Mapping Mountains – Hill Reclassifications – The Welsh P15s

 

Y Garreg Las (SN 768 098) – Welsh Sub-P15 reclassified to Welsh 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 Y Garreg Las (SN 768 098)

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 now listed by is Y Garreg Las and this was derived from the Tithe map and it is adjoined to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with a minor road to its immediate north-east, and west, the A4068 road farther to its north-east and the A4067 road to its south-east, and has the community of Y Gurnos towards the east south-east. 

Extract from the Ordnance Survey 1:25,000 Explorer map

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 the 157m 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 an estimated c 143m bwlch height, based on interpolation of 5m contouring between 140m – 145m that appeared on the OS Maps website.  Latterly the interactive mapping hosted on the WalkLakes website gives this hill a 158m summit spot height. 

Extract from the interactive mapping hosted on the WalkLakes website

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 from Welsh Sub-P15 status is due to LIDAR analysis, resulting in a 158.5m summit height and a 141.7m bwlch height, with these values giving this hill 16.75m of drop, which is sufficient for it to be included in the main list of The Welsh P15s. 

 

The full details for the hill are: 

Group:  Mynydd Du 

Name:  Y Garreg Las 

OS 1:50,000 map:  160

Summit Height:  158.5m (LIDAR)                                                           

Summit Grid Reference:  SN 76803 09804 (LIDAR) 

Bwlch Height:  141.7m (LIDAR) 

Bwlch Grid Reference:  SN 76797 09723 (LIDAR) 

Drop:  16.75m (LIDAR) 

 

Myrddyn Phillips (March 2024)