Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Carreg y Defaid (SH 341 325) – 30-99m Sub-Twmpau deletion

There has been a deletion to the 30-99m Twmpau due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

30-99m Twmpau - Welsh hills at and 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 and above 30m and below 100m in height with 20m and more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

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

Prior to LIDAR analysis this hill was listed with an estimated c 20m of drop based on the 33m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 13m based on bwlch contouring at 5m intervals and between 10m – 15m on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The name of the hill is Carreg y Defaid and it is adjoined to the Pen Llŷn group of hills, which is situated in the north-west part of North Wales (Region A, Sub-Region A1), and is positioned with the A 499 road to its west and between the village of Llanbedrog to the south-west and the town of Pwllheli towards the north-west.

If wanting to visit the hill permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so the Wales Coast Path passes close to its summit and would give easiest access.

The deletion of Carreg y Defaid from 30-99m Sub-Twmpau status is due to LIDAR analysis conducted by Myrddyn Phillips.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR analysis gives the hill the following details:

Carreg y Defaid

Summit Height:  33.4m

Summit Grid Reference:  SH 34109 32555

Bwlch Height:  13.7m (natural bwlch)

Bwlch Grid Reference:  SH 33751 32623 (natural bwlch)

Drop:  19.7m

LIDAR image of Craig y Defaid

Therefore, the 33.389m LIDAR analysis for the summit position at SH 34109 32555 and the 13.693m LIDAR analysis for the bwlch position at SH 33751 32623 gives this hill 19.696m of drop, which is insufficient for 30-99m Sub-Twmpau status.

There is a 13.747m higher hill to hill traverse positioned at SH 33818 32605, but as this is to an elevated road which is considered man-made the remaining natural bwlch is being taken for the drop value of this hill.

The full details for the hill are:

Group:  Pen Llŷn

Name:  Carreg y Defaid

Summit Height:  33.4m (LIDAR)

Summit Grid Reference:  SH 34109 32555 (LIDAR)

OS 1:50,000 map:  123

Drop:  19.7m (LIDAR)

Myrddyn Phillips (April 2018)