Wednesday, 1 April 2020

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

Craig Arthbry (SJ 031 455) – Trichant reclassified to Sub-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 confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Craig Arthbry (SJ 031 455)

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 the hill is listed by is Craig Arthbry, and it is adjoined to the Mynydd Hiraethog group of hills which are situated in the north-eastern part of North Wales (Region A, Sub-Region A2), and it has the B5105 road to its north-west, the A5 road to its south and the A494 road to its east, and has the town of Corwen towards the east south-east.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s 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 30m of drop, based on the 317m spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 287m based on interpolation of 10m contouring between 280m – 290m.

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 had many spot heights not on other publicly available Ordnance Survey maps and in certain areas also included 5m contouring.  As this mapping has a 286m spot height in the area of this hill’s bwlch, the drop value was revised to 31m.  This spot height is also shown on Ordnance Survey data that appears on the Magic Maps website.

Extract from the Magic 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.

LIDAR summit image for Craig Arthbry

Therefore, the reclassification of this hill from Trichant status is due to LIDAR analysis, resulting in a 315.4m summit height and a 286.8m bwlch height, with these values giving this hill 28.6m of drop, which is sufficient for it to be classified as a Sub-Trichant.

The full details for the hill are:

Group:  Mynydd Hiraethog

Name:  Craig Arthbry

OS 1:50,000 map:  116

Summit Height:  315.4m (LIDAR)

Summit Grid Reference:  SJ 03112 45567 (LIDAR)

Bwlch Height:  286.8m (LIDAR)

Bwlch Grid Reference:  SJ 03460 45898 (LIDAR)

Drop:  28.6m (LIDAR)

Myrddyn Phillips (April 2020)

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