Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Tomen y Mur (SH 705 386) 

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 and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Tomen y Mur (SH 705 386)

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 Tomen y Mur, and it is adjoined to the Arenig group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its immediate north, the A470 road to its west and further to its north, and has the village of Trawsfynydd towards the south. 

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 included in the sub list with an estimated c 14m of drop, based on an estimated c 300m summit height and an estimated c 286m bwlch height, with the former based on interpolation of the uppermost 300m ring contour that appears on the WalkLakes interactive map and the latter based on interpolation of 10m contouring between 280m – 290m that appears on the Ordnance Survey 1:25,000 Explorer map and the mapping on the WalkLakes website. 

Extract from 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 image of Tomen y Mur (SH 705 386)

The summit height produced by LIDAR analysis is 303.3m, and as the summit has now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes, 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. 

The Trimble GeoXH 6000 gathering data at the summit of Tomen y Mur

Therefore, this hill’s new listed summit height is 303.2m and this was derived from a Trimble GeoXH 6000 survey, this is 3.2m higher than the previous listed summit height of c 300m which was derived from interpolation of the uppermost 300m ring contour that appears on the WalkLakes interactive map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Arenig 

Name:  Tomen y Mur 

OS 1:50,000 map:  124

Summit Height (New Height):  303.2m (converted to OSGM15, Trimble GeoXH 6000)  

Summit Grid Reference:  SH 70544 38679 (Trimble GeoXH 6000) 

Bwlch Height:  288.1m (LIDAR) 

Bwlch Grid Reference:  SH 70674 38754 (LIDAR) 

Drop:  15.1m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

Myrddyn Phillips (April 2021)