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

Foel Gron (SH 583 621)

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 a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips.

Foel Gron (SH 583 621)

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

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 Foel Gron and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it positioned encircled by minor roads with the A4244 road to the north-west and the A4086 road and Llyn Padarn to the south-west, and has the villages of Deiniolen towards the north and Llanberis towards the south.

When the original Welsh 300m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with a 353m summit height, based on the spot height positioned at SH 58390 62231 that appears on the Ordnance Survey 1:25,000 Explorer map on the eastern side of a stone wall that crosses the upper part of this hill.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it also showed the 353m spot height on the eastern side of the upper stone wall.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website with the cursor indicating the position of the relocated summit

Three positions on the summit area of this hill were surveyed with the Trimble GeoXH 6000.  These are given below with the corresponding latest LIDAR analysis:

1^st position: 351.577 at SH 58388 62224 (LIDAR gives 351.760m at SH 58387 62224)

2^nd position:  351.482m at SH 58403 62175 (LIDAR gives 351.950m at SH 58404 62177)

3^rd position:  351.585m at SH 58359 62170 (LIDAR gives 351.850m at SH 58360 62171)

The Trimble GeoXH 6000 gives the third position as the higher, whilst the latest LIDAR analysis gives the second position as the highest.  The three heights produced by the Trimble are relatively close to one another, as indeed are the three heights produced by LIDAR analysis.  However, it is the Trimble data that is being prioritised for the summit of this hill.

The Trimble GeoXH 6000 gathering data at the summit of Foel Gron (SH 583 621)

The summit height produced by the Trimble GeoXH 6000 is 351.6m and is positioned at SH 58359 62170 and its position in relation to that previously given 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 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.

LIDAR summit image of Foel Gron (SH 583 621)

Therefore, the summit height produced by the Trimble GeoXH 6000 survey is 351.6m and is positioned at SH 58359 62170, 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 65 metres south south-westward from where the previously listed summit is positioned and where the 353m spot height appears on the Ordnance Survey 1:25,000 Explorer map.  

The full details for the hill are:

Group:  Glyder Fawr

Name:  Foel Gron

OS 1:50,000 map:  114, 115

Summit Height:  351.6m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference (New Position):  SH 58359 62170 (Trimble GeoXH 6000)

Bwlch Height:  319.4m (LIDAR)

Bwlch Grid Reference:  SH 58679 62182 (LIDAR)

  

Drop:  32.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (March 2019)