Mapping Mountains – Summit Relocations – The Welsh P15s

Y Lan Cwm Gwilym Fach (SO 070 364) 

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 Lan Cwm Gwilym Fach (SO 070 364)

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 Lan Cwm Gwilym Fach, and this was derived from the Tithe map, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads with the B4520 road farther to its west and the A470 road farther to its north-east and south-east, and has the small community of Capel Uchaf (Upper Chapel) towards the north-west.

When the listing that became known as The Welsh P15s was being compiled, this hill was included in the main list with an estimated c 14 of drop, based on the 367m summit spot height positioned at SO 07022 36442 that appears on the Ordnance Survey 1:25,000 Explorer map, and an estimated c 353m bwlch height, based on interpolation of 10m contouring between 350m – 360m. 

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 367.5m positioned at SO 07050 36413.  However, this is to the top of the remains of a boundary to an old conifer plantation 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 Y Lan Cwm Gwilym Fach (SO 070 364)

The height produced by LIDAR analysis to the natural summit of this hill is 367.4m and is positioned at SO 07042 36407, and this 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 either on Ordnance Survey maps or interactive mapping, 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 the de-twinning of a summit, 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 367.4m and is positioned at SO 07042 36407, 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 35 metres south-eastward from where the spot height is positioned and approximately 8 metres south-westward from where the high point of the boundary of the old conifer plantation is positioned. 

 

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  Y Lan Cwm Gwilym Fach 

OS 1:50,000 map:  160

Summit Height:  367.4m (LIDAR)                                                           

Summit Grid Reference (New Position):  SO 07042 36407 (LIDAR) 

Bwlch Height:  352.8m (LIDAR) 

Bwlch Grid Reference:  SO 07321 36626 (LIDAR) 

Drop:  14.6m (LIDAR) 

 

Myrddyn Phillips (May 2025)