Mapping Mountains – Significant Height Revisions – 600m Twmpau and The Welsh Highlands – Uchafion Cymru

Craig Ysgafn (SH 659 443) 

There has been a Significant Height Revision to a hill that is listed in the 600m Twmpau and the The Welsh Highlands – Uchafion Cymru, 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 Craig Ysgafn (SH 659 443)

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

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m 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.

600m 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 Craig Ysgafn and it is adjoined to the Moelwyn Mawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A4085 road to its west, the A487 road to its south and the A496 road to its east, and has the town of Blaenau Ffestiniog towards the east north-east.

When the list of Welsh 500m P15s that later became known as the The Welsh Highlands – Uchafion Cymru was first compiled, this hill was listed with an estimated c 26m of drop, based on the 689m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 663m bwlch height, based on interpolation of 10m contouring between 660m – 670m. 

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 Craig Ysgafn (SH 659 443)

LIDAR analysis gives the summit of this hill as 690.0m positioned at SH 65994 44332, and when compared to its originally listed summit height 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 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 690.0m and this was derived from LIDAR analysis, this is 1.0m higher than the originally listed summit height of 689m, which was based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map and importantly this height signifies that an uppermost 690m ring contour is missing from this map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Moelwyn Mawr

Name:  Craig Ysgafn

OS 1:50,000 map:  124

Summit Height (New Height):  690.0m (LIDAR)

Summit Grid Reference:  SH 65994 44332 (LIDAR)  

Bwlch Height:  663.4m (LIDAR)

Bwlch Grid Reference:  SH 66001 44596 (LIDAR)

Drop:  26.6m (LIDAR)

 

Myrddyn Phillips and Aled Williams (May 2026)