Mapping Mountains – Summit Relocations – 200m Twmpau

 

Bigni (SN 199 311) 

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 Bigni (SN 199 311)

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.  The list is authored by Myrddyn Phillips, 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 Bigni, and this was derived from the Ordnance Survey series of Six-Inch maps with the language protocol being used, and it is adjoined to the Mynydd Preseli group of hills, which are situated in the south-western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north, south and east, and the A478 road to its west, and has the village of Crymych towards the 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 included in the main P30 list with a 263m summit height, based on the spot height that appears on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

After the accompanying 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 51m of drop, based on the 263m summit spot height positioned at SN 19920 31148 and an estimated c 212m bwlch height, based on interpolation of 5m contouring between 210m – 215m. 

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 264.5m positioned at SN 19915 31110.  However, according to visits documents online this consists of a pile of dumped soil 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 Bigni (SN 199 311)

The height produced by LIDAR analysis to the what is deemed the natural summit of this hill is 262.9m and this is positioned at SN 19911 31149, with the caveat that due to the extensive soil heaps on the summit area of this hill it is hard to distinguish what is natural ground, 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 either on Ordnance Survey maps or interactive mapping, 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 262.9m and this is positioned at SN 19911 31149, and this position is approximately 39 metres northward from the highest pile of dumped soil and to a different feature. 

 

The full details for the hill are: 

Group:  Mynydd Preseli 

Name:  Bigni 

OS 1:50,000 map:  145

Summit Height:  262.9m (LIDAR) 

Summit Grid Reference (New Position):  SN 19911 31149 (LIDAR) 

Bwlch Height:  211.8m (LIDAR) 

Bwlch Grid Reference:  SN 19869 31901 (LIDAR) 

Drop:  51.1m (LIDAR) 

 

Myrddyn Phillips (February 2025)