Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Pt. 23.8m (SM 706 236) – Double Sub-Twmpau deletion

There has been a deletion to the list of 30-99m Twmpau, with the summit height and its position determined by a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips which took place on the 15^th May 2016, and subsequent LIDAR analysis for summit and bwlch height and the status of the hill conducted by Aled Williams.

Pt. 23.8m (SM 706 236)

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

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is 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, and the second sub category which this hill has been deleted from is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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.

As I do not know an appropriate name for this hill either from historic research or local enquiry it is being listed by the point (Pt. 23.8m) notation, and it is adjoined to the Carn Llidi group of hills which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), and it is positioned in the central eastern part of Ynys Dewi.

This hill did not appear in the accompanying Hills to be surveyed sub list when the original Welsh 30-99m P30 hills were published on Geoff Crowder’s v-g.me website, as it did not meet the criteria then used for this sub category, however this sub list has now been standardised, with interpolated drop values and interpolated summit heights also included.

The summit of this hill was subsequently surveyed with the Trimble GeoXH 6000 resulting in a 23.8m summit height, and with an estimated bwlch height of c 2m it was included as a Double Sub-Twmpau with c 22m of drop.

The Trimble GeoXH 6000 gathering data at the summit of Pt. 23.8m (SM 706 236)

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 image of Pt. 23.8m (SM 706 236).  1m contour (black), 10m contour (red) and sea level (yellow)

The deletion of this hill from Double Sub-Twmpau status is due to LIDAR analysis conducted by Aled Williams, resulting in a 23.8m summit height and a 5m bwlch height based on LIDAR contouring, with these values giving this hill 18.8m of drop which is insufficient for it to retain its Double Sub-Twmpau status.

The full details for the hill are:

Group:  Carn Llidi

Name:  Pt. 23.8m

OS 1:50,000 map:  157

Summit Height:  23.8m (converted to OSGM15)

Summit Grid Reference:  SM 70677 23686

Bwlch Height:  5.0m (based on LIDAR contouring)

Bwlch Grid Reference:  SM 70634 23719 (based on LIDAR contouring) 

  

Drop:  18.8m (Trimble summit and LIDAR summit and bwlch)

For details on the summit survey of this hill

My thanks to Aled Williams for forwarding his LIDAR analysis of this hill

Myrddyn Phillips (July 2019)

© Crown: CHERISH PROJECT 2019. Produced with EU funds through the Ireland Wales Co-operation Programme 2014-2020. All material made freely available through the Open Government Licence.