Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Cefn Coch (SH 610 396) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau 

There has been confirmation of a reclassification to the list of 30-99m 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 Cefn Coch (SH 610 396)

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 that have 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 30-99m Twmpau by Myrddyn Phillips

The name the hill is now listed by is Cefn Coch and it is adjoined to the Moelwynion group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A498 road and the B4410 road to its north-west, the A497 road and the A487 road to its south-west and the A4085 road to its east, and has the town of Penrhyndeudraeth towards the south. 

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, as it was considered not to meet the criteria then used for the P30 category.  This hill was listed with an 81m summit height, with an accompanying note stating; 86m on 1984 1:50,000 map. 

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

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 33m of drop, based on the 81m summit spot height and the 48m bwlch spot height that appear on the Ordnance Survey 1:25,000 Explorer map. 

The details for this hill were subsequently re-assessed as the bwlch contouring on the Ordnance Survey 1:25,000 Explorer map is between 50m – 60m, therefore according to map contouring the 48m spot height is below the position of the bwlch.  This reassessment resulted in the hill being listed with an estimated c 26m of drop, based on the 81m summit spot height and an estimated c 55m bwlch height. 

The details for this hill were later reassessed and the 86m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger map was favoured over that of the 81m spot height, and with the bwlch height amended to an estimated c 54m, these values gave this hill an estimated c 32m of drop. 

Extract from the Ordnance Survey 1:50,000 Landranger 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 confirmation of the reclassification of this hill from 30-99m Sub-Twmpau status is due to LIDAR analysis, resulting in an 86.6m summit height and a 51.5m bwlch height, with these values giving this hill 35.1m of drop, which is sufficient for it to be classified as a 30-99m Twmpau. 

 

The full details for the hill are: 

Group:  Moelwynion 

Name:  Cefn Coch 

OS 1:50,000 map:  124

Summit Height:  86.6m (LIDAR) 

Summit Grid Reference:  SH 61030 39620 (LIDAR)               

Bwlch Height:  51.5m (LIDAR) 

Bwlch Grid Reference:  SH 61461 39763 (LIDAR) 

Drop:  35.1m (LIDAR) 

 

Myrddyn Phillips (December 2021)