Saturday, 7 March 2020

Mapping Mountains – Summit Relocations – 100m Twmpau and Y Trechol – The Dominant Hills of Wales

Trostrey Hill (SO 369 051)

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau and 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 Trostrey Hill (SO 369 051)

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

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

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.  The list is authored by Myrddyn Phillips with the Introduction to the start of 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 listed by is Trostrey Hill and it 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 Afon Wysg (River Usk) and the B4598 road to its west, the A40 road to its north, the A472 road to its south 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.

When the original Welsh 100m P30 list was published on Geoff Crowder’s website this hill was listed with a 199m summit height based on the spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and which is adjoined to a triangulation pillar positioned at SO 36917 05185.

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

The details for this hill were re-evaluated when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online, and as this map gives no further details the summit location remained the same.

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 Trostrey Hill

The summit height produced by LIDAR analysis to remaining natural ground is 199.2m positioned at SO 36921 05189 with LIDAR also giving a 199.3m height positioned at SO 36931 05185 to a raised field boundary that is excluded from the height of this hill as it is considered a recent man-made construct.  The above detail 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 is 199.2m to ground meeting the criteria used within this list and this is positioned at SO 36921 05189, this is approximately 10 metres from where LIDAR gives the high point of the raised field boundary, but as this is considered a relatively recent man-made construct it is not taken as a part of the height of this hill.

The full details for the hill are:

Group:  Mynyddoedd Duon

Name:  Trostrey Hill

OS 1:50,000 map:  161

Summit Height:  199.2m (LIDAR, natural ground)

Summit Grid Reference (new position):  SO 36921 05189 (LIDAR, natural ground)

Bwlch Height:  129.0m (LIDAR)

Bwlch Grid Reference:  SO 37461 05937 (LIDAR)
Drop:  70.2m (LIDAR)

Dominance:  35.23% (LIDAR)

Myrddyn Phillips (March 2020)

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