Thursday, 5 March 2020

Mapping Mountains – Hill Reclassifications – Marilyns

Coed y Bwnydd (SO 366 069) – Submarilyn addition

This is one in a series of retrospective Hill Reclassification posts that detail hills whose status has altered in the listing of the Welsh Marilyns.  These reclassification posts will give details of hills where I have had direct association with their change of status, and they will tie in with a forthcoming Change Register giving detail to this list and its alterations since publication in the TACit Tables booklet

The listing of Welsh Marilyns was published in booklet format by TACit Tables in February 1997 and entitled The Hewitts and Marilyns of Wales.  The Marilyns were originally listed as an all British compilation and were published by Cicerone Press in 1992 in the book entitled The Relative Hills of Britain.  The list compiler for the Marilyns is Alan Dawson.

When the Welsh Marilyns list was published in booklet format there were 156 qualifying hills with a further 27 Welsh Submarilyns also included.  The criteria for Marilyn status being any hill that has a drop of at least 150m, irrespective of their height, with the criteria for Submarilyn being any hill that has 140m or more and below 150m of drop.

The Hewitts and Marilyns of Wales by Alan Dawson

The details for the hill reclassification appear below:

The name of the hill is Coed y Bwnydd, and it is situated in the Mynyddoedd Duon group of hills in the south-eastern part of Wales, and is positioned with the Afon Wysg (River Usk) and the B4598 road to its west, the A40 road to its north and the A449 road to its east, and has the village of Raglan towards the east north-east and the town of Brynbuga (Usk) towards the south south-east.

Prior to this hill’s reclassification to Submarilyn status it was listed by the name of Clytha Hill with an estimated c 68m of drop based on the 196m spot heights that appear on the Ordnance Survey 1:25,000 Explorer map at SO 36831 06935 and the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map at SO 36738 06931, and an estimated bwlch height of c 128m based on bwlch contouring between 125m – 130m.  

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 of Coed y Bwnydd (SO 366 069)

The result produced by LIDAR analysis also affects the adjoining hill named Trostrey Hill (SO 369 051), with LIDAR now showing that Coed y Bwnydd is higher and therefore the respective bylchau positions for each hill are swapped.  The swapping of bylchau has also altered the drop value for each hill, with the caveat that the drop value for the higher hill; Coed y Bwnydd has also been amended as there is a road cutting on the hill to hill traverse.  The LIDAR heights for each summit and bwlch are given below:

Coed y Bwnydd: 

201.0m summit at SO 36613 06921

59.4m bwlch at SO 39307 08899

141.6m drop

Trostrey Hill

199.2m summit at SO 36921 05189

129.0m bwlch at SO 37461 05937

70.2m drop  

LIDAR analysis for Coed y Bwnydd was conducted by Myrddyn Phillips resulting in a summit height of 201.0m positioned at SO 36613 06921 with this position to an embankment that is a part of an ancient hill fort, and the bwlch height is 59.4m positioned at SO 39307 08899 with this position to a road cutting, and the ground of both meets the criteria used within this list.

LIDAR bwlch image for Coed y Bwnydd

The addition of this hill to Submarilyn status was accepted by the list author; Alan Dawson, and its new classification augmented in to the listing of Marilyns in April 2019.

The full details for the hill are:

Name:  Coed y Bwnydd

OS 1:50,000 map:  161

OS 1:25,000 map:  13

Summit Height:  201.0m (LIDAR)

Summit Grid Reference:  SO 36613 06921 (LIDAR)

Bwlch Height:  59.4m (LIDAR)

Bwlch Grid Reference:  SO 39307 08899 (LIDAR)

Drop:  141.6m (LIDAR)

Myrddyn Phillips (March 2020)

No comments: