Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Twyni Mawr (SN 606 938)

There has been a Significant Height Revision to a hill that was listed in The Welsh P15s, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Twyni Mawr (SN 606 938)

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

The Welsh P15s – Welsh hills with 15m minimum drop, irrespective of their height, with an accompanying sub list entitled the Welsh Sub-P15s, with the criteria for this sub category being all Welsh hills with 14m or more and below 15m of drop.  The list is authored by Myrddyn Phillips, with the Introduction to the list appearing on Mapping Mountains on the 10^th May 2019. 

The Welsh P15s by Myrddyn Phillips

The name the hill is listed by is Twyni Mawr, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with the coast to its west, a minor road to its east and the B4353 road to its south-east, and has the village of Y Borth towards the south.

When the listing that became known as The Welsh P15s was being compiled, this hill was not included in either the main list or the accompanying sub list, as with no significant contours of note on the contemporary Ordnance Survey 1:25,000 map, it was hard to know whether any hill with qualifying prominence existed. 

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

Since the compilation of The Welsh P15s first started 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 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. 

Extract from the interactive mapping hosted on the WalkLakes website

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 a 20m uppermost contour is shown. 

Extract from the DataMapWales

Another resource now available online is the interactive mapping hosted on the Welsh Government website and entitled the DataMapWales.  This mapping has 5m contours and its detail matches that produced from the OS Terrain 5 product, which compliments much of that produced from LIDAR, and for this hill there are two 23m spot heights shown. 

LIDAR image of Twyni Mawr (SN 606 938)

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 Twyni Mawr (SN 606 938)

The summit height produced by LIDAR analysis is 22.8m and when compared to the detail on the Ordnance Survey 1:25,000 Explprer map, this comes within the parameters of the Significant Height Revisions used within this page heading, and these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 22.8m and this was derived from LIDAR analysis.  This is 22.8m higher than the detail on the Ordnance Survey 1:25,000 Explorer map, but is complimentary to the detail on the interactive mapping hosted on the WalkLakes website and also that on the DataMapWales.

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Twyni Mawr 

OS 1:50,000 map:  135

Summit Height (New Height):  22.8m (LIDAR)                                                           

Summit Grid Reference:  SN 60614 93828 (LIDAR)                                                  

Bwlch Height:  4.4m (LIDAR) 

Bwlch Grid Reference:  SN 60576 92484 (LIDAR) 

Drop:  18.4m (LIDAR) 

                                                           

Myrddyn Phillips (April 2026)

 

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

 

Carreg Rhoson (SM 664 253) – Double Sub-Twmpau addition

There has been confirmation of an addition to the list of 30-99m Twmpau, with the summit height and its location, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

Carreg Rhoson (SM 664 253) - second main hill from the right

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

30-99m Twmpau by Myrddyn Phillips

The name the hill is listed by is Carreg Rhoson, and it is adjoined to the Garn Fawr group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it forms part of an islet chain positioned to the west of mainland Wales with a minor road on the mainland to its east, and has the city of Tyddewi (St David’s) on the mainland also towards the east.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this island were re-evaluated and it was listed with 23m of drop, based on the 23m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

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

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 Carreg Rhoson (SM 664 253)

Therefore, the confirmation of the addition of this hill to Double Sub-Twmpau status is due to LIDAR analysis, resulting in a 21.7m summit height and as this hill is an island, this value also gives it 21.7m of drop, which is sufficient for it to be classified as a Double Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Garn Fawr 

Name:  Carreg Rhoson 

OS 1:50,000 map:  157

Summit Height:  21.7m (LIDAR) 

Summit Grid Reference:  SM 66449 25356 (LIDAR)               

Bwlch Height:  N/A (sea level) 

Bwlch Grid Reference:  N/A (sea level) 

Drop:  21.7m (LIDAR) 

 

Myrddyn Phillips (April 2026)

 

Mapping Mountains – Significant Height Revisions – The Fours – The 400m Hills of England

 

Mason’s Bank (SO 228 873)

There has been a Significant Height Revision to a hill that is listed in the The Fours – The 400m Hills of England, with the summit height, col height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Mason's Bank (SO 228 873)

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

The Fours – The 400m Hills of England - English hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main list is a sub list entitled the Sub-Fours, the criteria for which are all English hills at or above 400m and below 500m in height that have 15m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is available to download in Google Doc format from the Mapping Mountains site.

