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

 

Carn yr Hyrddod (SN 790 704) and Lan Ddu Fawr (SN 787 699) 

There has been a Summit Relocation due to a newly recognised twin top to a hill that is listed in the The 500-Metre Tops of England and Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips and Aled Williams. 

Lan Ddu Fawr (SN 787 699) from the summit of Carn yr Hyrddod (SN 790 704)

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.

The name the hill is now listed by is Carn yr Hyrddod for the northerly of these two tops and Lan Ddu Fawr for the southerly top, and it is adjoined to the Carn yr Hyrddod group of hills, which are situated in the northern part of South Wales, and it is positioned with a minor road to its south and the B4343 road to its west, and has the village of Pontrhydfendigaid towards the west south-west. 

Extract from the Ordnance Survey 1:25,000 Explorer 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 594m, based on the spot height positioned at SN 790 704 that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

To the south of the originally listed Dewey is the summit of Lan Ddu Fawr which is given a 593m summit spot height adjoined to a triangulation pillar that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.  As the northerly hill was given as 1m higher on Ordnance Survey maps it was this hill that was listed as the Dewey.

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 image of Lan Ddu Fawr (SN 787 699) and Carn yr Hyrddod (SN 790 704)

The numerical detail produced from LIDAR analysis for each hill appears below: 

 

For Carn yr Hyrddod LIDAR gives:

1m DSM:  593.059m at SN 79067 70424

1m DTM:  592.780m at SN 79069 70424

 

For Lan Ddu Fawr LIDAR gives:

1m DSM:  593.010m at SN 78757 69975

1m DTM:  592.809m at SN 78761 69976 and SN 78762 69977

 

As LIDAR has difficulty separating these summits, with 1m DSM giving Carn yr Hyrddod the higher by 5cm and 1m DTM giving Lan Ddu Fawr the higher by 3cm, these hills were prioritised for a Trimble GeoXH 6000 survey, the results of which appear below: 

 

Carn yr Hyrddod:

592.729m at SN 79069 70421

 

Lan Ddu Fawr:

592.724m at SN 78759 69974

 

The resulting Trimble survey gave Carn yr Hyrddod the higher by 5mm, and as all of these results are within the margin of uncertainty applicable to each method the qualifying Dewey is now listed as a twin top. 

Gathering data at the summit of Carn yr Hyrddod

Gathering data at the summit of Lan Ddu Fawr

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 when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill 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 the listing of a new twin summit or de-twinning of a summit, 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.

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 592.7m and this is positioned at SN 79069 70421 and SN 78759 69974.  The new twin top is given a 593m spot height adjoined to a triangulation pillar on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 1,500 metres south-westward from where the originally listed and now twin summit is positioned. 

 

The full details for the hill are: 

Group:  Carn yr Hyrddod 

Name:  Carn yr Hyrddod and Lan Ddu Fawr 

OS 1:50,000 map:  135, 147

OS 1:25,000:  213 

Summit Height:  592.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference (New Position):  SN 79069 70421 (Trimble GeoXH 6000 for Carn yr Hyrddod) and SN 78759 69974 (Trimble GeoXH 6000 for Lan Ddu Fawr)  

Bwlch Height:  424.8m (LIDAR)

Bwlch Grid Reference:  SN 79547 66955 (LIDAR)

Drop:  168.0m (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

Myrddyn Phillips (July 2025)