Myrddyn Phillips - Mapping Mountains: Mapping Mountains – Hill Reclassifications

Sunday, 18 June 2017

Mapping Mountains – Hill Reclassifications – 500m Twmpau

500m Twmpau – Hill Reclassifications

The 500m Twmpau (thirty welsh metre prominences and upward) are the Welsh hills at or above 500m and below 60om in height that have a minimum drop of 30m. Accompanying the main P30 list is a sub list entitled the 500m Sub-Twmpau with the qualification to this sub category being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop. The Introduction to this list is linked in the above title.

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 - 500m Twmpau

Llechwedd Deiliog (SH 785 398) – 500m Sub-Twmpau addition (22nd reclassification)

Survey post for Llechwedd Deiliog

There has been confirmation of an addition to the listing of the 500m Twmpau that is retrospective, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams and subsequent LIDAR analysis conducted by Aled Williams.

LIDAR image of Llechwedd Deiliog (SH 785 398)

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

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m 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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains.

500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Llechwedd Deiliog and it is adjoined to the Arenig Fach group of hills, which are situated in the central part of North Wales (Region C1, Sub-Region C1A), and it is positioned with the B4391 road to its south, and has the village of Trawsfynydd towards the west south-west.

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

When the list that later became known as the 500m Twmpau was first compiled, this hill was not included in the accompanying sub list, but was later added after a basic levelling survey conducted by Myrddyn Phillips on the 16.05.04 gave it 66½ft (20.3m drop).

This hill is also included in the listing of The Welsh Highlands – Uchafion Cymru co-authored by Myrddyn Phillips and Aled Williams and it formed one of a number of hills held back from publication until this list was published in its entirety. However, this policy has now changed; therefore the details for this hill have been put in the public domain.

The Trimble set-up position at SH 78452 39920 with subsequent LIDAR analysis prioritised

However, it was not until a Trimble GeoXH 6000 survey and 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.

The Trimble GeoXH 6000 gahering data at the bwlch of Llechwedd Deiliog

Therefore, the confirmation of the addition of this hill to 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey and LIDAR analysis, resulting in a 501.7m summit height and a 481.2m bwlch height, with these values giving this hill 20.6m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Arenig Fach

Name: Llechwedd Deiliog

OS 1:50,000 map: 124

Summit Height: 501.7m (LIDAR)

Summit Grid Reference: SH 78522 39815 (LIDAR)

Bwlch Height: 481.2m (converted to OSGM15, Trimble GeoXH 6000)

Bwlch Grid Reference: SH 78298 40196 (Trimble GeoXH 6000)

Drop: 20.6m (LIDAR summit and Trimble GeoXH 6000 bwlch)

Myrddyn Phillips (September 2026)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Esgair Saeson (SN 795 603) – 500m Sub-Twmpau deletion (21st reclassification)

There has been a deletion to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams.

LIDAR imge of Esgair Saeson (SN 795 603)

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Esgair Saeson and it is adjoined to the Esgair Wen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north-west and south, and has the small community of Pontrhydfendigaid towards the north-west and the town of Tregaron towards the west.

After the list that later became known as the 500m Twmpau was first compiled, this hill was included in the accompanying sub list with an estimated c 24m of drop, based on the 500m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and an estimated c 476m bwlch height, based on interpolation of 10m contouring between 470m – 480m.

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.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 475m bwlch spot height positioned at SN 79101 60658, and when coupled with its 500m summit spot height these values gave this hill 25m of drop.

One of the resources recently available online is the mapping on the OS Maps website and the details for this hill were subsequently re-assessed against this mapping. 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. This mapping had bwlch contouring between 470m – 475m, with interpolation placing the height of the bwlch as an estimated c 473m, resulting in its drop value being amended to an estimated c 27m.

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.

Therefore, the deletion of this hill from 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 498.4m summit height and a 473.8m bwlch height, with these values giving this hill 24.6m of drop, with its height insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Esgair Wen

Name: Esgair Saeson

OS 1:50,000 map: 146, 147

Summit Height: 498.4m (LIDAR)

Summit Grid Reference: SN 79504 60391 (LIDAR)

Bwlch Height: 473.8m (LIDAR)

Bwlch Grid Reference: SN 79174 60567 (LIDAR)

Drop: 24.6m (LIDAR)

Myrddyn Phillips (November 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Esgair Llyn Du (SN 769 620) – 500m Sub-Twmpau deletion (20th reclassification)

Survey post for Esgair Llyn Du

Esgair Llyn Du (SN 769 620)

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Esgair Llyn Du and it is adjoined to the Esgair Wen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north-west and south, and has the small community of Pontrhydfendigaid towards the north-west and the town of Tregaron towards the west south-west.

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

After the list that later became known as the 500m Twmpau was first compiled, this hill was included in the accompanying sub list with 20m of drop, based on the 534m summit spot height positioned at SN 767 617 and the 514m bwlch spot heights that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, with this mapping giving two positions of equal height for the bwlch.

Extract from the interactive mapping hosted on the WalkLakes website

The Ordnance Survey Vector Map Local hosted on the Geograph website gave the higher spot height to the middle of three larger 530m map contour rings, with the southerly and middle ring contours subsequently surveyed with the Trimble GeoXH 6000. However, the interactive mapping hosted on the WalkLakes website indicates that the most northerly of these is the higher.

