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

Clogwyn Mawr (SH 728 583) – 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 confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Clogwyn Mawr (SH 728 583)

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.

The name the hill is listed by is Clogwyn Mawr and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A5 road to its west and south, and has the village of Capel Curig towards the south-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 Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used for this sub category. 

When the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated.  Its details were also 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 available on any other publicly available Ordnance Survey map.  The hill was subsequently listed with an estimated c 23m of drop, based on the 347m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 324m based on bwlch contouring between 320m – 330m.

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

The details for this hill were also re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and the online Vector Map Local.  This latest re-assessment resulted in the hill being listed with an estimated c 24m of drop, based on the 347m summit spot height and an estimated bwlch height of c 323m based on bwlch contouring between 320m – 325m.

Extract from the OS Maps website

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. 

The confirmation of the addition of Clogwyn Mawr to Sub-Trichant status is due to LIDAR analysis, resulting in a 345.7m summit height and a 321.8m bwlch height, with these values giving this hill 23.9m of drop which is sufficient for it to be classified as a Sub-Trichant.

The full details for the hill are:

Group:  Carneddau

Name:  Clogwyn Mawr

OS 1:50,000 map:  115

Summit Height:  345.7m (LIDAR)

Summit Grid Reference:  SH 72874 58313 (LIDAR)

Bwlch Height:  321.8m (LIDAR)

Bwlch Grid Reference:  SH 72844 58514 (LIDAR)

Drop:  23.9m (LIDAR)

Myrddyn Phillips (November 2019)