Mapping Mountains – Hill Reclassifications – Y Trichant – The 300m Hills of Wales

Yr Allt (SO 196 358) – Sub-Trichant reclassified to Trichant

There has been confirmation of a reclassification to the list of Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Yr Allt (SO 196 358)

The criteria for the list that this reclassification 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 13th May 2017.

The name of the hill is Yr Allt and it is adjoined to the Mynyddoedd Duon group of hills which are situated in the eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with minor roads to its west, north and east, with the A4078 road further to its west and the A438 road further to its north-west, and has the village of Aberllynfi (Three Cocks) 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 included in the Hills to be surveyed sub list that accompanied the main P30 list, as it was considered not to meet the criteria then used for the main P30 list. 

When 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 31m of drop, based on an estimated c 335m summit height and an estimated c 304m bwlch height.

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

The details for this hill were re-examined when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many spot heights not on any other publicly available Ordnance Survey map and gives a 302m spot height on the area of this hill’s bwlch.  This spot height is also shown on Ordnance Survey data that appears on the Magic Maps website.

Extract from Ordnance Survey data hosted on the Magic Maps website

During the re-assessment of this hill’s details the summit height was amended to 334m based on the 1096ft (334.06m) imperial height that appears on the summit area of this hill on the series of Ordnance Survey Six-Inch maps.  These re-assessments resulted in the hill being listed with 32m of drop.

Extract from the series of Ordnance Survey Six-Inch maps

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 Yr Allt

LIDAR bwlch image for Yr Allt

The confirmation of the reclassification of Yr Allt from Sub-Trichant status is due to LIDAR analysis, resulting in a 334.0m height to a remaining natural summit and a 301.7m bwlch height, with these values giving this hill 32.3m of drop which is sufficient for it to be classified as a Trichant.

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Yr Allt

OS 1:50,000 map:  161

Summit Height:  334.0m (LIDAR, natural summit)

Summit Grid Reference:  SO 19677 35879 (LIDAR, natural summit)

Bwlch Height:  301.7m (LIDAR)

Bwlch Grid Reference:  SO 20120 35549 (LIDAR)

Drop:  32.3m (LIDAR)

Myrddyn Phillips (December 2019)