Mapping Mountains – Summit Relocations – 100m Twmpau

Twyn y Cryn (SO 325 008)

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 confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Twyn y Cryn (SO 325 008)

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.

100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Twyn y Cryn, and it is adjoined to the Cefn yr Ystrad group of hills, which are situated in the southern part of South Wales (Region B, Sub-Region B5), and it is positioned with the A4042 road to the west and the A472 road to the north, and has the town of Pont-y-pŵl (Pontypool) towards its west and Brynbuga (Usk) towards its east.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website this hill was listed with twin tops with the prioritised summit positioned at SO 325 005, with an accompanying note stating Two tops of same height – other at SO 326 009.

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

The details for this hill were re-evaluated when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online, however both tops were given an uppermost 145m contour ring and no spot height, therefore the details relating to this hill remained the same.

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.

LIDAR summit image of Twyn y Cryn (SO 325 008)

The height produced by LIDAR analysis to the natural summit of this hill is 148.3m positioned at SO 32531 00891,  this confirms the summit position and its relocation from its previously 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.

Therefore, the height produced by LIDAR analysis is 148.3m and this is positioned at SO 32531 00891, this position is not given a spot height on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and is approximately 350 metres northward from where the previously prioritised summit was listed.

The full details for the hill are:

Group:  Cefn yr Ystrad

Name:  Twyn y Cryn

OS 1:50,000 map:  171

Summit Height:  148.3m (LIDAR)

Summit Grid Reference (New Position):  SO 32531 00891 (LIDAR)

Bwlch Height:  113.0m (LIDAR)

Bwlch Grid Reference:  SO 32392 01153 (LIDAR)

Drop:  35.3m (LIDAR)

Myrddyn Phillips (September 2019)