Mapping Mountains – Hill Reclassifications – 100m Twmpau

Twyn y Cryn (SO 325 008) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification 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 reclassification 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 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list that accompanied the main P30 list, as it did not meet the criteria then used in this list. 

When 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 an estimated c 35m of drop, based on an estimated c 147m summit height and an estimated c 112m bwlch height, with both heights based on interpolation of 10m contouring between 110m - 120m that appears on the Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; and which is entitled the Interactive Coverage Map.  This mapping has additional spot heights not available on any other form of Ordnance Survey publicly available map, although no additional spot heights are given for this hill, this mapping does have contours at 5m intervals for this hill with bwlch contouring between 110m – 115m with an estimated bwlch height of c 112m and an uppermost summit contour of c 145 with an estimated summit height of c 147m, with these values confirming the interpolated values given this hill based on detail from the Ordnance Survey 1:25,000 Explorer map.

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. 

Therefore, the confirmation of the reclassification of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 148.3m summit height and a 113.0m bwlch height, with these values giving this hill 35.3m of drop which is sufficient for it to be classified as a 100m Twmpau.

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:  SO 32531 00891 (LIDAR)

Bwlch Height:  113.0m (LIDAR)

Bwlch Grid Reference:  SO 32392 01153 (LIDAR)

Drop:  35.3m (LIDAR)

Myrddyn Phillips (September 2019)