Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Deg Erw Llandafin (ST 003 731) – 30-99m Twmpau reclassified to 30-99m Sub-Twmpau

There has been confirmation of a reclassification to a hill listed in the 30-99m 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 Deg Erw Llandafin (ST 003 731)

The criteria for the list that this reclassification applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m 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 the hill is now listed by is Deg Erw Llandafin and this was derived from the Tithe map, and it is adjoined to the Bro Morgannwg group of hills, which are situated in the southern part of South Wales (Region C, Sub-Region C2), and it is positioned with the A4222 and A48 roads to its north with minor roads to its west, south and east, and has the town of Y Bont-faen (Cowbridge) to the north-west.

When the original 30-99m 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 the main P30 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 a 67m summit height based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map which is positioned outside of the uppermost 65m ring contour and an estimated bwlch height of c 37m based on interpolation of 5m contouring between 35m – 40m, with these values giving this hill an estimated c 30m of drop.

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 and its larger dimension enabled easier study.  It was at this stage that the 67m spot height was considered erroneous and the drop of the hill was listed as an estimated c 29m.

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 this hill to 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in a 66.4m summit height and a 37.2m bwlch height, with these values giving this hill 29.2m of drop, which is insufficient for continued 30-99m Twmpau status. 

The full details for the hill are:

Group:  Bro Morgannwg

Name:  Deg Erw Llandafin

OS 1:50,000 map:  170

Summit Height:  66.4m (LIDAR)

Summit Grid Reference:  ST 00313 73177 & ST 00315 73179 (LIDAR)

Bwlch Height:  37.2m (LIDAR)

Bwlch Grid Reference:  ST 00721 73222 (LIDAR)

Drop:  29.2m (LIDAR)

Myrddyn Phillips (October 2019)