Mapping Mountains – Hill Reclassifications – 100m Twmpau

 

Cae Bonc (SJ 129 121) – 100m Sub-Twmpau deletion 

There has been a deletion 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. 

Cae Bonc (SJ 129 121)

The criteria for the list that this deletion 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 Cae Bonc 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 south-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. 

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

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 20m of drop, based on the 145m summit spot height and the 125m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage 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 Cae Bonc (SJ 129 121)

LIDAR analysis gives this hill 20.3m of drop with a 145.2m summit height and a 124.9m bwlch height.  However, this hill has now been surveyed with the Trimble GeoXH 6000 resulting in a 144.8m summit height, and it is this result that is being prioritised for listing purposes. 

The Trimble GeoXH 6000 gathering data at the summit of Cae Bonc

Therefore, the deletion of this hill from 100m Sub-Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 144.8m summit height and a 124.9m bwlch height, with these values giving this hill 19.8m of drop, which is insufficient for it to be classified as a 100m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Y Berwyn 

Name:  Cae Bonc 

OS 1:50,000 map:  125

Summit Height:  144.8m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SJ 12918 12172 (Trimble GeoXH 6000)

Bwlch Height:  124.9m (LIDAR)

Bwlch Grid Reference:  SJ 12993 12315 (LIDAR)

Drop:  19.8m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

Myrddyn Phillips (July 2021)