Mapping Mountains – Summit Relocations – 100m Twmpau

Bron y Gaer (SN 699 326) 

There has been confirmation of a Summit Relocation to a hill that is listed in the 100m 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 Bron y Gaer (SN 699 326)

The criteria for the list 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. 

The 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Bron y Gaer and this was derived from the Tithe map, and it is adjoined to the Mynydd Mallaen group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its west and south, the A40 road farther to its south and the A482 road to its east, and has the town of Llanymddyfri (Llandovery) towards the east north-east. 

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website, this hill was listed with a 198m summit height positioned at SN 697 328, based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

Extract from the Ordnance Survey 1:25,000 Explorer map

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites. 

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map became available online.  This mapping had many spot heights not on other publicly available Ordnance Survey maps and for this hill it had a 199m summit spot height positioned at SN 69968 32669, therefore the listed summit position of this hill was relocated. 

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 summit image of Bron y Gaer (SN 699 326)

The summit height produced by LIDAR analysis is 199.7m and is positioned at SN 69948 32662 and SN 69950 32662, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are: 

The term Summit Relocations applies to when the high point is positioned in a different field, to a different feature such as a conifer plantation, within a different map contour, a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit 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 199.7m and is positioned at SN 69948 32662 and SN 69950 32662, 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 270 metres south-eastward from where the previously listed summit is positioned. 

 

The full details for the hill are: 

Group:  Mynydd Mallaen

Name:  Bron y Gaer 

OS 1:50,000 map:  146, 160

Summit Height:  199.7m (LIDAR) 

Summit Grid Reference (New Position):  SN 69948 32662 & SN 69950 32662 (LIDAR) 

Bwlch Height:  166.5m (LIDAR) 

Bwlch Grid Reference:  SN 69362 32822 (LIDAR) 

Drop:  33.2m (LIDAR) 

 

Myrddyn Phillips (April 2022)