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

 

Bryn Posteg Hill (SN 960 822) 

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 Bryn Posteg Hill (SN 960 822)

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 Bryn Posteg Hill, and it is adjoined to the Hirddywel group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B2), and it is positioned with the B4518 road to its east and north, the A470 road to its west and a minor road to its south, and has the town of Llanidloes towards the north.

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 c 370m summit height, based on non interpolation of its uppermost contour that appears on the Ordnance Survey 1:25,000 Exlorer map.  The summit height was subsequently amended to 371m and its position given as SN 96025 82266. 

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 371.5m positioned at SN 96027 82264.  However, this is a part of a raised hedgebank 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 Bryn Posteg Hill (SN 960 822)

The height produced by LIDAR analysis to the natural summit of this hill is 370.7m positioned at SN 96020 82261, and this position in relation to the raised hedgebank 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 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.

Therefore, the summit height produced by LIDAR analysis is 370.7m and this is positioned at SN 96020 82261, 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 7 metres south-westward from where the high point of the raised hedgebank boundary is positioned. 

 

The full details for the hill are:

Group:  Hirddywel 

Name:  Bryn Posteg Hill 

OS 1:50,000 map:  136

Summit Height:  370.7m (LIDAR)                                                           

Summit Grid Reference (New Position):  SN 96020 82261 (LIDAR) 

Bwlch Height:  326.7m (LIDAR) 

Bwlch Grid Reference:  SN 96584 82501 (LIDAR) 

Drop:  44.0m (LIDAR) 

 

Myrddyn Phillips (November 2024)