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

Mynydd Nodol (SH 865 393) 

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 a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips and LIDAR bwlch analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips. 

Mynydd Nodol (SH 865 393)

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 Mynydd Nodol 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 Llyn Celyn and the A4212 road to its north and a minor road to its south, and has the town of Y Bala towards the south-east.

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 530+m, based on the uppermost 530m ring contour that appeared on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

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

However, it was not until the survey with the Trimble GeoXH 6000 and 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. 

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Nodol (SH 865 393)

The survey with the Trimble GeoXH 6000 gives the summit of this hill as 540.0m positioned at SH 86514 39339, 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 540.0m and this was derived from a Trimble GeoXH 6000 survey, this is 10.0m higher than the originally listed summit height of 530+m, which was based on the non-interpolated uppermost 530m ring contour that appeared on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and 1m height than the 539m summit spot height that now appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and importantly this height signifies that an uppermost 540m ring contour is missing from these maps. 

 

The full details for the hill are:

Group:  Arenig Fawr

Name:  Mynydd Nodol

OS 1:50,000 map:  124, 125

Summit Height (New Height):  540.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 86514 39339 (Trimble GeoXH 6000)  

Bwlch Height:  370.8m (LIDAR)

Bwlch Grid Reference:  SH 85660 39014 (LIDAR)

Drop:  169.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (August 2026)