The Trimble GeoXH 6000 set-up position at the southerly of the three tops

The Trimble GeoXH 6000 set-up position at the middle of the three tops

At the northerly of the three tops and the summit of Esgair Llyn Du

LIDAR image of Esgair Llyn Du (SN 769 620)

Therefore, the deletion of this hill from 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 532.8m summit height and a 513.8m bwlch height, with these values giving this hill 19.0m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Esgair Wen

Name: Esgair Llyn Du

OS 1:50,000 map: 146, 147

Summit Height: 532.8m (LIDAR)

Summit Grid Reference: SN 76930 62044 (LIDAR)

Bwlch Height: 513.8m (LIDAR)

Bwlch Grid Reference: SN 76942 61196 (LIDAR)

Drop: 19.0m (LIDAR)

Myrddyn Phillips (November 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Carreg y Big (SH 927 201) – 500m Sub-Twmpau deletion (19th reclassification)

There has been a deletion to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail produced by Joe Nuttall in his surface analysis progamme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips.

LIDAR image of Carreg y Big (SH 927 201)

The criteria for the list that this deletion applies to are:500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m 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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains.

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg y Big and it is adjoined to the Esgeiriau Gwynion group of hills, which are situated in the southern part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north and west, and the B4393 road to its east north-east, and has the village of Llanwddyn towards the east.

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the accompanying sub list with an estimated c 21m of drop, based on the 589m summit height that appears on the contemporary Ordnance Survey 1:25,000 Outdoor Leisure map, and an estimated c 568m bwlch height, based on interpolation of 10m contouring between 560m – 570m.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 569m bwlch spot height, and therefore the drop value was revised to 20m.

Therefore, the deletion of this hill from 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 589.4m summit height and a 569.8m bwlch height, with these values giving this hill 19.6m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Esgeiriau Gwynion

Name: Carreg y Big

OS 1:50,000 map: 125

Summit Height: 589.4m (LIDAR)

Summit Grid Reference: SH 92712 20119 (LIDAR)

Bwlch Height: 569.8m (LIDAR)

Bwlch Grid Reference: SH 92769 19949 & SH 92769 19953 (LIDAR)

Drop: 19.6m (LIDAR)

Myrddyn Phillips (August 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Pt. 551.8m (SH 665 322) – 500m Sub-Twmpau addition (18th reclassification)

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Aled Williams.

LIDAR image of Pt. 551.8m (SH 665 322)

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

The 500m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 551.8m) notation and it is adjoined to the Y Llethr 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 B4573 road to its west, the A470 road to its east, and has a minor road to its west south-west, and has the village of Trawsfynydd towards the north-east.

When the list that later became known as the 500m Twmpau was first compiled, this hill was not included in the accompanying sub list due to a basic levelling survey conducted by Myrddyn Phillips on the 31^st August 2004 that resulted in 63ft (19.2m) of drop, and it was originally listed with an estimated c 552m summit height and an estimated c 533m bwlch height, with both heights based on interpolation of 10m contouring.

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

One of the mapping resources 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 maps and for this hill it has two 512m summit spot heights which enabled the numerical detail to be updated, but with 19.2m of listed drop it was still not included as a 500m Sub-Twmpau.

Extract from the interactive mapping hosted on the WalkLakes website

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 551.8m summit height and a 531.7m bwlch height, with these values giving this hill 20.1m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Y Llethr

Name: Pt. 551.8m

OS 1:50,000 map: 124

Summit Height: 551.8m (LIDAR)

Summit Grid Reference: SH 66588 32216 (LIDAR)

Bwlch Height: 531.7m (LIDAR)

Bwlch Grid Reference: SH 66333 32260 (LIDAR)

Drop: 20.1m (LIDAR)

Myrddyn Phillips (July 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Pt. 511.7m (SH 693 485) – 500m Sub-Twmpau addition (17th reclassification)

LIDAR image of Pt. 511.7m (SH 693 485)

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

The 500m Twmpau by Myrddyn Phillips

The hill is being listed by the point (Pt. 511.7m) notation and it is adjoined to the Moelwyn Mawr 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 east, and has the town of Blaenau Ffestiniog towards the south.

When the list that later became known as the 500m Twmpau was first compiled, this hill was not included in the accompanying sub list due to a basic levelling survey conducted by Myrddyn Phillips on the 13^th July 2004 that resulted in 65ft (19.8m) of drop, and it was subsequently listed with an estimated c 512m summit height and an estimated c 492m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map.

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

One of the mapping resources 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 maps and for this hill it has a 512m summit spot height which enabled the numerical detail to be updated, but with 19.8m of drop it was still not included as a 500m Sub-Twmpau.

Extract from the interactive mapping hosted on the WalkLakes website

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 511.7m summit height and a 491.7m bwlch height, with these values giving this hill 20.0m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Moelwyn Mawr

Name: Pt. 511.7m

OS 1:50,000 map: 115

Summit Height: 511.7m (LIDAR)

Summit Grid Reference: SH 69325 48552 (LIDAR)

Bwlch Height: 491.7m (LIDAR)

Bwlch Grid Reference: SH 69253 48542 (LIDAR)

Drop: 20.0m (LIDAR)

Myrddyn Phillips (July 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Foel y Geifr (SH 716 050) – 500m Twmpau reclassified to 500m Sub-Twmpau (16th reclassification)

There has been a reclassification to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail produced by Joe Nuttall in his surface analysis progamme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Aled Williams and Myrddyn Phillips.

Foel y Geifr (SH 716 050)

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Foel y Geifr and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with the B4405 road to its north-west, the A493 road to its south and the A487 road to its east, and has the town of Machynlleth towards the south-east.

