Mapping Mountains – Significant Height Revisions – 500m Twmpau and The Welsh Highlands – Uchafion Cymru

 

Mynydd Nodol (SH 865 393) 

There has been a Significant Height Revision that is retrospective to a hill that is listed in the 500m Twmpau and The Welsh Highlands – Uchafion Cymru, 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 two listings that this height revision applies to are:

500m Twmpau – Welsh hills at or above 500m and below 600m in height that have 30m minimum drop, with an accompanying sub category entitled the 500m Sub-Twmpau consisting of all Welsh hills at or above 500m and below 600m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  The list is authored by Myrddyn Phillips and is published on Mapping Mountains in Google Doc format.

500m Twmpau by Myrddyn Phillips

The Welsh Highlands – Uchafion Cymru – Welsh hills at or above 500m in height with 15m minimum drop, with an accompanying sub list entitled the Welsh Highland Subs, the criteria for which is all Welsh hills at or above 500m in height with 10m or more and below 15m of drop.  This list is authored by Myrddyn Phillips and Aled Williams with the Introduction to the list published on Mapping Mountains in November 2015 and the latest update relating to the list published on Mapping Mountains in January 2023.

The Welsh Highlands - Uchafion Cymru by Aled Williams and Myrddyn Phillips

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 (Region A, Sub-Region A2), 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 listings of the 500m Twmpau and The Welsh Highlands – Uchafion Cymru were first compiled, this hill was listed with a summit height of 539m, based on the spot height 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

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

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 1.0m higher than the originally listed summit height of 539m, which was based on the spot height that appears on the contemporary 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 this map.

 

ills of Wales, and are reproduced below@

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 and Aled Williams (July 2026)