Mapping Mountains – Hill Reclassifications – 100m Twmpau

Bryn y Fedwen (SJ 115 125) – 100m Sub-Twmpau addition 

There has been confirmation of an addition to the list of 100m 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. 

LIDAR image of Bryn y Fedwen (SJ 115 125) 

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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

The 100m Twmpau by Myrddyn Phillips

The name the hill is listed by is Bryn y Fedwen and this was derived from the Tithe map, and it is adjoined to the Y Berwyn group of hills, which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with minor roads to its north-east, north-west and south and the A495 road to its east, and has the village of Meifod towards the east 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 20m of drop, based on the 157m summit spot height and an estimated c 137m bwlch height, with the latter based on interpolation of 10m contouring between 130m – 140m that appear 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. 

LIDAR analysis confirms this hill as having over 20m of drop, but as this hill has now been surveyed with the Trimble GeoXH 6000 resulting in a 156.8m summit height, it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the summit of Bryn y Fedwen

Therefore, the confirmation of the addition of this hill to 100m Sub-Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 156.8m summit height and a 134.9m 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:  Y Berwyn 

Name:  Bryn y Fedwen 

OS 1:50,000 map:  125

Summit Height:  156.8m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SJ 11582 12548 (Trimble GeoXH 6000)

Bwlch Height:  134.9m (LIDAR)

Bwlch Grid Reference:  SJ 11523 12758 (LIDAR)

Drop:  21.8m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (July 2021)