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the main P30 list due to a basic levelling survey conducted by Myrddyn Phillips on the 26^th June 2000 that resulted in over 30m of drop, and listed with a 515m summit height, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and a 483m bwlch height; relative to the drop value ascertained from the basic levelling survey and the summit spot height.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 486m bwlch spot height and when coupled with its 515m summit spot height, these values gave this hill 29m of drop. This emphasised the uncertainty applicable to the result produced by the basic levelling survey.

LIDAR image of Foel y Geifr (SH 716 050)

Therefore, the reclassification of this hill to 500m Sub-Twmpau status is due to LIDAR analysis, resulting in a 515.6m summit height and a 486.8m bwlch height, with these values giving this hill 28.8m of drop, which is insufficient for it to be classified as a 500m Twmpau.

The full details for the hill are:

Group: Tarren y Gesail

Name: Foel y Geifr

OS 1:50,000 map: 124

Summit Height: 515.6m (LIDAR)

Summit Grid Reference: SH 71634 05092 (LIDAR)

Bwlch Height: 486.8m (LIDAR)

Bwlch Grid Reference: SH 71726 05403 (LIDAR)

Drop: 28.8m (LIDAR)

Myrddyn Phillips (July 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Craig Stwlan (SH 667 446) – 500m Sub-Twmpau addition (15th reclassification)

Survey post for Craig Stwlan

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Aled Williams.

The Trimble GeoXH 6000 gathering data at the summit of Craig Stwlan (SH 667 446) (Photo: Aled Williams)

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Craig Stwlan and it is adjoined to the Moelwyn Mawr 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 B4410 road to its south-west, the A487 road to its south and the A496 road to its east, and has the town of Blaenau Ffestiniog towards the east north-east.

When the list that became known as the 500m Twmpau was first compiled, this hill was listed with an estimated c 20m of drop, based on the 568m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 548m bwlch height, based on interpolation of 10m contouring between 540m – 550m.

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

The hill was subsequently deleted from 500m Sub-Twmpau status due to initial LIDAR analysis, resulting in a 568.2m summit height and a 548.5m bwlch height, with these values giving this hill 19.7m of drop. However, the hill has now been surveyed with the Trimble GeoXH 6000 and it is this result that is being prioritised.

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, resulting in a 568.4m summit height and a 548.3m bwlch height, with these values giving this hill 20.1m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Moelwyn Mawr

Name: Craig Stwlan

OS 1:50,000 map: 124

Summit Height: 568.4m (converted to OSGM15)

Summit Grid Reference: SH 66736 44654

Bwlch Height: 548.3m (converted to OSGM15)

Bwlch Grid Reference: SH 66693 44717

Drop: 20.1m

Myrddyn Phillips (March 2023)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Y Drum (SJ 082 378) – 500m Sub-Twmpau reclassified to 500m Twmpau (14th reclassification)

Survey post for Y Drum

There has been a reclassification to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips with assistance from Aled Williams and Mark Trengove, and a subsequent Leica RX1250 survey conducted by Alan Dawson.

Y Drum (SJ 082 378)

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Y Drum and it is adjoined to the Craig Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A3), and it is positioned with the A5 road to its north and the B4401 road to its west, and has the village of Cynwyd towards the north-west.

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the sub list with 28m of drop, based on the 579m summit and 551m bwlch spot heights that appear on the contemporary Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were then re-assessed when the OS Maps website became available online. This is the replacement for OS Get-a-map and it used to have contours at 5m intervals which proved 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. This mapping had bwlch contouring between 545m – 550m, with interpolation giving an estimated bwlch height of c 549m. It was the bwlch contouring on the OS Maps website that prioritised a GNSS survey of this hill.

The Trimble GeoXH 6000 gathering data at the bwlch area of Y Drum

The Trimble GeoXH 6000 gathering data at the summit of Y Drum

Therefore, the reclassification of this hill from 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, with confirmation from a subsequent Leica RX1250 survey, resulting in a 579.1m summit height and a 549.1m bwlch height, with these values giving this hill 30.0m of drop, which is sufficient for it to be classified as a 500m Twmpau.

The full details for the hill are:

Group: Craig Berwyn

Name: Y Drum

OS 1:50,000 map: 125

Summit Height: 579.1m (converted to OSGM15, Trimble GeoXH 6000 and Leica RX1250)

Summit Grid Reference: SJ 08247 37893 (Trimble GeoXH 6000 and Leica RX1250)

Bwlch Height: 549.1m (converted to OSGM15, Leica RX1250)

Bwlch Grid Reference: SJ 07986 38259 (Leica RX1250)

Drop: 30.0m (Trimble GeoXH 6000 and Leica RX1250 summit and Leica RX1250 bwlch)

Myrddyn Phillips (June 2021)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Carreg yr Ogof (SN 777 214) – 500m Sub-Twmpau addition (13th reclassification)

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from detail on recent and contemporary maps produced from Ordnance Survey data.

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

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg yr Ogof and it is adjoined to the Y Mynydd Du group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C1), and it is positioned with the A40 road to its north, the A4069 road to its west, the A4068 road to its south south-west and the A4067 road to its east, and has the small community of Llanddeusant towards the north.

When the list that later became known as the 500m Twmpau was first compiled this hill was not included as details on contemporary Ordnance Survey maps of the day give a 585m summit height adjoined to a triangulation pillar and an interpolated bwlch height of c 567m based on contours between 560m – 570m, with these values giving this hill an estimated c 18m of drop.

Extract from the Ordnance Survey 1:25,00 Explorer map

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available Ordnance Survey maps and gave a 566m spot height on the bwlch area of this hill, this when coupled with its 585m summit map height gave this hill 19m of drop.

