Mapping Mountains – Summit Relocations – 200m Twmpau

 

Grug (SN 441 328) 

There has been a Summit Relocation to a hill that is listed in the 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Grug (SN 441 328)

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

200m Twmpau - Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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. 

200m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Grug and this was derived from the Tithe map, and it is adjoined to the Mynydd Pencarreg group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with a minor road to its north and south-east, and the A485 road to its east, and has the town of Llandysul towards the north north-west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

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 25m of drop, based on the 266m summit spot height positioned on a road at SN 44142 32904 that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and the 241m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map. 

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 267.4m positioned at SN 44163 32897.  However, this is on the edge of a raised road 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 Grug (SN 441 328)

The height produced by LIDAR analysis to the natural summit of this hill is 266.6m and this is positioned at SN 44127 32866, 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 considered 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 summit height produced by LIDAR analysis is 266.6m and this is positioned at SN 44127 32866, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 38 metres south-westward from the previously listed summit position and the high point of the raised road. 

 

The full details for the hill are: 

Group:  Mynydd Pencarreg 

Name:  Grug 

OS 1:50,000 map:  146

Summit Height:  266.6m (LIDAR) 

Summit Grid Reference (New Position):  SN 44127 32866 (LIDAR) 

Bwlch Height:  240.6m (LIDAR) 

Bwlch Grid Reference:  SN 45100 33058 (LIDAR) 

Drop:  26.0m (LIDAR) 

 

Myrddyn Phillips (October 2024)