Mapping Mountains – Significant Height Revisions – Y Trichant – The 300m Hills of Wales

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

There has been a Significant Height Revision 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 height revision 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

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.

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. 

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 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 Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR.  Also included are hills whose summit map data is missing an uppermost ring contour when compared with the data produced by the Trimble or by LIDAR analysis.

Therefore, the new listed prioritised summit height of this hill is 351.9m and this was derived from LIDAR analysis, this is 6.9m higher than the previously listed 345m summit height, which was based on the spot height that 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 (New Height):  344.4m (natural Dual Summit) (LIDAR) and 351.9m (artificial Dual Summit (LIDAR)

Summit Grid Reference:  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)