However, online photographs of the summit area of this hill show ground a few metres from the base of the triangulation pillar as significantly higher. The trig pillar has a flush bracket height of 585.520m and is positioned at SN 77778 21455. Accepting the accuracy of this height the ground at the base of the trig pillar would be approximately 585.2m in height and the summit of the hill has been estimated to be approximately 0.8m higher.

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to detail derived from recent and contemporary maps produced from Ordnance Survey data, resulting in an estimated c 586m summit height and a 566m bwlch height, with these values giving this hill an estimated c 20m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Y Mynydd Du

Name: Carreg yr Ogof

OS 1:50,000 map: 160

Summit Height: c 586m (interpolation)

Summit Grid Reference: SN 77780 21461 (interpolation)

Bwlch Height: 566m (spot height)

Bwlch Grid Reference: SN 77792 21045 (spot height)

Drop: c 20m (interpolated summit and spot height bwlch)

Myrddyn Phillips (January 2021)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Trawsallt (SN 778 704) – 500m Sub-Twmpau addition (12th reclassification)

There has been an addition to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by detail on contemporary maps produced from Ordnance Survey data

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

The name the hill is listed by is Trawsallt, 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 west, and has the village of Ysbyty Ystwyth towards the west north-west.

When the list that later became known as the 500m Twmpau was first compiled this hill was not included as a basic levelling survey conducted by Myrddyn Phillips on the 8^th June 2005 resulted in 63ft / 19.3m of drop. With the hill’s details listed as having a 572m summit height that appears as a spot height on the Ordnance Survey 1:50,000 Landranger map and with 19m of drop.

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

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

However, although notes made at the time did not give detail from where the survey took place, it is likely that it was conducted from the bwlch to an intermediary 564m point positioned at SN 78299 70191, with 8m then added to the survey result to compensate between the 564m and 572m spot heights.

The accuracy of this survey is dependent upon the survey method and two spot heights ascertained from photogrammetry; therefore it had a relatively large margin of uncertainty applicable to it.

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online. This mapping had many spot heights not on other publicly available Ordnance Survey maps and gave a 551m spot height on the bwlch area of this hill and positioned at SN 78327 70061.

The details for this hill were then 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 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. This re-assessment checked the positioning of the 551m bwlch spot height and resulted in the eventual addition of this hill.

Extract from the OS Maps website

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to detail derived from contemporary maps produced from Ordnance Survey data, resulting in a 572m summit height and a 551m bwlch height, with these values giving this hill 21m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Elenydd

Name: Trawsallt

OS 1:50,000 map: 135, 147

Summit Height: 572m (spot height)

Summit Grid Reference: SN 77855 70410 (hand-held GPS via DoBIH)

Bwlch Height: 551m (spot height)

Bwlch Grid Reference: SN 78327 70061 (spot height)

Drop: 21m (spot height summit and bwlch)

Myrddyn Phillips (August 2020)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Tapiau y Gell (SH 927 168) – 500m Sub-Twmpau addition (11th reclassification)

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

The name the hill is listed by is Tapiau y Gell, and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the A470 road to its west, the A458 road to its south and the B4393 road to its north-east, and has the town of Dinas Mawddwy towards the west south-west.

When the list that later became known as the 500m Twmpau was first compiled the qualifying sub hill was listed as Drum Ddu (SH 931 167) with c 25m of drop based on the 538m summit spot height and an estimated bwlch height of c 513m based on interpolation of bwlch contouring between 510m – 520m that appear on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

However, as Drum Ddu has an adjacent hill named Tapiau y Gell (SH 927 168 [refer to Y Pellennig – The Remotest Hills of Wales by Aled Williams and Myrddyn Phillips]) of similar height that has no spot height on either the Ordnance Survey 1:50,000 Landranger or the 1:25,000 Explorer map, it was noted that if Tapiau y Gell is higher, it would also be the qualifying hill for the 500m Sub-Twmpau.

The details for this hill were re-evaluated when the WalkLakes interactive map became available online. This map originated from the Ordnance Survey Open Data programme and has many spot heights not on other publicly available Ordnance Survey maps and gives a 539m spot height on the summit area of Tapiau y Gell.

Extract from the WalkLakes website

Therefore, the addition of Tapiau y Gell to 500m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data with this hill given a 539m summit spot height as opposed to the 538m summit spot height given to Drum Ddu. Therefore, the position of the bylchau for these hills are swapped resulting in Tapiau y Gell now being listed with a 539m summit height and c 516m bwlch height, with these values giving this hill c 23m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Y Berwyn

Name: Tapiau y Gell

OS 1:50,000 map: 125

Summit Height: 539m (spot height)

Summit Grid Reference: SH 92715 16812 (spot height)

Bwlch Height: c 516m (interpolation)

Bwlch Grid Reference: SH 93132 16626 (interpolation)

Drop: c 23m (spot height summit and interpolated bwlch)

Myrddyn Phillips (July 2020)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Drum Ddu (SH 931 167) – 500m Sub-Twmpau deletion (10th reclassification)

There has been a deletion of a hill in the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by detail on contemporary maps produced from Ordnance Survey data

The summit of Drum Ddu (SH 931 167)

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

The name the hill is listed by is Drum Ddu, and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the A470 road to its west, the A458 road to its south and the B4393 road to its north-east, and has the town of Dinas Mawddwy towards the west south-west.

