Mapping Mountains – Hill Reclassifications – The Welsh P15s

Bonc (SH 539 796) – Welsh Sub-P15 deletion

There has been a deletion 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 Bonc (SH 539 796)

The criteria for the list that this deletion 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 Bonc 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 a minor road to its immediate north-west, and has the village of Pentraeth towards the south-west. 

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 32m summit height and an estimated c 18m bwlch height, with both heights based on interpolation of 5m 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 deletion of this hill from Welsh Sub-P15 status is due to LIDAR analysis, resulting in a 33.5m summit height and a 20.85m bwlch height, with these values giving this hill 12.7m of drop, which is insufficient for it to be classified as a Welsh Sub-P15. 

 

The full details for the hill are: 

Group:  Ynys Môn 

Name:  Bonc 

OS 1:50,000 map:  114, 115

Summit Height:  33.5m (LIDAR)                                                           

Summit Grid Reference:  SH 53915 79698 (LIDAR) 

Bwlch Height:  20.85m (LIDAR) 

Bwlch Grid Reference:  SH 53863 79583 & SH 53862 79582 (LIDAR) 

Drop:  12.7m (LIDAR) 

 

Myrddyn Phillips (May 2021)