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

Pt. 118.9m (SN 710 294)

There has been a Summit Relocation to a hill that is listed in the 100m Twmpau and the Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height and their locations, the drop, dominance and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Pt. 118.9m (SN 710 294)

The criteria for the two lists 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.

100m 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 inits entirety on Mapping Mountains in Google Doc format.

Y Trechol - The Dominant Hills of Wales by Myrddyn Phillips

The hill is being listed by the point (Pt. 118.9m) notation and it is adjoined to the Mynydd Du group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the Afon Tywi (River Towy) and the A40 road to its west, the B4069 road to its east, and has the village of Llangadog towards the south.

When the original 100m height band of Welsh P30hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 category and listed with a 119m summit height based on the spot height positioned at SN 72465 30422 and which appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

Extract from the 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-assessed and it was listed with an estimated c 50m of drop and 42.02% dominance, based on the 119m summit spot height an estimated c 69m bwlch height, based on interpolation of 5m contouring between 65m - 70m that appeared on the OS 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 analysis gives the highest ground on this hill as 119.8m positioned at SN 71062 29399.  However, this is a part of a raised field boundary 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 Pt. 118.9m (SN 710 294)

The height produced by LIDAR analysis to the natural ground of this hill is 118.0m and is positioned at SN 71046 29411, 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 natural summit of this hill is 118.9m and is positioned at SN 71046 29411, this position is clost to where the 119m spot height appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 18 metres north-westward from where the high point of the raised field boundary is positioned.  

The full details for the hill are:

Group:  Mynydd Du

Name:  Pt. 118.9m

OS 1:50,000 map:  146, 160

Summit Height:  118.9m (LIDAR)

Summit Grid Reference:  SN 71046 29411 (LIDAR)

Bwlch Height:  69.9m (LIDAR)

Bwlch Grid Reference:  SN 72466 30393 (LIDAR)

Drop:  48.9m (LIDAR)

Dominance:  41.16% (LIDAR)

Myrddyn Phillips (February 2019)