Mapping Mountains – Summit Relocations – Y Trichant – The 300m Hills of Wales

Rhos Garn Whilgarn (SN 451 517) 

There has been a Summit Relocation to a hill that is listed in the Y Trichant – The 300m Hills of Wales, 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 Rhos Garn Whilgarn (SN 451 517)

The criteria for the list that this summit relocation applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Rhos Garn Whilgarn, and it is adjoined to the Mynydd Bach group of hills, which are situated in the western part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its west and east, and the B4338 road to its south-west, and has the village of Post-mawr (Synod Inn) towards the north-west.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a summit height of 312m that appears as a spot height adjoined to a triangulation pillar positioned at SN 45117 51763 that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 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.

LIDAR analysis gives the highest ground on this hill as 313.1m positioned at SN 45149 51777.  However, this is a part of a raised field boundary and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Rhos Garn Whilgarn (SN 451 517)

The height produced by LIDAR analysis to the natural summit of this hill is 312.1m positioned at SN 45127 51754, and this position in relation to the raised field boundary comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill 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 312.1m and this is positioned at SN 45127 51754, 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 23 metres south-westward from where the high point of the raised field boundary is positioned and approximately 10 metres south-eastward from where the triangulation pillar is positioned, which is also situated on the same raised field boundary. 

 

The full details for the hill are:

Group:  Mynydd Bach 

Name:  Rhos Garn Whilgarn 

OS 1:50,000 map:  146

Summit Height:  312.1m (LIDAR)                                                           

Summit Grid Reference (New Position):  SN 45127 51754 (LIDAR) 

Bwlch Height:  273.3m (LIDAR) 

Bwlch Grid Reference:  SN 45739 50729 (LIDAR) 

Drop:  38.8m (LIDAR) 

 

Myrddyn Phillips (January 2024)