When the list that later became known as the 500m Twmpau was first compiled this hill was listed with c 25m of drop based on the 538m summit spot height and an estimated bwlch height of c 513m based on bwlch contouring between 510m – 520m that appear on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

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

Extract from the WalkLakes website

Therefore, the deletion of Drum Ddu from 500m Sub-Twmpau status is due to detail on contemporary maps produced from Ordnance Survey data with this hill given a 538m summit spot height as opposed to the 539m summit spot height given to Tapiau y Gell. Therefore, the position of the bylchau for these hills are swapped resulting in Drum Ddu now being listed with a 538m summit height and c 523m bwlch height, with these values giving this hill c 15m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Y Berwyn

Name: Drum Ddu

OS 1:50,000 map: 125

Summit Height: 538m (spot height)

Summit Grid Reference: SH 93180 16796 (hand-held GPS via DoBIH)

Bwlch Height: c 523m (interpolation)

Bwlch Grid Reference: SH 93019 16980 (interpolation)

Drop: c 15m (spot height summit and interpolated bwlch)

Myrddyn Phillips (June 2020)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Pen y Cerrig Serth (SH 970 429) – 500m Sub-Twmpau addition (9th reclassification)

Survey post for Pen y Cerrig Serth

Pen y Cerrig Serth (SH 970 429)

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

500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Pen y Cerrig Serth, and it is adjoined to the Carnedd y Filiast group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A2), and it is positioned with the A5 road to its north-east, the B4501 road to its west, the A4212 road to its south-west and the A494 road to its south-east, and has the town of Y Bala towards the south-west.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with a 522m summit height and a 503m bwlch height, with the former appearing as a spot height on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps. With the 19m drop value also ascertained from a basic levelling survey conducted by Myrddyn Phillips on the 1^st May 2004, resulting in a 64ft / 19.5m of drop.

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

However, as the margin of uncertainty applicable to the basic levelling method of surveying over the distance between bwlch and summit and the terrain of this hill is approximately +/- 1.5m, the hill was prioritised for a GNSS survey.

Gathering data at the bwlch area of Pen y Cerrig Serth

Gathering data at the summit of Pen y Cerrig Serth

Therefore, the addition of this hill to 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 summit survey and subsequent LIDAR bwlch analysis, resulting in a 520.7m summit height and a 500.0m bwlch height, with these values giving this hill 20.7m of drop, which is sufficient for it to be classified as a 500m Sub-Twmpau.

The full details for the hill are:

Group: Carnedd y Filiast

Name: Pen y Cerrig Serth

OS 1:50,000 map: 125

Summit Height: 520.7m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference: SH 97054 42933 (Trimble GeoXH 6000)

Bwlch Height: 500.0m (LIDAR)

Bwlch Grid Reference: SH 96806 42746 (LIDAR)

Drop: 20.7m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (June 2020)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Mynydd Pen y Cae (SN 887 032) – 500m Sub-Twmpau deletion (8th reclassification)

There has been a deletion of a hill that is listed in the 500m 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 conducted by Myrddyn Phillips with assistance from Aled Williams which took place on the 22^nd June 2019.

Mynydd Pen y Cae (SN 887 032)

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

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m 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 hill is Mynydd Pen y Cae, and it 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 is positioned with the A465 road and Cwm Nedd (Vale of Neath) to its north-west and the A4061 road to its east, and has the small town of Glyn-nedd (Glynneath) to the north.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with an estimated c 20m of drop based on the 573m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 553m based on 10m contouring between 550m – 560m.

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

The area taking in this hill 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. This mapping shows the bwlch contouring between 555m – 560m with interpolation placing the bwlch height as an estimated c 557m which when coupled with the 573m summit spot height on the 1:25,000 Explorer map gives this hill c 16m of drop.

Extract from the OS Maps website

The deletion of Mynydd Pen y Cae from 500m Sub-Twmpau status is due to a survey with the Trimble GeoXH 6000, resulting in a 574.6m summit height and a 557.0m bwlch height, with the latter in accordance with the 5m contouring on the OS Maps website, with these values giving this hill 17.6m of drop which is insufficient for its continued classification as a 500m Sub-Twmpau.

The Trimble GeoXH 6000 gathering data at the summit of Mynydd Pen y Cae

The Trimble GeoXH 6000 gathering data at the bwlch of Mynydd Pen y Cae

The full details for the hill are:

Group: Cymoedd Morgannwg

Name: Mynydd Pen y Cae

OS 1:50,000 map: 170

Summit Height: 574.6m (converted to OSGM15)

Summit Grid Reference: SN 88743 03214

Bwlch Height: 557.0m (converted to OSGM15)

Bwlch Grid Reference: SN 89494 03481

Drop: 17.6m

For details on the survey of this hill

Myrddyn Phillips (September 2019)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Cefn Cyfarwydd (SH 752 630) – 500m Sub-Twmpau reclassified to 500m Twmpau (7th reclassification)

There has been a reclassification of a hill that is listed in the 500m 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 conducted by Myrddyn Phillips and Aled Williams which took place on the 17^th February 2019.

Cefn Cyfarwydd (SH 752 630)

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

The name of the hill is Cefn Cyfarwydd, and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A5 road to its south and the B5106 and the A470 roads and the Afon Conwy to its east, and has the villages of Trefriw and Llanrwst towards the east.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with 26m of drop based on the 503m summit and 477m bwlch spot heights that appear on the Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map. As this mapping is larger in format it enables a better judgement of interpolated heights and it occasionally has contours at 5m intervals compared to the 10m contour intervals normally shown on the 1:25,000 Explorer map. However, this mapping has the same spot heights and 10m contouring as the 1:25,000 Explorer map and therefore the drop value for this hill remained listed as 26m.

The area taking in this hill 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. This mapping shows the 477m bwlch spot height to be positioned on a separate 475m contour ring implying that the height of this hill’s bwlch is substantially lower than 477m, with interpolation based on this contouring giving the hill c 30m of drop. But it was not until the survey with the Trimble GeoXH 6000 that the increased drop value was confirmed.

