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

Allt Ddu (SN 714 776) – Sub-Trichant addition

There has been confirmation of an addition 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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Allt Ddu (SN 714 776)

The criteria for the list that this addition 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

The name the hill is listed by is Allt Ddu, and it is adjoined to the Y Garn group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north-east and the A4120 road to its south, and has the village of Pontarfynach (Devil’s Bridge) towards the east south-east.

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 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 22m of drop, based on the 323m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 301m bwlch height based on interpolation of 10m contouring between 300m – 310m. 

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.

Therefore, the confirmation of the addition of this hill to Sub-Trichant status is due to LIDAR analysis, resulting in a 324.1m summit height and a 300.3m bwlch height, with these values giving this hill 23.8m of drop, which is sufficient for it to be classified as a Sub-Trichant. 

 

The full details for the hill are: 

Group:  Y Garn 

Name:  Allt Ddu 

OS 1:50,000 map:  135, 147

Summit Height:  324.1m (LIDAR)                                                           

Summit Grid Reference:  SN 71425 77687 (LIDAR) 

Bwlch Height:  300.3m (LIDAR) 

Bwlch Grid Reference:  SN 70947 77718 (LIDAR) 

Drop:  23.8m (LIDAR) 

 

Myrddyn Phillips (February 2023)