Mapping Mountains – Hill Reclassifications – 500m Twmpau

 

Llechwedd Deiliog (SH 785 398) – 500m Sub-Twmpau addition

There has been confirmation of an addition to the listing of the 500m Twmpau that is retrospective, 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 and Aled Williams and subsequent LIDAR analysis conducted by Aled Williams. 

LIDAR image of Llechwedd Deiliog (SH 785 398)

The criteria for the list that this addition 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.

500m Twmpau by Myrddyn Phillips

The name the hill is listed by is Llechwedd Deiliog and it is adjoined to the Arenig Fach group of hills, which are situated in the central part of North Wales (Region C1, Sub-Region C1A), and it is positioned with the B4391 road to its south, and has the village of Trawsfynydd towards the west south-west. 

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

When the list that later became known as the 500m Twmpau was first compiled, this hill was not included in the accompanying sub list, but was later added after a basic levelling survey conducted by Myrddyn Phillips on the 16.05.04 gave it 66½ft (20.3m drop).

This hill is also included in the listing of The Welsh Highlands – Uchafion Cymru co-authored by Myrddyn Phillips and Aled Williams and it formed one of a number of hills held back from publication until this list was published in its entirety.  However, this policy has now changed; therefore the details for this hill have been put in the public domain. 

The Trimble set-up position at SH 78452 39920 with subsequent LIDAR analysis prioritised

However, it was not until a Trimble GeoXH 6000 survey and 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. 

The Trimble GeoXH 6000 gahering data at the bwlch of Llechwedd Deiliog

Therefore, the confirmation of the addition of this hill to 500m Sub-Twmpau status is due to a Trimble GeoXH 6000 survey and LIDAR analysis, resulting in a 501.7m summit height and a 481.2m bwlch height, with these values giving this hill 20.6m of drop, which is insufficient for it to be classified as a 500m Sub-Twmpau.

 

The full details for the hill are:

Group:  Arenig Fach

Name:  Llechwedd Deiliog

OS 1:50,000 map:  124

Summit Height:  501.7m (LIDAR) 

Summit Grid Reference:  SH 78522 39815 (LIDAR) 

Bwlch Height:  481.2m (converted to OSGM15, Trimble GeoXH 6000) 

Bwlch Grid Reference:  SH 78298 40196 (Trimble GeoXH 6000) 

Drop:  20.6m (LIDAR summit and Trimble GeoXH 6000 bwlch)

Myrddyn Phillips (September 2026)