Mapping Mountains – Hill Reclassifications – The Welsh P15s

Pen yr Allt (SH 429 917) – Welsh P15 reclassified to Welsh Sub-P15 

There has been a reclassification 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 429 917)

The criteria for the list that this reclassification 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 this was derived from the Tithe map and it is adjoined to the Ynys Môn group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the coast to its north, and is encircled by minor roads with the A5025 road farther to its north and the B5111 road farther to its east, and has the town of Amlwch towards the north-east. 

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main P15 list with 15m of drop, based on the 69m 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 and the 54m bwlch 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. 

Therefore, the reclassification of this hill to Welsh Sub-P15 status is due to LIDAR analysis, resulting in a 69.4m summit height and a 55.1m bwlch height, with these values giving this hill 14.3m of drop, which is insufficient for it to be included in the main list of The Welsh P15s. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Pen yr Allt 

OS 1:50,000 map:  114

Summit Height:  69.4m (LIDAR)                                                           

Summit Grid Reference:  SH 42925 91717 (LIDAR) 

Bwlch Height:  55.1m (LIDAR) 

Bwlch Grid Reference:  SH 43244 91030 (LIDAR) 

Drop:  14.3m (LIDAR) 

 

Myrddyn Phillips (September 2021)