Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Coed Cerrig Mawr (SN 688 944) – 100m Sub-Twmpau addition

There has been an addition to 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 Coed Cerrig Mawr (SN 688 944) 

The criteria for the list that this addition 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 listed by is Coed Cerrig Mawr, and it is adjoined to the Banc Llechwedd Mawr group of hills, which are situated in the north-western part of South Wales (Region B, Sub-Region B1), and it is positioned with minor roads to its north and south-west, with the A487 road to its west, and has the town of Machynlleth towards the north-east.

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 estimated c 19m of drop, based on an estimated c 167m summit height and an estimated c 148m bwlch height, with both heights based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map. 

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 addition of this hill to 100m Sub-Twmpau status is due to LIDAR analysis, resulting in a 171.8m summit height and a 150.0m bwlch height, with these values giving this hill 21.8m of drop, which is sufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Banc Llechwedd Mawr 

Name:  Coed Cerrig Mawr 

OS 1:50,000 map:  135

Summit Height:  171.8m (LIDAR) 

Summit Grid Reference:  SN 68824 94413 (LIDAR) 

Bwlch Height:  150.0m (LIDAR) 

Bwlch Grid Reference:  SN 68905 94399 (LIDAR) 

Drop:  21.8m (LIDAR) 

 

Myrddyn Phillips (July 2025)