Mapping Mountains – Significant Height Revisions – Y Pedwarau – The 400m Hills of Wales

Grugfryn (SJ 248 356)

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau – The 400m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Aled Williams.

Grugfryn (SJ 248 356)

The criteria for the list this height revision affects are:

Y Pedwarau – The 400m Hills of Wales.  Welsh hills at or above 400m and below 500m in height with 30m minimum drop, the list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

The name of the hill is Grugfryn and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is encircled by minor roads with the B4579 road to its south-west, and has the village of Glyn Ceiriog towards the west north-west.

When the original 400m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website the summit height of this hill was listed as 403m which was based on the spot height positioned at SJ 24849 35660 that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The upper section of this hill is immersed in a conifer plantation and the 403m spot height appears adjacent to a forest track.  This area was visited and a mound on the western side of the track was surveyed with the Trimble GeoXH 6000, resulting in a 404.05m height positioned at SJ 24842 35652.

The Trimble GeoXH 6000 gathering data from the mound beside the indistinct forest track

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 Grugfryn

The summit height produced by LIDAR analysis is 405.1m positioned at SJ 24835 35615, this is not a dramatic height revision when compared to some revised heights, but it does come 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 or Harvey 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 to the data produced by the Trimble or by LIDAR analysis.

Therefore, this hill’s new listed summit height is 405.1m and this was produced by LIDAR analysis, this is 2.1m higher than the 403m spot height that appears on contemporary Ordnance Survey 1:25,000 Explorer maps.

The full details for the hill are:

Name:  Grugfryn

OS 1:50,000 map:  126

Summit Height (New Height):  405.1m (LIDAR)

Summit Grid Reference:  SJ 24835 35615 (LIDAR)

Bwlch Height: 361.9m (LIDAR) 

Bwlch Grid Reference:  SJ 24456 36336 (LIDAR) 

        

Drop:  43.2m (LIDAR)

For details on the Trimble GeoXH 6000 survey of this hill

Myrddyn Phillips and Aled Williams (November 2019)