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

Tirlenwiad Bryn Posteg (SN 971 820) and Pt. 344.4m (SN 970 825) 

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 Tirlenwiad Bryn Posteg (SN 971 820) and Pt. 344.4m (SN 970 825)

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

Dual Summit hills:

A hill classified as a Dual Summit is defined as one which has an extant natural summit coupled with that of a higher artificial summit, with the latter that can be described as stable in character.  In the main, these recent man-made constructions are the result of quarrying activities producing spoil tips, or as in this instance a landfill.  These recent man-made constructions are treated differently to ancient man-made constructions such as hill forts and tumuli, as if the latter are deemed stable and of an earthen character their age dictates that they can be viewed as being permanent in nature and are now effectively a part of the hill.  For those bagging Dual Summit hills, a visit to either the natural high point or the elevated man-made high point is sufficient to claim an ascent of the hill.  With the Dual Summit classification being a relatively new category and fist instigated in January 2018 for a Dual Summit Pedwar.

The name the hill is now listed by is 344.4m for the lower natural summit and Tirlenwiad Bryn Posteg for the higher man-made summit, 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 minor roads to its north-east and south-east, and the B4518 road to its south-west, and has the town of Llanidloes 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 not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

Extract from the Ordnance Survey 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 345m summit spot height positioned at SN 97010 82528 that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 319m bwlch height, based on interpolation of 10m contouring between 310m – 320m.

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 summit image of Tirlenwiad Bryn Posteg (SN 971 820)

The summit height produced by LIDAR analysis for the artificial summit is 351.9m and is positioned at SN 97109 82092 and SN 97114 82098, and this comes within the parameters of the Significant Height Revisions 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 listing of a new twin summit or 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 new listed prioritised summit height of this hill is 351.9m and this is positioned at SN 97109 82092 and SN 97114 82098, this is approximately 430 metres southward from where the 345m spot height appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

 

The full details for the hill are:

Group:  Hirddywel 

Name:  344.4m (natural Dual Summit) and Tirlenwiad Bryn Posteg (artificial Dual Summit)

OS 1:50,000 map:  136

Summit Height:  344.4m (natural Dual Summit) (LIDAR) and 351.9m (artificial Dual Summit (LIDAR)

Summit Grid Reference (New Position):  SN 97023 82535 (natural Dual Summit) (LIDAR) and SN 97109 82092 & SN 97114 82098 (artificial Dual Summit) (LIDAR)

Bwlch Height:  331.0m (LIDAR)

Bwlch Grid Reference:  SN 97021 81849 (LIDAR)

Drop:  13.4m (natural Dual Summit) (LIDAR) and 20.9m (artificial Dual Summit) (LIDAR)

 

Myrddyn Phillips (July 2025)