Mapping Mountains – Significant Height Revisions – 100m Twmpau

Pedwar Erw (SS 864 837)

There has been a Significant Height Revision to a hill that is now listed in the 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Pedwar Erw

The criteria for the list that this height revision applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name of the enclosed land that once comprised ground where the summit of this hill is situated is Pedwar Erw, and this was derived from the Tithe map and it is this name that the hill is now listed by.  The hill is adjoined to the Cymoedd Morgannwg group of hills, which are situated in the central part of South Wales (Region C, Sub-Region C2), and it forms a part of landscaped ground that once made up the Margam opencast mine, and is encircled by minor roads to its north, west and east and has the B4281 road and the small communities of Cefn Cribwr and Kenfig Hill towards its south.

This hill did not appear in the original 100m height band of Welsh P30 hills when they were published on Geoff Crowder’s v-g.me website as contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps of the day showed the opencast mine without any ring contours of note.

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

This area was re-examined when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and for the majority of land comprising old mine workings it shows present day contours, as opposed to the blank space showed on the counterparts of the 1:50,000 Landranger and 1:25,000 Explorer maps.

Extract from the OS Maps website

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 Pedwar Erw

The summit height produced by LIDAR analysis is 131.0m, this is a substantial revision compared to some revised heights, and it comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR analysis.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new listed summit height is 131.0m and this was produced by LIDAR analysis, this is 31m higher than the highest contour that appears on the Ordnance Survey 1:25,000 Explorer map, and this is only a partial contour as it soon disappears in to the blank space associated with mine workings that appears on many contemporary Ordnance Survey maps.

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Pedwar Erw

OS 1:50,000 map:  170

Summit Height (New height):  131.0m (LIDAR)

Summit Grid Reference:  SS 86413 83767 (LIDAR) 

 

Bwlch Height:  98.4m (LIDAR)

Bwlch Grid Reference:  SS 86635 84031 (LIDAR)

Drop:  32.5m (LIDAR)

Myrddyn Phillips (May 2019)

Mapping Mountains – Hill Reclassifications – 600m Twmpau

600m Twmpau – Hill Reclassifications

The 600m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 600m and below 70om in height that have a minimum drop of 30m.  Accompanying the main P30 list is a sub list entitled the 600m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 600m and below 700m in height with 20m or more and below 30m of drop.

The list is authored by Myrddyn Phillips and the posts that have appeared on Mapping Mountains detailing the additions, reclassifications and deletions to the main P30 list and the sub list appear below presented chronologically in receding order.

Mapping Mountains - Hill Reclassifications - 600m Twmpau

Chwarel Graig Ddu (SH 725 455) – 600m Sub-Twmpau addition (3rd reclassification)

Survey post for Chwarel Graig Ddu

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps. 

Chwarel Graig Ddu (SH 725 455)

The criteria for the list that this addition applies to are:

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 600m Twmpau by Myrddyn Phillips

The name the hill is listed by is Chwarel Graig Ddu, and it is adjoined to the Arenig Fach group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and the B4391 road to its south, and has the town of Blaenau Ffestiniog towards the west.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included in the accompanying sub list, as because it is a part of a working quarry the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map do not show any spot heights or contours, therefore the hill seemed not to exist. 

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

Since the original compilation of this list there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the resources recently available online is the mapping on the OS Maps website.  This is the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the online Vector Map Local.  Although the mapping on the OS Maps website no longer has contours at 5m intervals, such contours are represented on other mapping available online and it was Aled Williams who scrutinised this mapping which has an uppermost 610m ring contour and bwlch contouring between 590m – 595m, with these contours giving this hill a drop value exceeding the minimum 15m to qualify for this list. 

Extract from the online map showing 5m contouring

The hill has subsequently been inspected from adjacent hillsides and its validity confirmed and as a quarry road passes over its summit area it is considered solid and stable.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary online mapping with a subsequent inspection from adjacent hillsides undertaken to confirm its validity, resulting in a 613m summit height and a 592m bwlch height, with these values giving this hill 21m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Arenig Fach

Name:  Chwarel Graig Ddu

OS 1:50,000 map:  115

Summit Height:  613m

Summit Grid Reference:  SH 72505 45533

Bwlch Height:  592m

Bwlch Grid Reference:  SH 72456 45744   

Drop:  21m 

 

Myrddyn Phillips (June 2022)

 

Mapping Mountains - Hill Reclassifications - 600m Twmpau

Drum yr Eira (SN 851 589) – 600m Sub-Twmpau addition (2nd reclassification)

There has been an addition to the listing of the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on contemporary maps produced from Ordnance Survey data.

