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

 

South Hessary Tor (SX 597 723) 

There has been a Significant Height Revision that is retrospective and due to a summit relocation 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 South Hessary Tor (SX 597 723)

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 South Hessary Tor, and it is adjoined to the High Willhays group of hills, which are situated in Dartmoor in the south-west of the country, and it is positioned with the B3212 road to its north-west and a minor road to its east, and has the village of Princetown towards the north-west.

When the listing that is now known as The Fours – The 400m Hills of England was originally compiled this hill appeared under the name of South Hessary Tor North Top with a summit height of 451m positioned at SX 594 730, based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

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

When the 1^st edition of the The Fours was published by Europeaklist in December 2013, this hill was listed as South Hessary Tor with a 454m summit height positioned at SX 597 723, based on the spot height that appears on the Harvey Maps 1:40,000 British Mountain Map to Dartmoor.  This is also the summit height and position 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 Harvey Maps 1:40,000 British Mountain Map to Dartmoor

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 South Hessary Tor (SX 597 723)

LIDAR analysis gives the highest ground on this hill as 454.1m positioned at SX 59712 72361, as opposed to the originally listed summit position of 450.7m positioned at SX 59448 73056, 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 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 listed summit height of this hill is 454.1m and this was derived from LIDAR analysis, this is 3.1m higher than the originally listed summit height which was based on the 451m spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  High Willhays

Name:  South Hessary Tor

OS 1:50,000 map:  191

Summit Height (New Height):  454.1m (LIDAR)

Summit Grid Reference:  SX 59712 72361 (LIDAR)

Col Height:  418.9m (LIDAR)

Col Grid Reference:  SX 58746 73462 (LIDAR)

Drop:  35.2m (LIDAR)

 

Myrddyn Phillips and Aled Williams (February 2024)