Mapping Mountains – Significant Name Changes – The Welsh P15s

 

Allt y Brain (SN 660 927) 

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 Allt y Brain (SN 660 927)

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 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 8m of drop and listed under the point (Pt. c 27m) notation, based on an estimated c 27m summit height and an estimated c 19m 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

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map Ordnance Survey used for many decades leading to the production of the 1:10,000 Series of maps, both have now been superseded by the digitised Master Map.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 Allt y Brain 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 Allt y Brain, and this was derived from the Ordnance Survey series of Six-Inch maps.

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Allt y Brain 

Previously Listed Name:  Pt. c 27m 

OS 1:50,000 map:  135

Summit Height:  35.4m (LIDAR)                                                           

Summit Grid Reference:  SN 66012 92718 (LIDAR)                                                  

Bwlch Height:  18.5m (LIDAR) 

Bwlch Grid Reference:  SN 66123 92883 (LIDAR) 

Drop:  16.9m (LIDAR) 

 

Myrddyn Phillips (October 2025)

Mapping Mountains – Significant Height Revisions – The Welsh P15s

 

Allt y Brain (SN 660 927) 

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 Allt y Brain (SN 660 927)

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 Allt y Brain, and this was derived from the Ordnance Survey series of Six-Inch maps, 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 8m of drop, based on an estimated c 27m summit height and an estimated c 19m 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 Allt y Brain (SN 660 927)

The summit height produced by LIDAR analysis is 35.4m 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 35.4m and this was derived from LIDAR analysis.  This is 8.4m higher than the previous listed summit height of c 27m, which was based on interpolation of the uppermost 20m 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:  Allt y Brain 

OS 1:50,000 map:  135

Summit Height (New Height):  35.4m (LIDAR)                                                           

Summit Grid Reference:  SN 66012 92718 (LIDAR)                                                  

Bwlch Height:  18.5m (LIDAR) 

Bwlch Grid Reference:  SN 66123 92883 (LIDAR) 

Drop:  16.9m (LIDAR) 

 

Myrddyn Phillips (October 2025)

 

Mapping Mountains – Hill Reclassifications – The Welsh P15s

Allt y Brain (SN 660 927) – Welsh P15 addition 

There has been an addition to the list of 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 Allt y Brain (SN 660 927)

The criteria for the list that this addition 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 Allt y Brain, and this was derived from the Ordnance Survey series of Six-Inch maps, 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 8m of drop, based on an estimated c 27m summit height and an estimated c 19m 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 addition of this hill to Welsh P15 status is due to LIDAR analysis, resulting in a 35.4m summit height and an 18.5m bwlch height, with these values giving this hill 16.9m 0f drop, which is sufficient for it to be classified as a Welsh P15. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Allt y Brain 

OS 1:50,000 map:  135

Summit Height:  35.4m (LIDAR)                                                           

Summit Grid Reference:  SN 66012 92718 (LIDAR)                                                  

Bwlch Height:  18.5m (LIDAR) 

Bwlch Grid Reference:  SN 66123 92883 (LIDAR) 

Drop:  16.9m (LIDAR) 

 

Myrddyn Phillips (October 2025)