The criteria for the list that this addition applies to are:

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name the hill is listed by is Drum yr Eira and it is adjoined to the Elenydd group of hills, which are situated in the northern part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the B4343 road to its north-west, the A483 road to its south-east and the A470 road to its east, and has the town of Rhaeadr Gwy (Rhayader) towards the north-east.

When the list that later became known as the 600m Twmpau was first compiled this hill was not included but it was listed with an estimated c 19m of drop, based on a c 601m summit height and a c 582m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

Since the original compilation of this list there are now a number of interactive maps originated from Ordnance Survey data that are available online.  One of these was the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This map had many spot heights not on other publicly available Ordnance Survey maps and showed a 582m spot height at the area of this hill’s bwlch; substantiating the previously listed interpolated bwlch height.

Another resource now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available Ordnance Survey maps and for this hill it  shows a 602m spot height on the summit area of this hill.

Extract from the WalkLakes website

The details for this hill were also re-examined against the Ordnance Survey non-contour Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.  This is another map that has many spot heights that do not appear on other publicly available Ordnance Survey maps and it also gives a 602m summit spot height.

Extract from the Ordnance Survey non-contour Vector Map Local hosted on the Geograph website

The positions of these two spot heights were then compared to the contouring on the OS Maps website.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.

Therefore, the addition of this hill to 600m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data, resulting in a 602m summit height and a 582m bwlch height, with these values giving this hill 20m of drop, which is sufficient for it to be classified as a 600m Sub-Twmpau.

The full details for the hill are:

Group:  Elenydd

Name:  Drum yr Eira

OS 1:50,000 map:  147

Summit Height:  602m (spot height)

Summit Grid Reference:  SN 85120 58920 (spot height)

Bwlch Height:  582m (spot height)

Bwlch Grid Reference:  SN 85606 58495 (spot height)

Drop:  20m (spot height summit and bwlch)

Myrddyn Phillips (August 2020)

Mapping Mountains - Hill Reclassifications - 600m Twmpau

Foel Penolau (SH 661 347 and SH 661 348) – 600m Sub-Twmpau reclassified to 600m Twmpau (1st reclassification)

Survey post for Foel Penolau

There has been a reclassification to a hill that is listed in the 600m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by a survey with the Trimble GeoXH 6000 which was conducted over two days by Myrddyn Phillips and Aled Williams on the 17^th November 2018 and by Myrddyn Phillips on the 26^th November 2018.

Foel Penolau (SH 661 347 and SH 661 348)

The criteria for the list that this hill reclassification applies to are:

600m Twmpau – Welsh hills at or above 600m and below 700m in height that have 30m minimum drop, with an accompanying sub category entitled the 600m Sub-Twmpau consisting of all Welsh hills at or above 600m and below 700m in height that have 20m or more and below 30m of drop.  With the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The name of the hill is Foel Penolau and it is adjoined to the Rhinogydd group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A496 road to the west and the A470 road and the village of Trawsfynydd towards the east.

When the list that later became known as the 600m Twmpau was first compiled this hill was listed with c 26m of drop based on the 614m summit spot height that appears on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and an estimated bwlch height of c 588m based on interpolation of 10m contouring between 580m – 590m.

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

The details for this hill were re-evaluated when the Ordnance Survey Vector Map Local became available online; this map is hosted on the Geograph website and is entitled the Interactive Coverage Map.  As well as showing twin 614m map heighted summits it also gave a 588m spot height in the area of this hill’s bwlch, but as lower ground was known to exist on the hill to hill traverse compared to where this spot height is positioned the drop value was amended to an estimated c 27m.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

This hill’s details were again re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals and these showed that the 588m spot height that appears on the Ordnance Survey Vector Map Local is not positioned at the critical bwlch of this hill.  These contours placed the position of the critical bwlch in the boulder field below this hill’s eastern face and resulted in an estimated c 30m of drop, based on an estimated c 584m bwlch height based on interpolation of contours between 580m – 585m and the 614m spot height given to the two twin map heighted summits.

