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

Cefn Tŷ Mawr (SN 986 576) – 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 a Trimble GeoXH 6000 survey and subsequent LIDAR analysis conducted by Myrddyn Phillips.

Cefn Tŷ Mawr (SN 986 576)

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.

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Cefn Tŷ Mawr, and it is adjoined to the Elenydd group of hills, which are situated in the western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the A470 road to its north-east and the B4358 road to its south-east, and has the village of Newbridge-on-Wye towards the east north-east.

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 accompanying 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 29m of drop, based on the 323m summit spot height and an estimated c 294m bwlch height, with the latter based on interpolation of 10m contouring between 290m – 300m that appear on the Ordnance Survey 1:25,000 Explorer map.

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

Since publication of these P30 lists on Geoff Crowder’s v-g.me website there have been a number of Ordnance Survey maps made available online, some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website, whilst others are current and digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, and it was this mapping that had a 292m spot height on the area of this hill’s bwlch, which when coupled with its 323m summit spot height gave this hill 31m of drop.

As the listed drop value of this hill was close to the minimum of 30m required for Trichant status, this hill was prioritised for a GNSS survey, and this took place on the 23^rd May 2015, resulting in 31.8m of drop.

Gathering data at the summit of Cefn Tŷ Mawr

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed against the Trimble GeoXH 6000 survey result.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.  LIDAR analysis resulted in the height and position of this hill’s bwlch being amended.

LIDAR image of Cefn Tŷ Mawr

Therefore, the confirmation of the reclassification of this hill from Sub-Trichant status is due to a Trimble GeoXH 6000 survey and subsequent LIDAR analysis, resulting in a 324.2m summit height and a 292.0m bwlch height, with these values giving this hill 32.2m of drop, which is sufficient for it to be classified as a Trichant.

The full details for the hill are:

Group:  Elenydd

Name:  Cefn Tŷ Mawr

OS 1:50,000 map:  147

Summit Height:  324.2m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 98691 57665 (Trimble GeoXH 6000)

Bwlch Height:  292.0m (LIDAR)

Bwlch Grid Reference:  SN 98452 57819 (LIDAR)

Drop:  32.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

For details of the Trimble GeoXH 6000 survey of this hill

Myrddyn Phillips (November 2020)