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)

Mapping Mountains – Trimble Surveys – Arenig Fach

01.03.14  Carnedd Iago (SH 782 406) and Llechwedd Deiliog (SH 785 398)

Carnedd Iago (SH 782 406)

After visiting the two hills of Y Garn and Pt. 477m (SH 758 375 and SH 758 378) Aled and I drove to the junction of the B4391 and proceeded toward Ffestiniog, where we parked next to the forest boundary at SH 788 395.  Parking can be found either on the northern side of this road where a forest track enters the conifer plantation or on the southern side where there is a relatively large graveled area.

We wanted to visit Carnedd Iago (SH 783 406) which has a 538m spot height at the summit on the Ordnance Survey 1:50,000 map, and the 502m summit of Llechwedd Deiliog (SH 785 398) which looked as if it may require a little tree bashing to get to its highest point.

It was spotting with rain when we set off and the sky didn’t look very promising for the remainder of the walk.  At least by keeping to the forest track we were a little sheltered.

Carnedd Iago is situated in the expanse of Y Migneint between Arenig Fach (SH 820 415) to its east and Cerrig y Ieirch (SH 758 425) to its north-west.  It really is a lonely and forgotten spot; if not for the nearness of the B road it would also be relatively remote for a hill in Wales.

We soon came to the small clearing in the trees where access to the open hill side can be found.  From here it’s a few minutes uphill to the highest point of Carnedd Iago.  As we approached the top the views opened up, we could see that the greyness that was playing with the coastal hills to the west had escaped and was now rapidly taking over the inland hills to our east.  However, the threatened rain was miraculously being kept at bay. 

Approaching the summit of Carnedd Iago

As we picked what we thought to be the highest point for the Trimble to gather 12 minutes of data, the hill ridges to the west were trying to emerge from their dark grey enshroudment.  Sometimes when the weather is turning inclement the cloud conditions can give sudden glimpses of accentuated ridges that otherwise may be taken for granted.

The Trimble GeoXH 6000 gathering data at the summit of Carnedd Iago

Looking toward the Moelwynion

Away to the north-east one building stood out amongst the loneliness of desolate moor – Cefngarw.  We’d visited this old farm house toward the end of December 2013 on our way to the summit of Bryn Mawr (SH 801 442).

Lonely Cefngarw

Toward the west the Rhinogydd looked at their impressive best, tinged in a primeval dullness with dark cloud empathising their roughness.  All highlighted with the smallest flash of light on cloud.  The mountains are quite magical whatever the time or weather, they never seem to disappoint.

The Rhinogydd looking grey and inviting

Rhinog Fach (L) and Rhinog Fawr (R)

Once the Trimble was packed away we headed down beside the forest edge to the bwlch that connects with Llechwedd Deiliog.  Our plan was to survey the bwlch and if the forestry permitted; also its summit.  As Aled stood beside the fence I ventured in to the land of the bwlch, which not surprisingly consisted of a bog.  The more I squashed my way around its domain the more methane gas was released, this was foul spelling.

A foul spelling bog is not the best to crawl around on your hands and knees trying to estimated where the hill to hill traverse meets the valley to valley traverse.  After a particularly unpleasant few minutes I picked a spot for Trimble placement with Aled verifying that this looked the best point, and set the Trimble on the ground to gather its customary data.

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

The only place left to survey was the summit of Llechwedd Deiliog.  This is immersed in a conifer plantation which I had previously visited in May 2004 when conducting a basic leveling survey.  We pottered around in the forestry and having no clear sight from one possible high point to another, we eventually had to opt for a point to survey and found one that at least in its immediate vicinity looked to be the highest.  However, it was anyone's guess whether higher ground existed to our south.

Having set the Trimble up connected to its 2m pole we waited for nine minutes of data to be collected, then switched it off, packed the equipment away and headed back through to the trees to the forest track and the awaiting car below.  It proved an excellent couple of hours on the hill.

LIDAR image of Llechwedd Deiliog (SH 785 398)

Postscript:

Since surveying these hills full LIDAR coverage is now available.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height and positional data that is now freely available for England and Wales.  Consequently the numerical details for these hills have been analysed using this technique, and where appropriate these results appear below. 

Survey Result

Carnedd Iago

Summit Height:  538.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 78271 40653 (Trimble GeoXH 6000)

Bwlch Height:  463.85m (LIDAR)

Bwlch Grid Reference:  SH 76016 43204 (LIDAR)

Drop:  74.15m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  13.78% (Trimble GeoXH 6000 summit and LIDAR bwlch)

Llechwedd Deiliog

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) (500m Sub-Twmpau addition)

Dominance:  4.10% (LIDAR summit and Trimble GeoXH 6000 bwlch)

For details on the bwlch survey of Carnedd Iago

For further details please consult the Trimble Survey Spreadsheet