Mapping Mountains - Summit Relocations – The 500-Metre Tops of England and Wales – Deweys

Mynydd Trawsnant (SN 824 485) 

There has been a Summit Relocation to a hill that is listed in the The 500-Metre Tops of England and Wales, with the summit height, col height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams. 

LIDAR image of Mynydd Trawsnant (SN 824 485)

The criteria for the list that this summit relocation applies to are:

The 500-Metre Tops of England and Wales are affectionately known after their hill list compiler; Michael Dewey.  This list mixes metric and imperial height in its criteria to bookend up to the 2000ft height band and takes in all hills in England, Isle of Man and Wales that are 500m and above and below 2000ft (609.6m) in height that have 30m minimum drop. 

Mountain tables by Michael Dewey

Michael Dewey died on the 5^th November 2022.  His wife; Gillian wished for this list to be carried on and made provision for it to be co-authored by Myrddyn Phillips.  Michael will always be the compiler and originator, it’s just that he’s now got a co-author to safeguard and carry this list on. 

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

The name the hill is listed by is Mynydd Trawsnant and it is adjoined to the Drygarn Fawr group of hills, which are situated in the northern part of South Wales, and it is positioned with a minor road to its north-west and the A483 road to its south-east, and has the town of Llanwrtyd towards the east south-east. 

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

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed with a summit height of 517m, based on the spot height positioned at SN 821 484 that appears on 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 Mynydd Trawsnant (SN 824 485)

The details ascertained from LIDAR for the new and old summit position are given below, but as the upper section of this hill is forested the accuracy of LIDAR modelling may be compromised.  However, the pre-forestry levelled height on the Ordnance Survey series of Six-Inch maps substantiates the new LIDAR position. 

 

New summit:  516.559m at SN 82452 48564 

Old summit:  516.489m at SN 82162 48430 

 

The above detail compared to the summit position previously given comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies to when the high point is positioned in a different field, to a different feature such as a conifer plantation, within a different map contour, a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist. 

Extract from the Ordnance Survey series of Six-Inch maps

Therefore, the summit height produced by LIDAR analysis is 516.6m and this is positioned at SN 82452 48564.  This position is not given a spot height on the contemporary Ordnance Survey 1:25,000 Explorer map, but is close to where the 517m spot height appears on the contemporary 1:50,000 Landranger map and is approximately 320 metres north-eastward from where the previously listed summit is positioned. 

 

The full details for the hill are: 

Group:  Drygarn Fawr 

Name:  Mynydd Trawsnant 

OS 1:50,000 map:  147

OS 1:25,000:  187 

Summit Height:  516.6m (LIDAR)

Summit Grid Reference (new position):  SN 82452 48564 (LIDAR)  

Bwlch Height:  378.3m (LIDAR)

Bwlch Grid Reference:  SN 82781 50423 (LIDAR)

Drop:  138.3m (LIDAR)

 

Myrddyn Phillips (August 2023)