Mapping Mountains – Significant Height Revisions – The Fours – The 400m Hills of England

 

Mainstone Hill (SO 262 880) 

There has been a Significant Height Revision to a hill that is listed in the The Fours – The 400m Hills of England, with the summit height, col height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Mainstone Hill (SO 262 880)

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

The Fours – The 400m Hills of England - English hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main list is a sub list entitled the Sub-Fours, the criteria for which are all English hills at or above 400m and below 500m in height that have 15m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available to download in Google Doc format from the Mapping Mountains site.

The Fours - The 400m Hills of England by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Mainstone Hill, and this was derived from an 1847 inclosure award, and it is adjoined to the Cilfaesty group of hills, which are situated adjoined to the Welsh border, and it is positioned encircled by minor roads, with the B4385 road and the A489 road farther to its north, the B4368 road farther to its south-west and the A488 road farther to its east, and has the town of Bishop’s Castle towards the east.

When the Introduction to the first group of hills for the updated and revised listing of the The Fours – The 400m Hills of England was published by Mapping Mountains on the 10^th September 2022, it was announced that the accompanying sub lists were being revised with the two 390m categories dispensed with and the criteria and name of the 400m Sub-Fours revised.  The one accompanying sub list is now named the Sub-Fours with its criteria being all English hills 400m and above and below 500m in height that have 15m and more and below 30m of drop.

Prior to this revision this hill was listed with 15m of drop, based on the 418m summit spot height and the 403m col spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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 Mainstone Hill (SO 262 880)

LIDAR analysis gives the summit height of this hill as 421.3m and when compared to its originally listed 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 421.3m and this was derived from LIDAR analysis, this is 3.2m higher than the originally listed 418m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are:

Group:  Cilfaesty

Name:  Mainstone Hill

OS 1:50,000 map:  137

Summit Height (New Height):  421.3m (LIDAR)

Summit Grid Reference:  SO 26273 88041 (LIDAR)

Col Height:  404.3m (LIDAR)

Col Grid Reference:  SO 25878 88118 (LIDAR)

Drop:  17.0m (LIDAR)

 

Myrddyn Phillips and Aled Williams (April 2026)