Extract from the OS Maps website

The confirmation of the reclassification of Foel Penolau to 600m Twmpau status is due to a survey with the Trimble GeoXH 6000 resulting in a 614.4m summit height (with 1mm difference between the northern and southern summits the Trimble was unable to split these tops) and a 582.5m bwlch height, with these values giving this hill 31.9m of drop which is sufficient for it to be classified as a 600m Twmpau.

The Trimble set-up position at the northerly summit of Foel Penolau

The Trimble set-up position at the critical bwlch of Foel Penolau

The full details for the hill are:

Group:  Rhinogydd

Name:  Foel Penolau

OS 1:50,000 map:  124

Summit Height:  614.4m (converted to OSGM15)

Summit Grid Reference:  SH 66184 34832 and SH 66145 34774

Bwlch Height:  582.5m (converted to OSGM15)

Bwlch Grid Reference:  SH 66099 34735

Drop:  31.9m

For details on the survey of Foel Penolau

Myrddyn Phillips (May 2019)

Mapping Mountains – Trimble Surveys – Elenydd

27.02.19  Cefn Cwm Coel (SN 891 647), Craig Fawr (SN 879 638) and Pen Garn Ddu (SN 891 628 [bwlch only])

Cefn Cwm Coel (SN 891 647)

The hills of the Elenydd are some of the most tranquil, and yet also some of the roughest anywhere in the country.  They are in the main open hills, with only a few fences crossing their ridges, and consist of rough grassland which in summer can be energy sapping and soul destroying.  However, in late winter / early spring when Sky Larks sing overhead and a cooling breeze sweeps across their desolate land there are few other places I would rather be.

Today was forecast to be the last day of the unseasonably warm weather and therefore with clear conditions I decided to visit the hills between the Claerwen and Garreg Ddu / Caban Coch reservoirs to the south-west of Rhaeadr Gwy (Rhayader).

As I set off on the track leading toward the farm of Henfron blue sky radiated from above casting early morning colour with the sun glinting through the trees that sprung up beside the waters of the Garreg Ddu Reservoir.  The track led through old deciduous woodland and gave to a leisurely pace. 

Glinting light

The bridge spanning the Garreg Ddu and Caban Coch reservoirs

The bridge spanning the waters between the Garreg Ddu and Caban Coch reservoirs merged grey in the background giving an ethereal feel to the land.  Although the track was shaded with dappled light the sun and blue sky heralded a warm day ahead on the hill.

The track leading to Henfron farm

As I gained height past the farm of Henfron the wilderness of the Elenydd opened up with its bleached moor and rough grassland.  Ahead lay the summit of Cefn Cwm Coel which had been reclassified to 500m Sub-Uchaf status in February 2017 with 14m of drop based on the 489.6m LIDAR bwlch data analysed by Aled and the 504m Ordnance Survey summit spot height.  I wanted to take summit and bwlch data with the Trimble to ascertain this hill’s accurate drop value.

Beyond the last gate I followed vehicle tracks on the moor and only left these when the summit ridge came in to view.  Once off the vehicle track the underfoot conditions became rough with grassed tussocks and heather the order of the day.  Surrounding me the land shone a tinged subtle and bleached yellow, a marvellous sight.

I spent 40 minutes on the summit of Cefn Cwm Coel and took data sets from four potential summit positions.  During this I looked out on my next objective; this hill’s connecting bwlch to Craig Dyfnant, and it looked a wild inhospitable place consisting of an extremely large bog.

One of four data sets gathered on the summit area of Cefn Cwm Coel

The openness of these hills can take time to adjust to, as there are few features for the eye to follow, just rising land to more open hills and rounded grassed ridges leading in to the distance.  Once I had packed the survey equipment away I tried to follow a sheep track through the tussocks down to the connecting bwlch.  By now my mind was adjusting to the landscape with its openness a thing of beauty however rough the underfoot conditions.

