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

Allt Cilhywi (SO 035 358) – Sub-Trichant reclassified to Trichant

There has been 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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Allt Cilhywi (SO 035 358)

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 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 Cilhywi, and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north and the B4520 road to its west, and has the small community of Lower Chapel towards the 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, as it was considered not to meet the criteria then used for the main P30 category.

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 30m of drop, based on the 334m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 304m bwlch height, based on interpolation of 10m contouring between 300m – 310m.  The details for this hill were later re-assessed against the Ordnance Survey 1:25,000 Explorer map and the drop value amended to an estimated c 31m, based on an interpolated bwlch height of c 303m. 

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.

The initial reclassification of this hill from Trichant to Sub-Trichant status was based on analysis of the available 2m DTM LIDAR, at that stage no 1m DTM LIDAR was available, resulting in a 333.2m summit height and a 303.5m bwlch height, with these values giving this hill 29.7m of drop.  However, 1m LIDAR is now available and the details for this hill have been analysed using this.

Therefore, the reclassification of this hill from Sub-Trichant status is due to analysis of the latest available 1m DTM LIDAR, resulting in a 334.1m summit height and a 303.6m bwlch height, with these values giving this hill 30.5m of drop, which is sufficient for it to be classified as a Trichant. 

 

The full details for the hill are: 

Group:  Mynydd Epynt 

Name:  Allt Cilhywi 

OS 1:50,000 map:  160

Summit Height:  334.1m (LIDAR)                                                           

Summit Grid Reference:  SO 03505 35843 (LIDAR) 

Bwlch Height:  303.6m (LIDAR) 

Bwlch Grid Reference:  SO 03893 35958 (LIDAR) 

Drop:  30.5m (LIDAR) 

 

Myrddyn Phillips (July 2024)