Extract from the OS Maps website

The reclassification of Cefn Cyfarwydd to 500m Twmpau status is due to a survey with the Trimble GeoXH 6000, resulting in a 501.7m summit height and a 471.0m bwlch height, with these values giving this hill 30.7m of drop which is sufficient for it to be classified as a 500m Twmpau.

The Trimble GeoXH 6000 gathering data at the summit of Cefn Cyfarwydd

The full details for the hill are:

Group: Carneddau

Name: Cefn Cyfarwydd

OS 1:50,000 map: 115

Summit Height: 501.7m (converted to OSGM15)

Summit Grid Reference: SH 75201 63067

Bwlch Height: 471.0m (converted to OSGM15)

Bwlch Grid Reference: SH 74708 62751

Drop: 30.7m

For details on the survey of Cefn Cyfarwydd

Myrddyn Phillips (July 2019)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Garn Fach (SO 041 815) – 500m Sub-Twmpau addition (6th reclassification)

There has been an addition to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams. The Twmpau is the title for the list of Welsh P30s and takes in all Welsh hills that have a minimum 30m of drop, with this addition affecting the 500m height band of hills within the Twmpau.

Accompanying the main list of Twmpau hills is a sub category, with the criteria for this sub list in the 500m height band being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop. It is this sub category that this addition affects and details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

Prior to analysis of LIDAR data the hill had been listed with 19m of drop based on the 503m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and the 484m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

The name of the hill is Garn Fach and it is situated in the Pegwn Mawr group of hills with its Cardinal Hill being Pegwn Mawr (SO 023 812) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B1). The hill is situated between the small community of Llanbadarn Fynydd towards its south-east and the town of Llanidloes towards its west north-west.

If wanting to visit the hill it is situated on open access land which is covered in a conifer plantation, however on my only visit I followed a forest track leading in from the east which gave access to a forest ride that led towards the summit.

The addition of Garn Fach to 500m Sub-Twmpau status is due to the analysis of LIDAR data by Aled Williams. LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Garn Fach

Summit Height: 504.3m

Summit Grid Reference: SO 04184 81512

Bwlch Height: 484.3m

Bwlch Grid Reference: SO 03783 81771

Drop: 20.0m

Therefore, the 504.3m LIDAR data produced for the summit position at SO 04184 81512 and the 484.3m LIDAR data produced for the bwlch position at SO 03783 81771 gives this hill 20.0m of drop which is sufficient for its inclusion as a 500m Sub-Twmpau.

The full details for the hill are:

Cardinal Hill: Pegwn Mawr

Summit Height: 504.3m (LIDAR data)

Name: Garn Fach

OS 1:50,000 map: 136

Summit Grid Reference: SO 04184 81512

Drop: 20.0m (LIDAR data)

The overall total for the 500m Twmpau remains at 225 hills with the overall total for the 500m Sub-Twmpau increasing by one to 97 hills. The list of the 500m Twmpau was published on 15^th June 2015 with all subsequent changes having been detailed on Mapping Mountains.

My thanks to Aled Williams for sending the details of this hill to me

Myrddyn Phillips (September 2017)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

THIS HILL HAS SUBSEQUENTLY BEEN REINSTATED TO 500m SUB-TWMPAU STATUS

Craig Stwlan (SH 667 446) – 500m Sub-Twmpau deletion (5th reclassification)

There has been a deletion to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams. The Twmpau is the title for the list of Welsh P30s and takes in all Welsh hills that have a minimum 30m of drop, with this deletion affecting the 500m height band of hills within the Twmpau.

Accompanying the main list of Twmpau hills is a sub category list of hills, with the criteria for this sub list in the 500m height band of hills being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop. Details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

Prior to analysis of LIDAR data the hill had been listed with c 20m of drop based on the 568m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 548m based on interpolation of bwlch contouring between 540m – 550m.

The hill is situated in the Moelwynion group of hills with its Cardinal Hill being Moelwyn Mawr (SH 658 448) and is placed in the Region of North Wales (Region A, Sub-Region A1). The hill is situated with the town of Blaenau Ffestiniog to the east north-east and the small village of Croesor to the west.

If wanting to visit the hill it can be easily accessed from the east north-east from the end of the paved public road which is situated above the small community of Tanygrisiau, beyond a locked gate the paved road continues up toward the reservoir of Llyn Stwlan, and Craig Stwlan is positioned just above the north-eastern part of this lake.

The name of the hill is Craig Stwlan and its deletion from 500m Sub-Twmpau status is due to the analysis of LIDAR data by Aled Williams. LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Craig Stwlan

Summit Height: 568.2m

Summit Grid Reference: SH 66735 44655

Bwlch Height: 548.5m

Bwlch Grid Reference: SH 66693 44715

Drop: 19.7m

Therefore, the 568.2m LIDAR data produced for the summit position at SH 66735 44655 and the 548.5m LIDAR data produced for the bwlch position at SH 66693 44715 gives this hill 19.7m of drop which is insufficient for it to retain its status as a 500m Sub-Twmpau.

The full details for the hill are:

Cardinal Hill: Moelwyn Mawr

Summit Height: 568.2m (LIDAR data)

Name: Craig Stwlan

OS 1:50,000 map: 124

Summit Grid Reference: SH 66735 44655

Drop: 19.7m (LIDAR data)

The overall total for the 500m Twmpau remains at 225 hills with the overall total for the 500m Sub-Twmpau decreasing by one to 96 hills. The list of the 500m Twmpau was published on 15^th June 2015 with all subsequent changes having been detailed on Mapping Mountains.

