Wednesday, 8 April 2020

Mapping Mountains – Summit Relocations – Y Pedwarau – The 400m Hills of Wales

Twyn Croes Gwallter (SN 949 145)

There has been a Summit Relocation to a hill that is listed in the Y Pedwarau – The 400m 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 Aled Williams.

LIDAR image of Twyn Croes Gwallter (SN 949 145)

The criteria for the list that this summit relocation applies to are:

Y PedwarauThe 400m Hills of Wales.  Welsh hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main Y Pedwarau list are five categories of sub hills, with this hill listed in the 400m Sub-Pedwar category.  The criteria for 400m Sub-Pedwar status being all Welsh hills at or above 400m and below 500m in height that have 20m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

The name the hill is listed by is Twyn Croes Gwallter and it is adjoined to the Fforest Fawr group of hills, which are situated in the western part of South Wales (Region C, Sub-Region C2), and it is positioned with the A470 road to its north north-east and the A4059 road to its south-east, and has the town of Merthyr Tudful towards the south-east.

When the 1st edition of the Y Pedwarau was published by Europeaklist in May 2013 this hill was listed with c 22m of drop based on the 458m summit height that appears as a spot height positioned at SN 946 144 on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 436m based on interpolation of 10m contouring between 430m – 440m.

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

When the Y Pedwarau- The 400m Hills of Wales list was published on Mapping Mountains the summit position for this hill was given as a ten figure grid reference of SN 94626 14403, based on the position of the spot height.

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 Twyn Croes Gwallter

The summit height produced by LIDAR analysis is 456.6m at SN 94995 14573, and this position in relation to that previously given comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when it is positioned to a different feature such as in a conifer plantation, or when the high point of the hill is placed within a different map contour compared to its previous listed position, or when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.

Therefore, the summit height produced by LIDAR analysis for this hill is 456.6m and is positioned at SN 94995 14573, this position is not given a spot height on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and is approximately 370 metres eastward from where the previously listed summit is positioned.

ills of Wales, and are reproduced below@
The full details for the hill are:

Group:  Fforest Fawr

Name:  Twyn Croes Gwallter

OS 1:50,000 map:  160

Summit Height:  456.6m (LIDAR)

Summit Grid Reference (new position):  SN 94995 14573 (LIDAR) 
Bwlch Height:  434.5m (LIDAR)

Bwlch Grid Reference:  SN 95917 15369 (LIDAR)

Drop:  22.1m (LIDAR)

Aled Williams and Myrddyn Phillips (April 2020)

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