Mapping Mountains – Significant Name Changes – 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 Significant Name Change 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. 

Carn yr Hyrddod (SN 790 704)

The criteria for the list that this name change 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 twin top hill 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. 

Lan Ddu Fawr (SN 787 699)

When the original list of Deweys appeared in the Mountain tables book published by Constable in 1995, this hill was listed under the name of Waun Claerddu 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 1:25,000 Explorer 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. 

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

However, these two summits cannot be split in height, with the difference produced by LIDAR analysis and a Trimble GeoXH 6000 survey being within the margin of uncertainty applicable to each method.  Therefore, these summits are now listed as a Twin Top Dewey.

When Michael originally compiled this list the priority hill was listed as Waun Claerddu.  This name relates to the large water basin/bog to the south-east of the southerly of these two summits.

The northerly hill has the name Carn yr Hyrddod applied to its ancient cairn on the contemporary Ordnance Survey 1:25,000 Explorer and 1:50,000 Landranger map and this is also the locally known name for this hill.

The southerly summit has the name Llan Ddu Fawr applied to it on the contemporary Ordnance Survey 1:25,000 Explorer map, whilst the Ordnance Survey Six-Inch map has Lan Ddu Fawr.  The word Lan can have the meaning of side/edge/up/hill and it is this word that is preferred, with Lan Ddu Fawr also the locally known name for the southerly of these summits.  Therefore, the word Lan is preferred to Llan, as per Ordnance Survey Six-Inch maps. 

Extract from the Ordnance Survey series of Six-Inch maps

Therefore, this hill is now listed as a Twin Top in the Deweys, with the name of the northerly twin top now listed as Carn yr Hyrddod and the name of the southerly twin top now listed as Lan Ddu Fawr, and this was derived from a combination of sources including contemporary Ordnance Survey maps and the series of Ordnance Survey Six-Inch maps, local enquiry and advice given by Aled Williams relating to the confines of the use of the words Lan and Llan in Wales. 

 

The full details for the hill are: 

Group:  Carn yr Hyrddod 

Name:  Carn yr Hyrddod and Lan Ddu Fawr 

Previously Listed Name:  Waun Claerddu

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:  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)

 

My thanks to Aled Williams for advice relating to the listed name of this hill 

Myrddyn Phillips (August 2025)

  

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)

 

 

Mapping Mountains – Trimble Surveys – Carn yr Hyrddod

 

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

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

Due to independent surveying and LIDAR analysis there are few important hill surveys that remain.  Lan Ddu Fawr and Carn yr Hyrddod are two of these hills.  They are placed in the open grasslands of the Elenydd, approximately 0.5km apart.  Both are topped by the remains of ancient cairns, with scattered rock at their summits.

Considering the wild surrounds of each hill, they are relatively easy to access, with the minor road leading toward the Llynnau Teifi to their south.  The paved section of this road turns to track and continues to Claerwen; one of the remotest farms in Wales.  The track continues from this farm around the northern section of the Claerwen Reservoir, before turning back to a paved road and its continuation east toward Rhayader Gwy. 

The Llynnau Teifi

Lan Ddu Fawr is given a 593m spot height on contemporary Ordnance Survey maps with a triangulation pillar perched in the remains of the ancient cairn which is now fashioned as a wind shelter.  With the flush bracket adjoined to the trig pillar recorded as 592.836m in height.  Whilst, Carn yr Hyrddod is given a 594m summit spot height, which is coloured beige on contemporary Ordnance Survey maps, this indicates an air height, the accuracy of which is not as good compared to that of the flush bracket height.

As current maps give Carn yr Hyrddod 1m higher, it is this hill that is prioritised for listing purposes.  Its classification can be viewed as important as it is listed as a Marilyn, Dewey, Dodd, a Welsh Highland P15 and a 500m Twmpau.  Confusingly the hill originally appeared in the Marilyn and Dewey listings as Waun Claerddu, which applies to land incorporating a large bog to the south-east of the summit of Lan Ddu Fawr.  To add confusion upon confusion the Marilyn list has recently changed its listed name to Lan Ddu Fawr, which applies to the lower map heighted hill and not the higher. 

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

These hills had been on our surveying radar for over a year as the higher map heighted hill is one of our named hills for our Groups within our Regions and Sub Regions.  Whilst, the lower hill is also close to Welsh Highland P15 status with LIDAR giving it 14.6m of drop.  Therefore, our plan was to survey each summit, gathering at least two - three data sets from different positions and also to survey the connecting bwlch between the hills.  The weather forecast for the day’s adventure gave clear blue skies, albeit with a brisk chilling easterly wind.

I met Aled at Dinas Mawddwy at 7.30am and proceeded to drive toward Aberystwyth and then the roads heading south before connecting with the narrow road leading to Llynnau Teifi.  We parked close to a cattle grid and fence that headed up toward our hills.  As I opened the car door, the wind hit me and I quickly changed from shorts to walking trousers.  Thankfully Aled had brought spare gloves which I needed as I had dispensed with my winter gear in favour of a thin pair that would only protect from a slight chill, and that easterly wind definitely needed extra protection as it was chilling to the bone.

