Mapping Mountains – Hill Reclassifications – 100m Twmpau

Mynydd y Castell (SS 806 864) – 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 Mynydd y Castell

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.

The name of the hill is Mynydd y Castell, 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 it has the A48 road and the M4 motorway to its south-west and the town of Aberafan towards its north-west and the village of Y Pîl (Pyle) towards its south south-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 was thought at that stage not to meet the criteria 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 38m of drop, based on an estimated summit height of c 121m and an estimated bwlch height of c 83m, with both heights based on interpolation of 10m contouring on the Ordnance Survey 1:25,000 Explorer map.

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

The drop value for this hill was re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map.  Although this mapping still has no summit spot height it does have an 83m bwlch spot height and the advantage of 5m contouring which enabled a better interpolation of the drop value, and the hill was subsequently listed with c 37m of drop.

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 confirmation of the reclassification of Mynydd y Castell to 100m Twmpau status is due to LIDAR analysis, resulting in a 120.1m summit height and an 83.8m bwlch height, with these values giving this hill 36.2m of drop which is sufficient for it to be classified as a 100m Twmpau.

The full details for the hill are:

Group:  Cymoedd Morgannwg

Name:  Mynydd y Castell

OS 1:50,000 map:  170

Summit Height:  120.1m (LIDAR, natural ground)

Summit Grid Reference:  SS 80681 86460 (LIDAR, natural ground)

Bwlch Height:  83.8m (LIDAR)

Bwlch Grid Reference:  SS 80753 86593 (LIDAR)

Drop:  36.2m (LIDAR)

Myrddyn Phillips (May 2019)