Mapping Mountains – Significant Height Revisions – The Welsh P15s

Coed Bryn Twr (SH 539 400) 

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 Coed Bryn Twr (SH 539 400)

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 Coed Bryn Twr, and it is adjoined to the Moel Hebog group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with a minor road and the A487 road to its north and the A497 road to its south, and has the town of Porthmadog towards the east south-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 19m of drop, based on the 66m 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 and an estimated c 47m bwlch height, based on interpolation of 10m contouring between 40m – 50m. 

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 Coed Bryn Twr (SH 539 400)

The summit height produced by LIDAR analysis is 63.7m and when compared to detail on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 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, the new listed summit height of this hill is 63.7m and this was derived from LIDAR analysis.  This is 2.3m lower than the 66m spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage. 

 ills of Wales, and are reproduced below@

The full details for the hill are: 

Group:  Moel Hebog 

Name:  Coed Bryn Twr 

OS 1:50,000 map:  124

Summit Height (New Height):  63.7m (LIDAR)                                                           

Summit Grid Reference:  SH 53944 40079 (LIDAR) 

Bwlch Height:  46.3m (LIDAR) 

Bwlch Grid Reference:  SN 53944 40155 (LIDAR) 

Drop:  17.4m (LIDAR) 

 

Myrddyn Phillips (August 2024)