Mapping Mountains – Hill Reclassifications – 200m Twmpau

Ffridd Eithaf (SN 745 977) – 200m Twmpau deletion

There has been a deletion to the list of the 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips. 

Ffridd Eithaf (SN 745 977)

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

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 200m and below 300m 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 200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Ffridd Eithaf and it is adjoined to the Pumlumon group of hills, which are situated in the north-western part of Mid and West Wales (Region B, Sub-Region B2), and it is positioned with the A489 road to its north north-east, minor roads to its west, south and east, and the A487 road farther to its north-west and has the town of Machynlleth towards the north. 

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a summit height of 272m.

When 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 35m of drop, based on the 272m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated c 237m bwlch height, based on interpolation of 10m contouring between 230m – 240m.  

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

When the listing that became known as The Welsh P15s was being compiled, the adjacent hill listed as Ffridd Cae Crydd (SN 749 976) was included in the accompanying P14 sub list with an estimated c 14m of drop, based on an estimated c 271m summit height and an estimated c 257m bwlch height, with both heights based on interpolation of 10m contouring that appear on the Ordnance Survey 1:25,000 Explorer map. 

LIDAR image of Ffridd Cae Crydd (SN 749 976) on right and Ffridd Eithaf (SN 745 977) on left

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 Ffridd Eithaf (SN 745 977)

LIDAR analysis confirms Ffridd Cae Crydd as higher than Ffridd Eithaf, therefore their respective bylchau are swapped and as the summit of both hills have now been surveyed with the Trimble GeoXH 6000 it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the summit of Ffridd Eithaf

The Trimble GeoXH 6000 survey result for each summit is given below: 

 

Ffridd Cae Crydd:  272.289m at SN 74910 97692 

Ffridd Eithaf:  272.175m at SN 74578 97750 

 

Therefore, the confirmation of the deletion of this hill from 200m Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 272.2m summit height and a 255.7m bwlch height, with these values giving this hill 16.5m of drop, which is sufficient for it to retain its Welsh P15 status. 

 

The full details for the hill are: 

Group:  Pumlumon 

Name:  Ffridd Eithaf 

OS 1:50,000 map:  135

Summit Height:  272.2m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 74578 97750 (Trimble GeoXH 6000) 

Bwlch Height:  255.7m (LIDAR) 

Bwlch Grid Reference:  SN 74763 97683 (LIDAR) 

Drop:  16.5m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (October 2021)