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

Pt. 574.9m (SH 663 500) 

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 TrimbleGeoXH 6000 survey conducted by Aled Williams. 

Pt. 574.9m (SH 663 500).  Photo: Aled Williams

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 hill is being listed by the point (Pt. 574.9m) notation and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region C1, Sub-Region C1A), and it is positioned with the A498 road to its north-west, a minor road to its west and the A470 road to its east, and has the town of Blaenau Ffestiniog 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 an estimated summit height of c 578m, based on a 70ft (21.3m) basic levelling survey conducted by Myrddyn Phillips and interpolation of 10m contouring that appears on the Ordnance Survey 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 that the details for this hill could be accurately re-assessed.  This survey gives the summit of this hill as 574.9m positioned at SH 66301 50001, 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. 

The Trimble GeoXH 6000 gathering data at the summit of Pt. 574.9m (SH 663 500).  Photo: Aled Williams

Therefore, the new listed summit height of this hill is 574.9m and this was derived from a Trimble GeoXH 6000 survey, this is 3.1m lower than the originally listed summit height of c 578m, which was based on a 70ft (21.3m) basic levelling survey conducted by Myrddyn Phillips and interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Pt. 574.9m

OS 1:50,000 map:  115

Summit Height (New Height):  574.9m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 66301 50001 

Bwlch Height:  552.9m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference:  SH 66287 50113

Drop:  22.0m

 

Myrddyn Phillips and Aled Williams (September 2026)