Mapping Mountains – Summit Relocations – The Welsh P15s

Y Garreg Las (SN 768 098) 

There has been a Summit Relocation to a hill that is listed in the 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 summit relocation 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 positioned at SN 76897 09754 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. 

LIDAR summit image of Y Garreg Las (SN 768 098)

LIDAR analysis gives the highest ground on this hill as 158.5m positioned at SN 76803 09804, and this position comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to when the high point of the hill is positioned in a different field, to a different feature such as a conifer plantation, within a different map contour, a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the summit height produced by LIDAR analysis is 158.5m and this is positioned at SN 76803 09804, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 95 metres north-westward from where the originally listed summit is positioned. 

 

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 (New Position):  SN 76803 09804 (LIDAR) 

Bwlch Height:  141.7m (LIDAR) 

Bwlch Grid Reference:  SN 76797 09723 (LIDAR) 

Drop:  16.75m (LIDAR) 

 

Myrddyn Phillips (April 2024)