Mapping Mountains – Summit Relocations – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Hill Park (SM 955 102) 

There has been a Summit Relocation to a hill that is listed in the 30-99m 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 prompted by detail produced by Joe Nuttall in his surface analysis programme, with subsequent LIDAR analysis conducted by the DoBIH team and independently by Myrddyn Phillips. 

LIDAR image of Hill Park (SM 955 102)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height that have 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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. 

The 30-99m Twmpau by Myrddyn Phillips

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 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format. 

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The name the hill is now listed by is Hill Park, and this was derived from the Tithe map, and it is adjoined to the Garn Fawr group of hills which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned encircled by minor roads, with the A4076 road farther to its west, and the A477 road to its south-west, and has the village of Johnston towards the west.

When the original 30-99m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a 99m summit height positioned at SM 95593 10231, based on the spot height that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with an estimated c 31m of drop, based on the 99m summit spot height and an estimated c 68m bwlch height, based on interpolation of 5m contouring between 65m – 70m, resulting with the drop value being insufficient in relation to the summit height for consideration to Lesser Dominant status. 

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 analysis gives the highest ground on this hill as 101.7m positioned at SM 95588 10281.  However, LIDAR contouring indicates this is a mound constituting disturbed ground, implying that it is not natural and protocols dictate that as this is deemed a relatively recent man-made construct such ground is discounted from the height of a hill. 

LIDAR summit image of Hill Park (SM 955 102)

The height produced by LIDAR analysis to the highest remaining natural ground on this hill is 99.5m and this is positioned at SM 95585 10244, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, 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 or covered reservoir that is judged to be a relatively recent man-made construct, 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.

Therefore, the height produced by LIDAR analysis to the highest remaining natural ground on this hill is 99.5m and is positioned at SM 95585 10244, this position is close to where the 99m spot height appears on the contemporary Ordnance Survey 1:25,000 Explorer map, and is approximately 37 metres southward from where the mound is positioned. 

 

The full details for the hill are: 

Group:  Garn Fawr

Name:  Hill Park

OS 1:50,000 map:  157, 158

Summit Height:  99.5m (LIDAR)

Summit Grid Reference (New Position):  SM 95585 10244 (LIDAR)

Bwlch Height:  63.6m (LIDAR)

Bwlch Grid Reference:  SM 93281 10622 (LIDAR)

Drop:  35.9m (LIDAR)

Dominance:  36.16% (LIDAR)

 

Myrddyn Phillips (January 2024)