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 – Significant Height Revisions – The Welsh P15s

Grinllwm (SH 772 625) 

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. 

LIDAR image of Grinllwm (SH 772 625)

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 now listed by is Grinllwm, and its name placement was derived from the Ordnance Survey series of Six-Inch maps, and it 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 included in the main list 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

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 Grinllwm (SH 772 625)

The summit height produced by LIDAR analysis is 267.7m and when compared to the previously listed summit height, 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 267.7m and this was derived from LIDAR analysis.  This is 3.7m higher than the previous listed summit height of c 264m, which was based on interpolation of the uppermost 260m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Grinllwm 

OS 1:50,000 map:  115

Summit Height (New 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 (March 2026)

 

Mapping Mountains – Significant Height Revisions – The Welsh P15s

Bryn Caniadau (SH 764 617) 

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. 

LIDAR image of Bryn Caniadau (SH 764 617)

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 Bryn Caniadau, and this was derived from the Ordnance Survey series of Six-Inch maps, and it 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-west and east, with the A5 road to its south and the B5106 road and the A470 road towards its east, and has the town of Llanrwst towards the east. 

Extract from the Ordnance Survey series of Six-Inch maps

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the P14 sub list with an estimated c 14m of drop, based on an estimated c 213m summit height and an estimated c 199m 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. 

LIDAR summit image of Bryn Caniadau (SH 764 617)

The summit height produced by LIDAR analysis is 215.1m and when compared to the previously listed summit height, 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 215.1m and this was derived from LIDAR analysis.  This is 2.1m higher than the previous listed summit height of c 213m, which was based on interpolation of the uppermost 210m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Carnedd Llywelyn 

Name:  Bryn Caniadau 

OS 1:50,000 map:  115

Summit Height (New Height):  215.1m (LIDAR)                                                           

Summit Grid Reference:  SH 76492 61758 (LIDAR)                                                  

Bwlch Height:  201.2m (LIDAR) 

Bwlch Grid Reference:  SH 76439 61684 (LIDAR) 

Drop:  13.9m (LIDAR) 

 

Myrddyn Phillips (February 2026)

  

Mapping Mountains – Significant Height Revisions – The Welsh P15s

Cae Grib (SN 679 992) 

There has been a Significant Height Revision 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 Cae Grib (SN 679 992)

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 Cae Grib, and this was derived from the Tithe map, 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 A493 road to its immediate north-west, and has the small community of Cwrt towards the north-east.

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, and was subsequently documented with an estimated c 5m of drop, based on an estimated c 38m summit height and an estimated c 33m 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. 

LIDAR summit image of Cae Grib (SN 679 992)

The summit height produced by LIDAR analysis is 45.4m and when compared to the previously listed summit height, 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 45.4m and this was derived from LIDAR analysis.  This is 7.4m higher than the previous listed summit height of c 38m, which was based on interpolation of the uppermost 30m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Tarren y Gesail 

Name:  Cae Grib 

OS 1:50,000 map:  135

Summit Height (New Height):  45.4m (LIDAR)                                                           

Summit Grid Reference:  SN 67980 99200 (LIDAR)                                                  

Bwlch Height:  29.95m (LIDAR) 

Bwlch Grid Reference:  SN 67971 99264 (LIDAR) 

Drop:  15.4m (LIDAR) 

 

Myrddyn Phillips (November 2025)

  

Mapping Mountains – Significant Height Revisions – The Welsh P15s

Coed Felin Lodge (SN 663 932) 

There has been a Significant Height Revision 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 Coed Felin Lodge (SN 663 932)

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 Coed Felin Lodge, 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 B4353 road to its south-west and the A487 road to its east, and has the town of Machynlleth towards the north-east.

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, and was subsequently documented with an estimated c 12m of drop, based on an estimated c 66m summit height and an estimated c 54m 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. 

LIDAR summit image of Coed Felin Lodge (SN 663 932)

The summit height produced by LIDAR analysis is 70.9m and when compared to the previously listed summit height, 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 70.9m and this was derived from LIDAR analysis.  This is 4.9m higher than the previous listed summit height of c 66m, which was based on interpolation of the uppermost 60m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map.

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Coed Felin Lodge 

OS 1:50,000 map:  135

Summit Height (New Height):  70.9m (LIDAR)                                                           

Summit Grid Reference:  SN 66384 93253 (LIDAR)                                                  

Bwlch Height:  52.1m (LIDAR) 

Bwlch Grid Reference:  SN 66457 93532 (LIDAR) 

Drop:  18.8m (LIDAR) 

 

Myrddyn Phillips (November 2025)