Mapping Mountains – Hill Reclassifications – 200m Twmpau

Pt. 218.8m (SO 122 978) – 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. 

Pt. 218.8m (SO 122 978)

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 hill is being listed by the point (Pt. 218.8m) notation as an appropriate name for it either through local enquiry and / or historic research has not been found by the author, 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 minor roads to its south-east and north, and has the B4389 road to its south-west, and has the small community 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 included in the accompanying Hills to be surveyed sub list, and listed with an estimated c 230m summit height. 

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 27m of drop, based on an estimated c 231m summit height and a 204m bwlch height, with the latter based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.  An anomaly was also noted with the heights given the contours on the upper part of this hill on the 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 image of Pt. 218.8m

The result produced by LIDAR analysis gives this hill a 218.8m summit height and 13.6m of drop, confirming that the uppermost contour on the 1:25,000 Explorer map is not 230m, 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 Pt. 218.8m

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 218.8m summit height and a 205.3m bwlch height, with these values giving this hill 13.5m 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:  Pt. 218.8m 

OS 1:50,000 map:  136

Summit Height:  218.8m (converted to OSGM15, Trimble GeoXH 6000) 

Summit Grid Reference:  SO 12216 97806 (Trimble GeoXH 6000) 

Bwlch Height:  205.3m (LIDAR) 

Bwlch Grid Reference:  SO 12899 97950 (LIDAR) 

Drop:  13.5m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (January 2021)