Mapping Mountains – Hill Reclassifications – 500m Twmpau

 

Y Drum (SJ 082 378) – 500m Sub-Twmpau reclassified to 500m Twmpau

There has been a reclassification to the listing of the 500m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips with assistance from Aled Williams and Mark Trengove. 

Y Drum (SJ 082 378)

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

500m Twmpau - Welsh hills at or above 500m and below 600m in height with 30m minimum drop, with an accompanying sub list entitled the 500m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 500m and below 600m 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 list is authored by Myrddyn Phillips and is available to download in Google Doc format from Mapping Mountains.

The 500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Y Drum 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 the A5 road to its north and the B4401 road to its west, and has the village of Cynwyd towards the north-west. 

When the list that later became known as the 500m Twmpau was first compiled, this hill was included in the sub list with 28m of drop, based on the 579m summit and 551m bwlch spot heights that appear on the contemporary Ordnance Survey 1:25,000 Explorer map. 

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

The details for this hill were then re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and it used to have contours at 5m intervals which proved consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.  This mapping had bwlch contouring between 545m – 550m, with interpolation giving an estimated bwlch height of c 549m.  It was the bwlch contouring on the OS Maps website that prioritised a GNSS survey of this hill. 

The Trimble GeoXH 6000 gathering data at the bwlch of Y Drum

The Trimble GeoXH 6000 gathering data at the summit of Y Drum

Therefore, the reclassification of this hill from 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey, resulting in a 579.1m summit height and a 549.0m bwlch height, with these values giving this hill 30.1m of drop, which is sufficient for it to be classified as a 500m Twmpau. 

 

The full details for the hill are:

Group:  Y Berwyn

Name:  Y Drum

OS 1:50,000 map:  125

Summit Height:  579.1m (converted to OSGM15)

Summit Grid Reference:  SJ 08247 37893

Bwlch Height:  549.0m (converted to OSGM15)

Bwlch Grid Reference:  SJ 08003 38256

Drop:  30.1m

 

Myrddyn Phillips (June 2021)