My thanks to Aled Williams for sending the details of this hill to me.

Myrddyn Phillips (May 2017)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Pt. 499.1m (SN 863 760) - 500m Sub-Twmpau reclassified to 400m Sub-Pedwar and 490m Sub-Uchaf (4th reclassification)

There has been a reclassification to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams. The hill was previously listed in the 500m height band of Sub-Twmpau hills and it has now been reclassified to a 400m Sub-Pedwar and 490m Sub-Uchaf, with the criteria for the sub hills in the 500m Twmpau listing being 500m or more and below 600m in height with 20m or more and below 30m of drop. Details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

The hill had been listed with 27.9m of drop and an estimated c 500m summit height based on the flat topped nature of its summit area in relation to the dimension of the small uppermost 500m ring contour that appears on current Ordnance Survey mapping.

The hill is situated in the Elenydd group of hills with its Cardinal Hill being Draws Drum (SN 790 811) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B2). The hill is situated with the small community of Llangurig to the north-east and Cwmystwyth to the west south-west and is positioned directly above and to the north of the narrow mountain road that heads north-westward from Rhaeadr Gwy (Rhayader).

As the hill is immediately above a road access to it is relatively easy from where a public footpath is indicated on Ordnance Survey maps leaving the road to the west south-west of the summit and following a stream valley up toward the hill’s west, from here the ascent to the featureless flat topped summit area is on ground that is relatively easy compared to other hills in the vicinity.

The name of the hill is currently being listed by the point (Pt. 499.1m) notation and its reclassification from the ranks of 500m Sub-Twmpau is due to analysis of LIDAR data by Aled Williams. LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Pt. 499.1m

Summit Height: 499.1m

Summit Grid Reference: SN 86343 76073

Bwlch Height: 471.0m

Bwlch Grid Reference: SN 86323 76358

Drop: 28.1m

Therefore, the 499.1m LIDAR data produced for the summit position at SN 86343 76073 and the 471.0m LIDAR data produced for the bwlch position at SN 86323 76358 gives this hill 28.1m of drop, which is in good accordance with the value produced for this hill when surveyed by John Barnard, Graham Jackson, David Purchase and myself in June 2008 using a level and staff resulting in a drop of 27.9m. Therefore with a summit height below 500m this hill is not high enough to retain its classification as a 500m Sub-Twmpau. This now revises the total in the 500m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill: Draws Drum

Summit Height: 499.1m (LIDAR data)

Name: Pt. 499.1m

OS 1:50,000 map: 135, 136, 147

Summit Grid Reference: SN 86343 76073

Drop: 27.9m (line survey)

My thanks to Aled Williams for sending the details of this hill to me.

Myrddyn Phillips (March 2017)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Mynydd Llangatwg (SO 202 144) - 500m Twmpau reclassified to 500m Sub-Twmpau (3rd reclassification)

There has been a reclassification to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams. The hill was previously listed in the 500m height band of Twmpau hills and it has now been reclassified to a 500m Sub-Twmpau, with the criteria for the sub hills being 500m or more and below 600m in height with 20m or more and below 30m of drop. Details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

LIDAR image of Mynydd Llangatwg (SO 202 144)

The hill had been listed with c 31m of drop, based on the 529m 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 498m based on bwlch contouring between 490m – 500m. The summit area of the hill has two ancient cairns on it as well as a triangulation pillar with an adjoined flush bracket height of 529.044m.

The hill is situated in the easterly part of the Bannau Brycheiniog (Brecon Beacons) group of hills with its Cardinal Hill being Pen y Fan (SO 012 215) and is placed in the Region of South Wales (C-2). It is situated between the towns of Bryn-mawr to its south, Gilwern to its east and Crug Hywel (Crickhowell) to its north north-east.

The hill can be accessed from public footpaths originating from its north and from its south; however one of the easiest ways to its summit is from the east where a track heads toward the summit from a minor road that reaches over 350m high.

The name of the hill is Mynydd Llangatwg, however it is also recorded as Mynydd Pencyrn on Ordnance Survey maps and its reclassification to a Sub-Twmpau is due to the analysis of LIDAR data by Aled. LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Mynydd Llangatwg

Summit Height: 528.9m

Summit Grid Reference: SO 20281 14491

Bwlch Height: 499.7m

Bwlch Grid Reference: SO 18531 15394

Drop: 29.3m

Therefore, the 528.9m LIDAR data produced for the summit position at SO 20281 14491 and the 499.7m LIDAR data produced for the bwlch position at SO 18531 15394 is sufficient for this hill to be reclassified from a 500m Twmpau to a 500m Sub-Twmpau with 29.3m of drop.

This now revises the total in the 500m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill: Pen y Fan

Summit Height: 528.9m (LIDAR)

Name: Mynydd Llangatwg

OS 1:50,000 map: 161

Summit Grid Reference: SO 20281 14491 (LIDAR)

Drop: 29.3m (LIDAR)

My thanks to Aled Williams for sending the details of this hill to me.

Myrddyn Phillips (January 2017)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Cwar yr Ystrad (SO 084 142) - 500m Sub-Twmpau addition (2nd reclassification)

There has been a new addition to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams with a subsequent Trimble GeoXH 6000 survey. The hill is listed in the 500m height band of Sub-Twmpau, with the criteria for the sub hills being 500m or more and below 600m in height with 20m or more and below 30m of drop. Details to this height band of the Twmpau were published on the Mapping Mountains site on 15^th June 2015.

