Mapping Mountains – Hill Reclassifications – 200m Twmpau

 

Henfaes Common (SO 120 974) – 200m Sub-Twmpau deletion

There has been a deletion to the list of 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. 

Henfaes Common (SO 120 974)

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 Henfaes Common and this was derived from the Tithe map, and it is adjoined to the Carnedd Wen group of hills which are situated in the south-eastern part of North Wales (Region A, Sub-Region A4), and it is positioned with the B4390 road to its north, the B4389 road to its south and the A483 road to its east, and has the village of Betws Cedewain towards the south. 

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

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 20m of drop, based on the 214m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map, and the 194m bwlch spot height that appeared on the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage 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 image of Henfaes Common

The result produced by LIDAR analysis gives this hill 16.4m of drop, and as the summit has 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 Henfaes Common

Therefore, the deletion of this hill from 200m Sub-Twmpau status is due to LIDAR analysis and a Trimble GeoXH 6000 survey, resulting in a 215.2m summit height and a 198.8m bwlch height, with these values giving this hill 16.4m of drop, which is insufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Carnedd Wen 

Name:  Henfaes Common 

OS 1:50,000 map:  137

Summit Height:  215.2m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SO 12003 97409 (Trimble GeoXH 6000) 

Bwlch Height:  198.8m (LIDAR) 

Bwlch Grid Reference:  SO 12171 97587 (LIDAR) 

Drop:  16.4m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (February 2021)