Mapping Mountains – Significant Height Revisions – 200m Twmpau

Coed Bryn y Garth Lwyd (SH 725 576) 

There has been a Significant Height Revision to a hill that is listed in the 200m Twmpau, 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 y Garth Lwyd (SH 725 576)

The criteria for the list that this height revision applies to are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Coed Bryn y Garth Lwyd, and this was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Moel Siabod group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A5 road to its north and the A4086 road to its west north-west, and has the village of Capel Curig towards the north-west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category, as with no significant contours of note on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map it was difficult to know whether any hill of note existed. 

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

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill it shows an uppermost 200m ring contour and bwlch contouring between 180m – 190m. 

Extract from the interactive mapping hosted on 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 summit image of Coed Bryn y Garth Lwyd (SH 725 576)

LIDAR analysis gives the summit of this hill as 202.2m positioned at SH 72557 57676, 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 202.2m and this was derived from LIDAR analysis, this is 22.2m higher than the highest contour in the vicinity of this hill that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moel Siabod 

Name:  Coed Bryn y Garth Lwyd 

OS 1:50,000 map:  115

Summit Height (New Height):  202.2m (LIDAR) 

Summit Grid Reference:  SH 72557 57676 (LIDAR) 

Bwlch Height:  179.7m (LIDAR) 

Bwlch Grid Reference:  SH 72372 57543 (LIDAR) 

Drop:  22.55m (LIDAR) 

 

Myrddyn Phillips (July 2025)