Mapping Mountains – Hill Reclassifications – The 500-Metre Tops of England and Wales – Deweys

Twynwalter (SN 828 175) – Dewey deletion

This is one in a series of retrospective Hill Reclassification posts that detail hills whose status has altered in the listing of the Deweys and where I have had direct association with the status change.  These posts will tie in with a forthcoming Change Register giving detail to this list and its alterations since publication in the Mountain tables book.

LIDAR image of Twynwalter (SN 828 175)

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.

This list formed one of a number of lists that appeared in the Mountain tables book published by Constable in 1995 and at the time of publication comprised 373 hills with 164 in England, 5 in the Isle of Man and 204 in Wales.  The Deweys have undergone extensive revision since first publication with the initial stages forming the basis of this revision given below:

1995    Mountain tables published by Constable with 373 hills listed as Deweys.

April 2000    Strider (LDWA quarterly booklet) publishes contact details for David Purchase and Myrddyn Phillips who have found and list 24 and 14 possible new 500m tops respectively.

It was expanded versions of the above two lists that formed the basis of the next publication:

25^th May 2000    List of Possible 500 Metre Tops by Michael Dewey listing 44 hills.

David Purchase expands his Additional Dewey 500m Hills and Myrddyn Phillips produces lists of English 500m hills to measure and Welsh 500m hills to measure.

These lists formed the basis of the next publication:

29^th June 2000    Possible/Probable 500’s by Michael Dewey listing 77 hills.  Michael adopts following protocol; if one person proposes that a top should qualify as a 500 by personal survey, and is then confirmed by a second person, it should then be promoted to the main list.

April 2002    The 500+ Tops of England and Wales – The ‘New Deweys’ published in the Strider booklet and listing 66 new qualifying hills.

25^th May 2006    Rob Woodall republishes Michael’s main and possible/probable lists on the RHB Yahoo group file database. 

 

Mountaintables by Michael Dewey

The details for this deletion appear below:

The name the hill was listed by in the Deweys is Twynwalter, and it is adjoined to the Mynydd Du group of hills in the Brecon Beacons which are situated in south Wales, and it is positioned with the A4067 road to its east and south, and has the hamlet of Glyntawe towards the south-east and the village of Abercraf towards the south.

This hill was included in the original 1995 Constable publication, and listed with a 509m summit height which appears as a spot height on Ordnance Survey 1:25,000 Explorer maps and the Outdoor Leisure maps of the day.  Its drop value was later given as an estimated c 32m with its bwlch height an estimated c 477m based on interpolation of 10m contouring between 470m – 480m.

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

As the listed drop value was near the minimum 30m required for qualification to this list the hill was prioritised for a GNSS survey and this took place on the 10^th July 2014.  The summit of this hill was surveyed by Myrddyn Phillips using a Trimble GeoXH 6000.  However, this was the last of many surveys conducted during the day that included visiting the remotest spot in mainland Wales as well as one of the remotest hills in Wales and lack of daylight hours dictated that only the summit could be surveyed.  The survey resulted in a 502.1m summit height, which is 6.9m lower than the 509m spot height on the Ordnance Survey 1:25,000 Explorer map.

The Trimble GeoXH 6000 gathering data at the summit of Twynwalter (SN 828 175)

However, it was not until LIDAR became available that the details for the bwlch of 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 bwlch image of Twynwalter

The summit and bwlch heights for this hill were subsequently ascertained from LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips, resulting in a bwlch height of 475.9m and when coupled with the 502.1m summit height from the Trimble GeoXH 6000 survey this gives the hill 26.25m of drop.

These details were forwarded to the list author; Michael Dewey and the hill was subsequently deleted from his list on 19^th February 2019.

 

The full details for the hill are:

Name:  Twynwalter

OS 1:50,000 map:  160

OS 1:25,000 map:  12

Summit Height:  502.1m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 82837 17506 (Trimble GeoXH 6000)

Bwlch Height:  475.9m (LIDAR)

Bwlch Grid Reference:  SN 82597 17917 (LIDAR)

Drop:  26.25m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (October 2020)