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:
25th
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:
29th
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.
25th
May 2006 Rob Woodall republishes
Michael’s main and possible/probable lists on the RHB Yahoo group file
database.
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 10th 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 19th
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)
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