Nearing the bwlch the land turned from bleached tussock to dulled brown heather, and the grid reference produced for the critical point from Aled’s LIDAR analysis led me through a never ending land of heather and bog, which today after the warm weather of late was thankfully relatively dry.

I stood and enjoyed my surroundings as the Trimble gathered its customary five minutes of data.  The bog only ended as the land rose toward the eastern slopes of Craig Dyfnant, and I wondered where my best route lay toward the next survey point; the bwlch of Craig Fawr.  Once the Trimble had gathered its allotted data I set off on a direct course toward the next bwlch and soon regretted my decision as the underfoot conditions proved extremely rough, soon however I found another sheep track that thankfully led me toward another relatively dry bog.

Gathering data at the critical bwlch of Cefn Cwm Coel

This next bwlch proved to be situated in another stunning place, with a slender water course indicating the slow ebb of the ups and downs.  I took two data sets and after activating the equipment to log data I marched off across the bog so as not to get in the way of satellite reception and straight in front of me a large bird silently took flight, I had a glimpse of a rounded head as it flew away to nestle back on the moor.  I had disturbed a Short-eared Owl.

Gathering data at one of two positions surveyed for the critical bwlch of Craig Fawr

It proved a slow plod up to the summit of Craig Fawr which has a number of contenders for its summit position.  I took data from three points, during which I looked out on the blued waters of the Claerwen Reservoir as the first signs of delicate westerly mist slowly rolled in.

The Claerwen Reservoir

Once all points judged to be worthy of summit contention had been surveyed I packed the Trimble away and headed east toward a slight rise overlooking steepening ground.  I’d overheated on my ascent but had welcomed the morning’s cooling breeze as height was gained, but now I needed to convert my walking trousers to shorts and take my spring fleece off as it was so warm, I cannot remember having to do this on the hills in February before.

Gathering data at the summit of Craig Fawr

I did wonder if I should continue to Pen Garn Ddu and Gurnos but decided that time and the warmth of the day dictated that it was time to descend, but not before one last survey of the critical bwlch of Pen Garn Ddu.

This was again placed in a large bog that was thankfully relatively dry.  Its yellowed tinge almost glowed in the afternoon sunlight.  I sat during data collection and let my mind wander, as I also did once the equipment was packed away and I walked the few metres back toward a gate.

Gathering data at the critical bwlch of Pen Garn Ddu

It had proved a beautiful day on these hills, and again I stopped by the next gate entrance and rested in the warmth of the day.  This gave access on to a track which led through the conifer plantation to my awaiting car.

Having followed the forest track down I lingered beside the road bridge spanning the Garreg Ddu and Caban Coch reservoirs taking a series of photos and talking with a number of people who were out enjoying the warmth of the day.

The Garreg Ddu Reservoir and Valve Tower

The subtle greys of the morning against this bridge had been replaced with succulent colour and dappled reflections.  I was in no rush and savoured the time here.  Just looking and enjoying.  But as with everything, there comes a time to leave.  It had been a good day on the hill with the Elenydd beckoning for another visit shortly.           

      

Survey Result:

Cefn Cwm Coel

Summit Height:  503.0m (converted to OSGM15)

Summit Grid Reference:  SN 89142 64733

Bwlch Height:  489.6m (converted to OSGM15)

Bwlch Grid Reference:  SN 88436 65372

Drop:  13.4m (500m Sub-Uchaf deletion)

Dominance:  2.66%

Craig Fawr

Summit Height:  517.9m (converted to OSGM15)

Summit Grid Reference:  SN 87939 63846

Bwlch Height:  475.7m (converted to OSGM15)

Bwlch Grid Reference:  SN 88241 64748

Drop:  42.1m

Dominance:  8.14%

Pen Garn Ddu

Summit Height:  466.5m (LIDAR)

Summit Grid Reference:  SN 89125 62814

Bwlch Height:  415.2m (converted to OSGM15)

Bwlch Grid Reference:  SN 88854 63617

Drop:  51.3m (LIDAR summit and Trimble bwlch)

Dominance:  11.00% (LIDAR summit and Trimble bwlch)

For further details please consult the Trimble Survey Spreadsheet