Mapping Mountains – Hill Reclassifications – The Welsh P15s

 

Pen yr Allt (SH 748 011) – Welsh Sub-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 Pen yr Allt (SH 748 011)

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 Pen yr Allt, 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 A487 road to its west and the A489 road to its south, and has the town of Machynlleth 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, and was documented with an estimated c 13m of drop, based on an estimated c 74m summit height and an estimated c 61m 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 Sub-P15 status is due to LIDAR analysis, resulting in a 73.8m summit height and a 59.7m bwlch height, with these values giving this hill 14.1m 0f drop, which is sufficient for it to be classified as a Welsh Sub-P15. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Pen yr Allt 

OS 1:50,000 map:  135

Summit Height:  73.8m (LIDAR)                                                           

Summit Grid Reference:  SH 74846 01144 (LIDAR)                                                  

Bwlch Height:  59.7m (LIDAR) 

Bwlch Grid Reference:  SH 74986 01253 (LIDAR) 

Drop:  14.1m (LIDAR) 

 

Myrddyn Phillips (August 2025)