Friday, 3 July 2020

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

Chepstow Hill (ST 355 905)

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 Chepstow Hill (ST 355 905)

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 Chepstow Hill and it is adjoined to the Gwent Is Coed group of hills which are situated in the south-eastern part of South Wales (Region C, Sub-Region C3), and it is positioned with the Afon Wysg (River Usk) to its north-west, the M4 motorway to its south and the A449 road to its east, and has the city of Casnewydd (Newport) towards the south-west.

When the original Welsh 100m P30 list was published on Geoff Crowder’s website this hill was listed with a 111m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map and which is positioned at ST 36372 91144.

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

The details for this hill were re-assessed 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 gives a 114m spot height on the summit area of this hill and positioned at ST 35626 90589.  This 114m spot height also appears on Ordnance Survey data available 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 Chepstow Hill

The summit height produced by LIDAR analysis is 116.6m and is positioned at ST 35547 90567.  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 116.6m and this is positioned at ST 35547 90567, this is approximately 1000 metres south-westward from where the originally listed summit position was given which was to the position of the 111m spot height.

The full details for the hill are:

Group:  Gwent Is Coed

Name:  Chepstow Hill

OS 1:50,000 map:  171

Summit Height:  116.6m (LIDAR)

Summit Grid Reference (new position):  ST 35547 90567 (LIDAR)

Bwlch Height:  63.2m (LIDAR)

Bwlch Grid Reference:  ST 36979 90843 (LIDAR)

Drop:  53.4m (LIDAR)

Dominance:  45.76% (LIDAR)

Myrddyn Phillips (July 2020)

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