The Fours - The 400m Hills of England by Myrddyn Phillips and Aled Williams

The name the hill is listed by is Mason’s Bank, and it is adjoined to the Cilfaesty group of hills, which are situated adjoined to the Welsh border, and it is positioned with minor roads to its north and south, and the A489 road farther to its north and the B4368 road farther to its south, and has the village of Yr Ystog (Churchstoke) towards the north-east.

When the Introduction to the first group of hills for the updated and revised listing of the The Fours – The 400m Hills of England was published by Mapping Mountains on the 10^th September 2022, it was announced that the accompanying sub lists were being revised with the two 390m categories dispensed with and the criteria and name of the 400m Sub-Fours revised.  The one accompanying sub list is now named the Sub-Fours with its criteria being all English hills 400m and above and below 500m in height that have 15m and more and below 30m of drop.

Prior to this revision this hill was listed with 17m of drop, based on the 478m summit spot height that appears on the Ordnance Survey 1:25,000 Explrer map and the 461m col spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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

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 Mason's Bank (SO 228 873)

LIDAR analysis gives the summit height of this hill as 480.0m and when compared to its originally listed height this 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 480.0m and this was derived from LIDAR analysis, this is 2.0m higher than the originally listed 478m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are:

Group:  Cilfaesty

Name:  Mason’s Bank

OS 1:50,000 map:  137

Summit Height (New Height):  480.0m (LIDAR)

Summit Grid Reference:  SO 22897 87344 (LIDAR)

Col Height:  460.9m (LIDAR)

Col Grid Reference:  SO 22384 87926 (LIDAR)

Drop:  19.0m (LIDAR)

 

Myrddyn Phillips and Aled Williams (April 2026)

Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

 

Troed Rhiw Bylchau (SN 761 471)

There has been a Significant Height Revision to a hill that used to be listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Troed Rhiw Bylchau (SN 761 471)

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

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Troed Rhiw Bylchau, 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 minor roads to its north-east and south-west, and has the town of Llanwrtyd towards the east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with 19m of drop, based on the 383m summit spot height that appears n the Ordnance Survey 1:25,000 Explorer map and the 364m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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

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 Troed Rhiw Bylchau (SN 761 471)

The summit height produced by LIDAR analysis is 387.8m and is positioned at SN 76197 47177, and this 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, Harvey or other interactive 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 with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed summit height of this hill is 387.8m and this was derived from LIDAR analysis, this is 4.8m higher than the previously listed 383m summit height, which was based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

 

The full details for the hill are: 

Group:  Esgair Wen 

Name:  Troed Rhiw Bylchau 

OS 1:50,000 map:  146, 147

Summit Height (New Height):  387.8m (LIDAR)                                                           

Summit Grid Reference:  SN 76197 47177 (LIDAR) 

Bwlch Height:  360.6m (LIDAR) 

Bwlch Grid Reference:  SN 76205 47017 & SN 76217 47008 (LIDAR) 

Drop:  27.1m (LIDAR) 

 

Myrddyn Phillips (April 2026)

 

Monthly Synopsis

 

Synopsis of all major changes detailed on Mapping Mountains during the last calendar month.  These can include new, confirmation and retrospective changes

 

Hill Reclassifications: 

Carreg Rhoson (SM 665 254) - 30-99m Double Sub-Twmpau deletion

Twyni Mawr (SN 606 938) - Welsh P15 addition

 

Significant Height Revisions: 

Grinllwm (SH 772 625) - The Welsh P15s

Coed Penllyn (SH 786 696) - 100m Twmpau

Bryn Berw (SN 714 505) - Y Pedwarau - The 400m Hills of Wales

Summit Relocations: 

Llanfair Hill (SO 255 785) - The Fours - The 400m Hills of England

Disgwylfa (SO 172 841) - The Fours - The 400m Hills of England

Yr Eifl (SH 364 447) - 500m Twmpau, Y Trechol - The Dominant Hills of Wales and The Welsh Highlands - Uchafion Cymru

Significant Name Changes: 

Chwarel Penrhyn (SH 621 658) - 200m Twmpau

Disgwylfa (SO 172 841) - The Fours - The 400m Hills of England