Mapping Mountains – Significant Height Revisions – 100m Twmpau and Y Trechol – The Dominant Hills of Wales

 

Bolton Beacon (SM 917 112) 

There has been a Significant Height Revision to a hill that is listed in the 100m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from detail produced by Joe Nuttall in his surface analysis progamme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Bolton Beacon (SM 917 112)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 100m Twmpau by Myrddyn Phillips

Y Trechol – The Dominant Hills of Wales – Welsh P30 hills whose prominence equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height.  The list is authored by Myrddyn Phillips with the Introduction to the start of the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Bolton Beacon and this was derived from a variety of Ordnance Survey maps, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads, with the B4327 road farther to its north-west and the A4076 road farther to its east, and has the village of Johnston towards the east south-east.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was listed with a summit height of 110m, based on the spot height adjoined to a triangulation pillar positioned at SM 91681 11268 which appeared on the contemporary 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 Bolton Beacon (SM 917 112)

The summit height produced by LIDAR analysis is 113.4m positioned at SM 91710 11288, 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, the new listed summit height of this hill is 113.4m and this was derived from LIDAR analysis, this is 3.4m higher than the originally listed summit height of 110m, which was based on the spot height adjoined to a triangulation pillar which appeared 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:  Garn Fawr 

Name:  Bolton Beacon 

OS 1:50,000 map:  157, 158

Summit Height (New Height):  113.4m (LIDAR) 

Summit Grid Reference:  SM 91710 11288 (LIDAR) 

Bwlch Height:  66.8m (LIDAR) 

Bwlch Grid Reference:  SM 89719 12333 & SM 89721 12333 (LIDAR) 

Drop:  46.7m (LIDAR) 

Dominance:  41.13% (LIDAR) 

 

Myrddyn Phillips (September 2023)