Mapping Mountains – Significant Name Changes – The Welsh P15s

Grinllwm (SH 772 625) 

There has been a Significant Name Change to a hill that is 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. 

LIDAR image of Grinllwm (SH 772 625)

The criteria for the list that this name change 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 hill is adjoined to the Carnedd Llywelyn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with minor roads to its north, south and east, with the B5106 road and the A470 road farther to its east, and has the town of Llanrwst towards the east south-east.

When the listing that became known as The Welsh P15s was being compiled, this hill was listed under the point (Pt. c 264m) notation, with an estimated c 15m of drop, based on an estimated c 264m summit height and the 249m 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

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, and it is the series of Ordnance Survey Six-Inch maps that form the basis of the change in the listed name of this hill.

The Ordnance Survey series of Six-Inch maps formed the base map Ordnance Survey used for many decades leading to the production of the 1:10,000 Series of maps, both have now been superseded by the digitised Master Map.  The series of Six-Inch maps are excellent for name placement and especially so compared to the contemporary Ordnance Survey 1:25,000 Explorer map, and it is the series of Six-Inch maps that position the name of Grinllwm to land where the summit of this hill is situated.

Extract from the Ordnance Survey series of Six-Inch maps

Therefore, the name this hill is now listed by in the The Welsh P15s is Grinllwm, and this was derived from the Ordnance Survey series of Six-Inch maps. 

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Grinllwm 

Previously Listed Name:  Pt. c 264m 

OS 1:50,000 map:  115

Summit Height:  267.7m (LIDAR)                                                           

Summit Grid Reference:  SH 77258 62516 (LIDAR)                                                  

Bwlch Height:  249.1m (LIDAR) 

Bwlch Grid Reference:  SH 77295 62465 (LIDAR) 

Drop:  18.6m (LIDAR) 

 

Myrddyn Phillips (April 2026) 

Mapping Mountains – Hill Reclassifications – 200m Twmpau

Rhos Swydd (SO 117 650) – 200m Sub-Twmpau addition

There has been confirmation of an addition to the list of 200m Twmpau, 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 Rhos Swydd (SO 117 650)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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. 

200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Rhos Swydd, and this was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Fforest Glud group of hills, which are situated in the eastern part of South Wales (Region B, Sub-Region B3), and it is positioned with a minor road to its north-west, the A44 road to its south-west and the A488 road to its south-east, and has the town of Llandrindod towards the south-west.

When the original 200m 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 an estimated c 20m of drop, based on an estimated c 267m summit height and an estimated c 247m bwlch height, with both heights based on interpolation of 10m contouring 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.

Therefore, the confirmation of the addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 268.0m summit height and a 244.8m bwlch height, with these values giving this hill 23.2m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Fforest Glud 

Name:  Rhos Swydd 

OS 1:50,000 map:  148

Summit Height:  268.0m (LIDAR) 

Summit Grid Reference:  SO 11706 65038 (LIDAR) 

Bwlch Height:  244.8m (LIDAR) 

Bwlch Grid Reference:  SO 12253 65143 (LIDAR) 

Drop:  23.2m (LIDAR) 

 

Myrddyn Phillips (April 2026) 

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

 

Mainstone Hill (SO 262 880) 

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 Mainstone Hill (SO 262 880)

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 Mainstone Hill, and this was derived from an 1847 inclosure award, and it is adjoined to the Cilfaesty group of hills, which are situated adjoined to the Welsh border, and it is positioned encircled by minor roads, with the B4385 road and the A489 road farther to its north, the B4368 road farther to its south-west and the A488 road farther to its east, and has the town of Bishop’s Castle towards the 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 15m of drop, based on the 418m summit spot height and the 403m 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 Mainstone Hill (SO 262 880)

LIDAR analysis gives the summit height of this hill as 421.3m 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 421.3m and this was derived from LIDAR analysis, this is 3.2m higher than the originally listed 418m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are:

Group:  Cilfaesty

Name:  Mainstone Hill

OS 1:50,000 map:  137

Summit Height (New Height):  421.3m (LIDAR)

Summit Grid Reference:  SO 26273 88041 (LIDAR)

Col Height:  404.3m (LIDAR)

Col Grid Reference:  SO 25878 88118 (LIDAR)

Drop:  17.0m (LIDAR)

 

Myrddyn Phillips and Aled Williams (April 2026)

Mapping Mountains – Summit Relocations – 100m Twmpau

Coed Penllyn (SH 786 696)

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau, 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 Coed Penllyn (SH 786 696)

The criteria for the list that this summit relocation applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height that have 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.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

100m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Coed Penllyn, and this was derived from the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, and it is adjoined to the Mynydd Hiraethog group of hills, which are situated in the northern part of North Wales (Region A, Sub-Region A2), and it is positioned with the A470 road to its west and a minor road to its east, and has the town of Llanrwst towards the south.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for the main P30 list, and listed with a 193m summit height positioned at SH 786 697 with an accompanying note stating; Height from 1989 1:50000 map. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed as a twin top with an estimated c 25m of drop, based on the 193m spot height that appears on the 1:50,000 Landranger map and a twin 193m map heighted top positioned at SH 78713 69823 that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 168m bwlch height, based on interpolation of 10m contouring between 160m – 170m. 

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 Coed Penllyn (SH 786 696)

LIDAR analysis gives the highest ground on this hill as 198.0m positioned at SH 78630 69665, and compared to its previously listed twin summit position this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or the listing of a new twin summit or de-twinning of a summit, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the height produced by LIDAR analysis to the summit of this hill is 198.0m and is positioned at SH 78630 69665, this position is close to the spot height that appears on the Ordnance Survey 1:50,000 Landranger map and approximately 158 metres south-westward from where the twin map heighted spot height appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are: 

Group:  Mynydd Hiraethog 

Name:  Coed Penllyn 

OS 1:50,000 map:  115

Summit Height:  198.0m (LIDAR) 

Summit Grid Reference (New Position):  SH 78630 69665 (LIDAR) 

Bwlch Height:  170.9m (LIDAR) 

Bwlch Grid Reference:  SH 78892 70134 (LIDAR) 

Drop:  27.1m (LIDAR)

Myrddyn Phillips (April 2026) 

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)