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

Cwm Helyg (SO 121 460) 

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 detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Cwm Helyg (SO 121 460)

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 now listed by is Cwm Helyg and this was derived from the Tithe map, and it is adjoined to the Gwaun Ceste group of hills, which are situated in the eastern part of South Wales (Region B, Sub-Region B3), and it is positioned with a minor road to its north and south-west, the A470 road farther to its west and the B4594 road to its south-east, and has the small community of Erwyd (Erwood) towards the south-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 Hills to be surveyed sub list with a summit height of 332m, based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 26m of drop, based on the 332m summit spot height positioned at SO 12205 46007 and an estimated c 306m bwlch height, based on interpolation of 10m contouring between 300m – 310m. 

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 333.8m positioned at SO 12132 46026.  However, this is a part of an old 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 Cwm Helyg (SO 121 460)

The height produced by LIDAR analysis to the natural summit of this hill is 333.5m positioned at SO 12120 46030, and this position in relation to the old 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 333.5m and this is positioned at SO 12120 46030, 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 85 metres west north-westward from where the 332m spot height appears and approximately 12 metres west north-westward from the high point of the raised field boundary. 

 

The full details for the hill are:

Group:  Gwaun Ceste 

Name:  Cwm Helyg 

OS 1:50,000 map:  148

Summit Height:  333.5m (LIDAR)                                                           

Summit Grid Reference:  SO 12120 46030 (LIDAR) 

Bwlch Height:  303.8m (LIDAR) 

Bwlch Grid Reference:  SO 12449 46144 (LIDAR) 

Drop:  29.7m (LIDAR) 

 

Myrddyn Phillips (February 2024)