From where we parked there are two options for a relatively easy route toward the first of our hills; Lan Ddu Fawr.  One is a wide shaly track rising steeply above the road; the other is a comfortably gradiented green vehicle track that headed up beyond the cattle grid and would in time follow the course of a fence.  I was in following Aled mode and he soon headed up the steep shaly section.  Beyond was open hillside and miles of it in every direction.  This is one of the appeals of the Elenydd and few places in Wales can compare. 

Heading to the hill

Our route soon joined the remains of a leet that contoured around grassed hummocks to our west.  Aled soon left me to bag a P10 sub.  He had a number of these on his planned route, which would leave me on the summit of Carn yr Hyrddod and take a circuitous route across the grass and bog toward Bryn Dafydd, before heading back to Trawsallt and continuing to Disgwylfa and eventually more sub hills before reaching the relative comfort of the paved road below.  This would leave me to survey the summits of Carn yr Hyrddod and Lan Ddu Fawr, via the connecting bwlch, surveying all necessary points on the way, before heading back on our inward route to meet Aled back at the car. 

Lan Ddu Fawr above Waun Claerddu

Aled soon bagged the P10 sub and rejoined me as the leet headed toward a forlorn looking gate.  Here we rested whilst I set the Trimble up on a gate post to attune it with satellite configuration.  From here we contoured around the upper part of Waun Claerddu before heading up the steep southerly slopes of Lan Ddu Fawr, which took us to its summit.  By now we were exposed to the wind and it whisked across the hill, so much so that unless hunkered down behind the wind shelter it proved difficult to communicate.  We spent ten minutes or so assessing the summit area and judging what points needed to be surveyed.  Having picked three candidates we headed off following a sheep track on the moor down toward the connecting bwlch. 

Leaving the summit of Lan Ddu Fawr and heading toward Carn yr Hyrddod

If not for the ten figure grid reference produced by LIDAR it would be difficult to pinpoint where the critical point of this bwlch lay, as it consists of much bog, which today was thankfully almost bone dry.  Writing this post on the 6^th April follows the driest month of March ever recorded in Wales and England, and the first few days of April have been the same.  This helped us during the walk as few places were boggy and that included the bwlch we were now walking through.  Remembering the LIDAR lay-out of this bwlch, Aled pointed toward its critical point as we headed through its stunningly wild surrounds toward the steepening slopes of Carn yr Hyrddod. 

The bwlch between the two hills

As we crested the upper ridge the wind hit us again.  At least the confines of the bwlch had given some respite from its chilled extremes.  This summit consists of a relatively modern cairn and beside it the jumbled remains of the ancient cairn.  We again assessed the lay of land and pinpointed three positions to survey.  At each summit we looked for large rocks that could be deemed as being part of the natural hill.  There is plenty of rock at each summit, but none that could be described as large enough to be a part of the natural hill and therefore surveys were taken to either grass or the ground at the base of the rock. 

Aled at the summit of Carn yr Hyrddod

As we made our way to the summit a figure appeared beside the wind shelter on Lan Ddu Fawr, by the time our summit assessment was complete he had joined us.  Kevin Nicholls had driven up from Cardiff and was part of the SOTA – Summits on the air community, and planned on setting up his gear back on the summit of Lan Ddu Fawr.  He then headed back toward this summit, and soon afterward Aled was off on his wild hill bagging trek.   I watched as he headed down toward a near bog before claiming height on a ridge leading toward the summit of Bryn Dafydd. 

Aled heads off in to the wilds of the Elenydd 

This left me to my surveying activities.  I knew what needed to be done and even though the wind was proving fierce I had faced similar conditions on a few hills before.  However, with six planned surveys on the two summits I knew that the wind would hinder me more so than the Trimble, as even writing the notes in my surveying notebook proved cumbersome and especially so when the wind was blowing my camera strap and Trimble bag back over my head! 

I soon found a suitable rock to place above the first point to survey and positioned the Trimble on top of it, wedging it in place with two smaller rocks.  This proved an ideal solution to the wind and would be employed on the remaining summit surveys.  Having taken a measurement offset from the equipment’s internal antenna to the ground at its base, I waited patiently for the 0.1m accuracy level to be attained before data should be logged, and once achieved I activated it to gather data. 

Gathering data at the summit of Carn yr Hyrddod

During data collection I stood a safe distance away from the equipment so as not to hinder satellite reception, scribbled all necessary detail in my survey notebook, whose pages were flapping about in the wind.  Once five minutes of data were collected and stored I turned the equipment off, took a series of photographs and repeated the process at two further points.  Progress was being made.

Leaving the summit of Carn yr Hyrddod I headed back down to the sheltered confines of the connecting bwlch.  Using the Trimble as a hand-held device I zeroed in to the ten figure grid reference produced by LIDAR analysis.  The DTM and DSM model give the critical bwlch approximately 5 metres apart.  I favoured using the DTM model as the critical point, and this led me to a small channel amongst copious amounts of rather beautifully coloured grass tussocks. 

