Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Creigiau Cochion (SN 611 989) – 100m Sub-Twmpau deletion

There has been a deletion from the list of 100m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Creigiau Cochion (SN 611 989)

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

100m Twmpau – Welsh hills at or above 100m and below 200m in height that have 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.  The list is authored by Myrddyn Phillips, 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 now listed by is Creigiau Cochion and this was derived from the Tithe map, and it is adjoined to the Tarren y Gesail group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), and it is positioned with a minor road to its north and the A493 road to its west and south, and has the town of Tywyn towards the north-west.

When the original 100m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category.

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-assessed and it was listed with an estimate c 20m of drop, based on the 168m summit spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map and an estimated c 148m bwlch height, based on interpolation of 10m contouring between 140m – 150m. 

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

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 deletion of this hill from 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 168.3m summit height and a 148.3m bwlch height, with these values giving this hill 19.98m of drop, which is insufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Tarren y Gesail 

Name:  Creigiau Cochion 

OS 1:50,000 map:  135

Summit Height:  168.3m (LIDAR) 

Summit Grid Reference:  SN 61176 98937 (LIDAR) 

Bwlch Height:  148.3m (LIDAR) 

Bwlch Grid Reference:  SN 61236 98904 (LIDAR) 

Drop:  19.98m (LIDAR) 

 

Myrddyn Phillips (April 2025)