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

 

Stonewall Hill (SO 318 696) 

There has been a Significant Height Revision to a hill that is listed in the Y Pedwarau – The 400m Hills of Wales under the classification of National Top status (for details relating to the concept of National Tops see separate post), with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams. 

Stonewall Hill (SO 318 696)

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 that have 30m minimum drop, accompanying the main Y Pedwarau list are five categories of sub hills, with this hill being included in the 400m Sub-Pedwar category.  The criteria for 400m Sub-Pedwar status being all Welsh hills at or above 400m and below 500m in height that have 20m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

Y Pedwarau - The 400m Hills of Wales by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Stonewall Hill and it is adjoined to the Beacon Hill group of hills, which are situated in Mid and West Wales (Region B, Sub-Region B1), and it is positioned with the A4113 road to its north, a minor road to its immediate west and the B4355 road farther to its west, and has the town of Trefyclo (Knighton) towards the north-west.

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

When the original 400m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included as the concept of National Tops had not been devised.  The hill was subsequently analysed using contemporary Ordnance Survey maps and LIDAR for height and drop.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of Stonewall Hill (SO 318 696)

LIDAR analysis confirms this hill as over 400m in height, and as the summit has now been surveyed with the Trimble GeoXH 6000, it is this result that is being prioritised for listing purposes 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 summit 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. 

The Trimble GeoXH 6000 gathering data at the summit of Stonewall Hill

Therefore, this hill’s new listed summit height is 400.1m and this was derived from a Trimble GeoXH 6000 survey, this is 5.1m higher than the uppermost 395m ring contour taking in the summit of this hill on contemporary Ordnance Survey maps, although an erroneous 400m ring contour is shown toward the west. 

 

The full details for the hill are:

Group:  Beacon Hill 

Name:  Stonewall Hill

OS 1:50,000 map:  137, 148

Summit Height (New Height):  400.1m (converted to OSGM15)

Summit Grid Reference:  SO 31823 69655

Bwlch Height:  376.4m (converted to OSGM15)

Bwlch Grid Reference:  SO 31273 70279

Drop:  23.8m

 

Myrddyn Phillips and Aled Williams (October 2021)