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

Cae Gwar Allt (SN 455 133) 

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 derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Cae Gwar Allt (SN 455 133)

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 that have 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 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 Cae Gwar Allt and this was derived from the Tithe map, and it is adjoined to the Mynydd Sylen group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with minor roads to its north, west and south-east, and has the A4306 road farther to its north-east and the A4309 road to its south-west, and has the town of Caerfyrddin (Carmarthen) towards the north-west.

When the original 100m 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 111m summit height, based on the spot height that appears at SN 45492 13327 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-assessed and it was listed with an estimated c 44m of drop, based on the 111m summit spot height and an estimated c 67m bwlch height, based on interpolation of 5m contouring between 65m – 70m.  With these values giving this hill 39.64% dominance. 

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 109.8m positioned at SN 45495 13312.  However, this is probably excavated ground from construction of a pool 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 Cae Gwar Allt (SN 455 133)

The height produced by LIDAR analysis to the natural summit of this hill is 109.7m and is positioned at SN 45512 13322, 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 109.7m and is positioned at SN 45512 13322, this position is relatively close to where the spot height appears on the contemporary Ordnance Survey 1:25,000 Explorer map, and is approximately 17 metres north-eastward from the high point of the excavated ground. 

 

The full details for the hill are: 

Group:  Mynydd Sylen 

Name:  Cae Gwar Allt 

OS 1:50,000 map:  159

Summit Height:  109.7m (LIDAR) 

Summit Grid Reference (New Position):  SN 45512 13322 (LIDAR) 

Bwlch Height:  67.5m (LIDAR) 

Bwlch Grid Reference:  SN 46302 13388 (LIDAR) 

Drop:  42.2m (LIDAR) 

Dominance:  38.46% (LIDAR) 

 

Myrddyn Phillips (July 2024)