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

 

Three Barrows (SX 652 626) 

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 Three Barrows (SX 652 626)

The criteria for the list this height revision affects 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 are three categories of sub hills, the 400m Sub-Four category, the 390m Sub-Four category and the 390m Double Sub-Four category.  The list is co-authored by Myrddyn Phillips and Aled Williams and the 2^nd edition of the booklet containing this list was published by Mapping Mountains Publications on the 24^th April 2018.

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

The name the hill is listed by is Three Barrows, and it is adjoined to the Ryder’s Hill group of hills, which are situated in Dartmoor in the south-west of the country, and it is positioned with a series of minor roads to its south-west and south-east, and the B3213 road and the A38 road to its south, and has the town of Ivybridge towards the south. 

Extract from the Ordnance Survey 1:50,000 Landranger map

When the 1^st edition of the The Fours was published by Europeaklist in December 2013, this hill was listed with a 464m summit height, based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and this is also the summit height used when the 2^nd edition of the The Fours – The 400m Hills of England was published by Mapping Mountains Publications in April 2018. 

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.

The highest measurement recorded from LIDAR analysis is 464.1m positioned at SX 65310 62600.  However, LIDAR contouring and this position corresponds with that of an ancient cairn and as this is deemed not permanent in nature it is discounted from the summit height of this hill. 

LIDAR summit image of Three Barrows (SX 652 626)

LIDAR analysis gives the highest natural ground on this hill as 461.3m positioned at SX 65286 62610, and when compared to its originally listed summit height of 464m 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 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, the new listed summit height of this hill is 461.3m and this was derived from LIDAR analysis, this is 2.7m lower than its originally listed 464m summit height, which appears as a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Ryder’s Hill

Name:  Three Barrows

OS 1:50,000 map:  202

Summit Height (New Height):  461.3m (LIDAR)

Summit Grid Reference:  SX 65286 62610 (LIDAR)

Col Height:  426.7m (LIDAR)

Col Grid Reference:  SX 65153 63316 (LIDAR)

Drop:  34.6m (LIDAR)

 

Myrddyn Phillips and Aled Williams (March 2024)