Mapping Mountains – Significant Height Revisions – The 500-Metre Tops of England and Wales – Deweys

Rhobell Ganol (SH 785 274) 

There has been a Significant Height Revision to a hill that is listed in the The 500-Metre Tops of England and Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Rhobell Ganol (SH 785 274)

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

The 500-Metre Tops of England and Wales are affectionately known after their hill list compiler; Michael Dewey.  This list mixes metric and imperial height in its criteria to bookend up to the 2000ft height band and takes in all hills in England, Isle of Man and Wales that are 500m and above and below 2000ft (609.6m) in height that have 30m minimum drop. 

Mountain tables by Michael Dewey

Michael Dewey died on the 5^th November 2022.  His wife; Gillian wished for this list to be carried on and made provision for it to be co-authored by Myrddyn Phillips.  Michael will always be the compiler and originator, it’s just that he’s now got a co-author to safeguard and carry this list on.

The name the hill is listed by is Rhobell Ganol and it is adjoined to the Arenig Fawr group of hills, which are situated in the central part of North Wales, and it is positioned with a minor road to its north, the A470 road to its west and the A494 road to its east, and has the small community of Rhyd-y-main towards the south south-east.

Extract from the Ordnance Survey 1:50,000 Landranger map

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed with a non-interpolated summit height of 520+m, based on the uppermost 520m ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

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

Since the original publication of this list 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 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 Magic Maps website which hosts an interactive map originated from Ordnance Survey data.  This mapping has a 522m summit spot height applied to the southerly top that matches the summit position given the hill by LIDAR analysis. 

Extract from the interactive mapping hosted on the Magic Maps website

Another 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 gives a 522m spot height to a northerly top positioned at SH 78603 27533. 

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 Rhobell Ganol (SH 785 274)

LIDAR analysis gives the summit of this hill as 524.8m positioned at SH 78578 27406, 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 other GNSS equipment 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 524.8m and this was derived from LIDAR analysis, this is 4.8m higher than the originally listed summit height of 520+m, which was based on the non-interpolated uppermost 520m ring contour that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and 2.8m higher than the spot heights that appear on the Magic Maps interactive map and the interactive mapping hosted on the WalkLakes website. 

 

The full details for the hill are:

Group:  Arenig Fawr

Name:  Rhobell Ganol

OS 1:50,000 map:  124

Summit Height (New Height):  524.8m (LIDAR)

Summit Grid Reference:  SH 78578 27406 (LIDAR)  

Bwlch Height:  485.7m (LIDAR)

Bwlch Grid Reference:  SH 78549 26977 (LIDAR)

Drop:  39.1m (LIDAR)

 

Myrddyn Phillips (July 2026)