Mapping Mountains – Summit Relocations – 100m Twmpau

Bryn Bach (SS 903 875)

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 being confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Bryn Bach

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 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.

The name of the hill is Bryn Bach, and this was derived from the Ordnance Survey One-Inch ‘Old Series’ map and it is this name that the hill is now listed by, and it is adjoined to the Cymoedd Morgannwg group of hills, which are situated in the central part of South Wales (Region C, Sub-Region C2), and is positioned with the A4063 road and the Afon Llynfi to its south-west and the A4064 road and the Afon Garw to its south-east, and has the small community of Betws towards the south. 

When this hill was originally listed in the 100m height band of Welsh P30 hills on Geoff Crowder’s v-g.me website, its summit was listed at SS900871 and it appeared with an accompanying note stating; Two points of same height – other at SS903875, with these details being taken from the Ordnance Survey 1:25,000 Explorer map.

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

Prior to LIDAR analysis this hill was listed with an estimated c 31m of drop, based on the 164m summit spot height that appears on the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map.  This spot height appears beside a field boundary at SS 90354 87551.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph 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.  

The height produced by LIDAR analysis is 167.0m positioned at SS 90364 87531, and is in a different field and not beside the field boundary compared to the position of the 164m spot height, and confirms this land is higher than the prioritised position originally given in the 100m P30 list of Geoff Crowder’s v-g.me website, and as such comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, 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, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these relocations.

LIDAR summit image of Bryn Bach

Therefore, the height produced by LIDAR analysis is 167.0m to what is constituted natural ground and this is positioned at SS 90364 87531, this position is not given a spot height on Ordnance Survey maps and is approximately 600 metres north-eastward from where the prioritised listed summit was originally given and is positioned in a different field to where the 164m spot height appears on the Ordnance Survey Vector Map Local hosted on the Geograph website.

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Bryn Bach

OS 1:50,000 map:  170

Summit Height:  167.0m (LIDAR)

Summit Grid Reference (New Position):  SS 90364 87531 (LIDAR)

Bwlch Height:  135.2m (LIDAR)

Bwlch Grid Reference:  SS 90511 87849 (LIDAR)

Drop:  31.9m (LIDAR)

Myrddyn Phillips (June 2019)