Wednesday, 22 January 2020

Mapping Mountains – Hill Reclassifications – Y Trechol – The Dominant Hills of Wales

Ash Wood (SO 480 122) – Lesser Dominant reclassified to Dominant

There has been a reclassification to the list of the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Ash Wood (SO 480 122)

The criteria for the list that this reclassification applies to are:

Y Trechol - The Dominant Hills of Wales - Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

The name the hill is now listed by is Ash Wood and this was derived from contemporary Ordnance Survey maps with the name of the bounded land where the summit of the hill is situated confirmed via the Tithe map.  The hill is adjoined to the Mynyddoedd Duon group of hills which are situated in the eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the B4233 road to its north-east and the A40 road to its south, and has the town of Trefynwy (Monmouth) towards the east.

When the original 200m height band of Welsh P30 hills published on Geoff Crowder’s website were standardised with interpolated heights and drop values this hill was listed with an estimated c 101m of drop based on an estimated c 206m summit height and an estimated c 105m bwlch height, with these heights derived from interpolation of 5m contouring on the Ordnance Survey 1:25,000 Explorer map.  When the original Dominants list was compiled this hill was listed as a Lesser Dominant based on the detail given above which gave this hill 49.03% dominance.  These values were later amended to an estimated c 207m summit height and an estimated c 104m bwlch height, giving this hill c 103m of drop and 49.76% dominance.  

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.

LIDAR summit image for Ash Wood

LIDAR bwlch image for Ash Wood

Therefore, the reclassification of Ash Wood from Lesser Dominant status is due to LIDAR analysis, resulting in a 206.4m summit height positioned at SO 48011 12229 and a 97.6m bwlch height positioned at SO 46851 13845, with the latter taken to a track cutting which complies with the criteria used within this list, with the caveat that the natural 105.6m high bwlch is intact and positioned at SO 46851 13803, with these values giving this hill 108.8m of drop and 52.69% dominance, which is sufficient for Dominant status.

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Ash Wood

OS 1:50,000 map:  161

Summit Grid Reference:  SO 48011 12229 (LIDAR)

Summit Height:  206.4m (LIDAR)

Bwlch Grid Reference:  SO 46851 13845 (LIDAR)

Drop Summit to Bwlch:  108.8m (LIDAR)

Drop Bwlch to ODN:  97.6m (LIDAR)

Dominance:  52.69% (LIDAR)

Myrddyn Phillips (January 2019)

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