, this hill’s new listed summit height is 20.3m and this was derived from LIDAR analysis, this is 2.3m higher than the previous listed summit height of 18m which was based on the summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Ynys Môn 

Name:  Plas Cemlyn 

OS 1:50,000 map:  114

Summit Height (New Height):  20.3m (LIDAR)                                                           

Summit Grid Reference:  SH 32729 93641 (LIDAR) 

Bwlch Height:  2.3m (LIDAR) 

Bwlch Grid Reference:  SH 32454 93517 (LIDAR) 

Drop:  18.0m (LIDAR) 

 

Myrddyn Phillips (June 2021)