The hill did not appear in the Sub-List which accompanied the original Welsh P30 listings on Geoff Crowder’s v-g.me website as the Sub-List only included hills whose map details showed that when surveyed they may stand a chance of qualification to the main P30 list, with the title of the Sub-List being Hills to be surveyed. As this hill is a part of a quarry contemporary 1:50,000 and 1:25,000 Ordnance Survey maps do not give it any ring contours, as is the norm for areas of land that have been quarried. This hill is now listed as a Sub-Twmpau through the quarrying activities of past years that has resulted in producing a new bwlch for the hill. Since first publication this Sub-List has been standardised and now includes all hills that have a minimum of 20m of drop and below 30m of drop.

The hill is situated in the Cymoedd Gwent group of hills and is placed in the Region of South Wales (C-2). As its name suggests the hill is a part of a quarry with its qualification as a Sub-Twmpau partly due to past quarry activities, it is situated between the Pentwyn and Pontsticill Reservoirs to its west and the Talybont Reservoir to its north north-east, with the town of Merthyr Tudful to its south south-west and the small community of Trefil to its east south-east.

The hill can be accessed from a number of directions on public footpaths including from its south-west and from its north, whilst the route from the paved road north of Trefil also gives access onto the quarry track, but if wishing to ascend from this direction it should be noted that there is a barrier across the road beyond Trefil that is locked at 5.00pm and unlocked at 6.30am with online write ups stating this barrier is locked at weekends.

The name of the hill is Cwar yr Ystrad and its inclusion as a Sub-Twmpau is due to LIDAR analysis conducted by Aled Williams. LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Cwar yr Ystrad

Summit Height: 579.9m

Summit Grid Reference: SO 08358 14245

Bwlch Height: 559.0m

Bwlch Grid Reference: SO 08376 14189

Drop: 21.1m

The subsequent Trimble GeoXH 6000 survey produced a summit height of 579.3m and a bwlch height of 558.8m, with these values giving this hill 20.5m of drop, which is sufficient for this hill to be classified as a 500m Sub-Twmpau.

This now revises the total in the 500m Twmpau which will be updated accordingly.

The full details for the hill are:

Cardinal Hill: Pen y Fan

Summit Height: 579.3m (converted to OSGN15)

Name: Cwar yr Ystrad

OS 1:50,000 map: 160

Summit Grid Reference: SO 08445 14278

Drop: 20.5m

Cwar yr Ystrad (SO 084 142)

My thanks to Aled Williams for sending the details of this hill to me.

Myrddyn Phillips (January 2017)

Mapping Mountains - Hill Reclassifications - 500m Twmpau

Twyn Walter (SN 828 175) - 500m Twmpau reclassified to 500m Sub-Twmpau (1st reclassification)

Survey post for Twyn Walter

Significant Height Revisions post for Twyn Walter

There has been a reclassification of a 500m Twmpau hill by surveying with the Trimble. The hill is situated in the south-eastern lands of Y Mynydd Du in South Wales, and is positioned north-west of Glyntawe and the A4067 as it speeds its way between Pontsenni (Sennybridge) to the north and Ystradgynlais to the south.

These details are retrospective as the Trimble GeoXH 6000 survey that resulted in the reclassification of this hill from 500m Twmpau to 500m Sub-Twmpau was conducted on 10^th July 2014, in the company of Mark Trengove when we visited the remotest spot in mainland Wales, as well as the remotest hill in mainland Wales.

Access to the hill from the south-east of its summit is relatively easy as a broad path ascends from near to the Campsite entrance that is adjacent to Dan yr Ogof Showcaves. As height is gained this path resembles a vehicle track and eventually heads north-west into the remote grasslands of Mynydd Du, on its way it passes within approximately 260 metres of the summit.

The hill is named Twyn Walter (written as Twynwalter on current Ordnance Survey maps) and its summit is positioned at SN 828 175. The hill was listed with c 31m of drop based on a 509m summit spot height and an estimated bwlch height of c 478m. The area of the bwlch has a number of sink holes on it which complicates pinpointing where the critical bwlch is positioned. However, contouring suggests that there are two distinct positional possibilities for where this critical bwlch lies, these are at SN 82525 17584 and SN 82611 17986.

The survey with the Trimble GeoXH 6000 resulted in a 502.1m (converted to OSGM15) summit height, unfortunately daylight was quickly disappearing and this was the last survey of a long day on the hill and this dictated that the two options for the position of the bwlch were not surveyed. However, the dramatic decrease in summit height from the 509m map height to the 502.1m (converted to OSGM15) surveyed height and the re-evaluated and estimated bwlch height from contour interpolation of c 476m gives this hill a drop value of only c 26m.

This now brings the overall total for the 500m Twmpau hills to 226, please click {here} to access the list and {here} for its detailed Introduction.

Since this reclassification was announced the height and position of this bwlch has been determined via LIDAR analysis conducted by Aled Williams, the details for which are given below.

The full details for the hill are:

Group: Mynydd Du

Name: Twyn Walter

OS 1:50,000 map: 160

Summit Height: 502.1m (converted to OSGM15)

Summit Grid Reference: SN 82837 17506

Bwlch Height: 475.9m (LIDAR)

Bwlch Grid Reference: SN 82597 17917 (LIDAR)

Drop: 26.25m (Trimble summit and LIDAR bwlch)

Gathering data at the summit of Twyn Walter, which resulted in this hill being reclassified from a 500m Twmpau to a 500m Sub-Twmpau

For details on the survey that reclassified this hill from 500m Twmpau to 500m Sub-Twmpau please click {here}

Myrddyn Phillips (July 2014)

Sunday, 18 June 2017

Mapping Mountains – Hill Reclassifications – 500m Twmpau

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