Gathering data at the connecting bwlch between the two hills

Wanting a rest from the buffeting wind I decided to take two data sets, both from the same position, with the Trimble placed on top of my rucksack, with its base firmly wedged in the bottom of the channel.  After the first data set was complete I re-positioned the rucksack over the same point but with the Trimble now facing a different direction and set it to gather another five minute data set, during which I sat and looked at some extraordinary purple coloured bog enjoying lichen encrusted bulbous beauty that covered the edge of a near peat hag.  This and looking up toward the summit of Carn yr Hyrddod occupied the next few minutes.  Sometimes surveying can take you to some out of the way places, that in their own right are extraordinarily peaceful and have a tranquility all their own, and this connecting bwlch was one of those places.

Once the second bwlch survey was complete I packed the equipment away and headed around any bog and joined the ridge leading to the summit of Lan Ddu Fawr.  Nearing the top I looked east toward the ridge connecting with Trawsallt and spotted a small figure heading toward its summit.  It was Aled.  He had made good progress and I thought that he’d probably get back to the car before me as I still had three data sets to gather atop Lan Ddu Fawr. 

Aled on his way to Trawsallt

When I arrived back at the wind shelter Kevin Nicholls was hunkered down with his aerials already set up and transmitting, talking to friends in various parts of the country.  I said hi and explained that I wanted to take a series of data sets and would be doing so on the far side of the wind shelter for around 25 minutes.  He indicated that he would remain where he was whilst I did my stuff. 

Kevin Nicholls hunkered own with his SOTA equipment in place

I soon had the first position set up, again using a rock to place the Trimble on and two others to wedge it in place.  Once the measurement offset was taken I retreated as it collected another data set.  Once complete I headed back around the cairn to tell Kevin that the first was now complete and two more were to be taken.

This routine of using a rock to slightly elevate the Trimble and give it a solid platform and to protect it from any wind disturbance with two wedged rocks, proved ideal and soon the second data set was complete.  Before moving the Trimble to the last position I again headed off to tell Kevin that only one remained to take. 

Carn yr Hyrddod from Lan Ddu Fawr

The third position was going to be the most difficult to get a good data set from as Aled and I had decided that one needed to be taken at the entrance to the wind shelter.  This meant that even with its slight elevation on its rock the Trimble was faced with higher rock immediately above on three sides.  This is not conducive to either attaining the 0.1m accuracy level before data should be logged, let alone attaining a good data set, we will have to wait and see what the processed data gives. 

Gathering data at the summit of Lan Ddu Fawr

As the last data set was nearing completion Kevin was packing up his gear and therefore a quick bit of wind driven conversation later and we decided to head down the hill together.  All that remained was to take a few photos of the last of the eight survey positions taken during the day, pack the equipment away, get my rucksack on and join Kevin as we headed down the hill.

Our route took us back to the gate where on the ascent I had taken a data set to attune the Trimble to the local satellite reception.  From here we opted to follow Kevin’s inward route, walking on a good quad bike track beside the fence line.  We took our time on our outward walk with lots of good conversation. 

Kevin Nicholls

By the time we arrived back at the two awaiting cars, Aled was already there, flat on his back resting in the sun.  He’d made good time on his route around the hills.  Before leaving we chatted with Kevin for quite sometime about the various hills he’d transmitted from and those he hoped to visit.  It had been an excellent day on the hill, with all plans fulfilled.  It should be an interesting survey result! 

 

Postscript: 

The processed Trimble result could not split the heghts of these hills and therefore they are now listed as a twin top in the Deweys and 500m Twmpau lists.  However, for convenience the data given below uses Carn yr Hyrddod as the prioritised hill.  Please click here for more information relating to the Dewey twin top announcement. 

 

Survey Result: 

 

Lan Ddu Fawr (significant name change for the Deweys)

Summit Height:  592.7m (converted to OSGM15) (summit relocation based on twin top for the Deweys) (summit relocation based on twin top for the 500m Twmpau)

Summit Grid Reference:  SN 78759 69974

Bwlch Height:  578.1m (converted to OSGM15, average of two surveys)

Bwlch Grid Reference:  SN 78931 70288

Drop:  14.6m

Dominance:  2.47%

 

 

Carn yr Hyrddod (significant name change for the Deweys)

Summit Height:  592.7m (converted to OSGM15, Trimble GeoXH 6000) (summit relocation based on twin top for the Deweys) (summit relocation based on twin top for the 500m Twmpau)

Summit Grid Reference:  SN 79069 70421 (Trimble GeoXH 6000)

Bwlch Height:  424.8m (LIDAR)

Bwlch Grid Reference:  SN 79547 66955 (LIDAR)

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

Dominance:  28.34% (Trimble GeoXH 6000 summit and LIDAR bwlch)

 

 

For further details please consult the Trimble Survey Spreadsheet