Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ynys Gyffylog (SH 629 138) – Double Sub-Twmpau addition

There has been confirmation of an addition to the 30-99m Twmpau list due to LIDAR analysis conducted by Myrddyn Phillips and subsequently confirmed via a survey with the Trimble GeoXH 6000, with the criteria for this list being:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category which this hill is a part of is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m 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.

This hill did not appear in the accompanying Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website, as it did not meet the criteria then used for this sub category, however this sub list has now been standardised including the addition of interpolated drop values and interpolated summit heights.

This hill was included as a Double Sub-Twmpau after the Ordnance Survey Interactive Coverage Map became available on the Geograph website, and prior to LIDAR analysis it was listed with an estimated c 20m of drop based on the 23m summit spot height that appears on this map and an estimated bwlch height of c 3m based on interpolation of bwlch contouring between 0 – 10m.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

The name of the hill is Ynys Gyffylog and as its name implies it has association with being an island, or more strictly speaking a tidal estuary island that is now landlocked due to sea defences.  The hill is adjoined to the Cadair Idris group of hills, which are situated in the south-western part of North Wales (Region A, Sub-Region A3), with the hill positioned beside the Arthog Bog and immediately behind a row of terraced houses, and has the Morfa Mawddach Station towards its north-west, the A 493 road to its south and the village of Y Friog (Fairbourne) to its south-west. 

Ynys Gyffylog (SH 629 138)

If wanting to visit the hill permission to do so should be sought as it is not a part of designated open access land, for those wishing to do so it can be a rough experience, especially so in summer as it has copious amounts of undergrowth.

The confirmation of the addition of Ynys Gyffylog to Double Sub-Twmpau status is due to LIDAR analysis conducted by Myrddyn Phillips.  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 Ynys Gyffylog

The 2m DTM LIDAR analysis gives the hill the following details:

Name:  Ynys Gyffylog

Summit Height:  22.9m

Summit Grid Reference:  SH 62965 13851

Bwlch Height:  2.4m

Bwlch Grid Reference:  SH 63071 13758

Drop:  20.4m

Therefore, the 22.9m LIDAR analysis for the summit position at SH 62965 13851 and the 2.4m LIDAR analysis for the bwlch position at SH 63071 13758 gives this hill 20.4m of drop, with the details from the Trimble GeoXH 6000 survey being 23.1m (converted to OSGM15) summit height at SH 62964 13848, and when coupled with the LIDAR bwlch height it gives this hill 20.6m of drop, which confirms its addition to Double Sub-Twmpau status.

Somewhere amongst the undergrowth is the Trimble GeoXH 6000 gathering data at the summit of Ynys Gyffylog 

Group:  Cadair Idris

Name:  Ynys Gyffylog

OS 1:50,000 map:  124

Summit Height:  23.1m (converted to OSGM15)

Summit Grid Reference:  SH 62964 13848

Bwlch Height:  2.4m (LIDAR)

Bwlch Grid Reference:  SH 63071 13758 (LIDAR)

Drop:  20.6m (Trimble summit and LIDAR bwlch)

The Trimble set-up position at the summit of Ynys Gyffylog

For details on the survey of Ynys Gyffylog

Myrddyn Phillips (September 2018)

Mapping Mountains – Trimble Surveys – Cadair Idris

23.05.18  Ynys Gyffylog (SH 629 138)

The wooded summit of Ynys Gyffylog (SH 629 138)

Having visited some of the higher hills in the Cadair Idris range and with beautiful sun drenched weather continuing in to the early afternoon, I thought I’d re-visit the area surrounding the Arthog Bog and see if I could get to the top of another one of its landlocked islands.

I’d recently visited Fegla Fawr and Fegla Fach, both of which are the main and higher landlocked islands in this compact area, with many of these small islands wooded and rocky in nature, and the one I chose for this afternoon’s little adventure proved a rough place to visit and full of unsavoury undergrowth.

I parked adjacent to a row of terraced houses and set the Trimble on top of my car roof to log in to satellite reception, doing so would give me a ten figure grid reference to follow when on the summit area and compare to that produced by having LIDARed the hill, if indeed I made it to the summit, as what I could see of the hill looked particularly dense in foliage.

As I set off aiming for the most northerly of the terraced row of houses where I hoped a track would give me access to the hill, a man appeared walking his dog from the side of the most southerly of the houses, I smiled and said hello and asked if he was local and if so, did he know of a path leading up the wooded lump at the back of the houses, he saved the day and kindly took me to the side of his house where he directed me up steps to what he described as a path of sorts that would lead via his blue barrowed potatoes and discarded tyres and a mound of bramble and trees toward two sets of stone steps to the wilderness above.  If not for this kind gesture I doubt I would have found a way to the top.

The path starts at the front of the car on the left

The path heads up toward the blue barrel on the left and makes its way past the barrels on the right

The path crept past his potatoes and a mound of foliage above the backyards of the terraced houses toward a wooden hand rail and stone flagged steps leading steeply upward to a second set of steps that were hardly distinguishable from the undergrowth surrounding them, these led to the brackened and tree infested upper part of the hill.

Spot the path

Looking back at the blue barreled potatoes

The hand rail indicates the way upward

As I popped out on the summit ridge which could hardly be seen due to thick undergrowth the sun blazed from above, a radiant blue sky cast down warmth.  I stumbled my way through bracken trying to get to the high point which was immersed in bramble, bracken and gorse with trees obstructing any direct upward view, I thought the chances of satellite reception for the Trimble was slim.

The summit ridge

I spent a few minutes stamping down as much foliage as I could before setting the Trimble atop my rucksack, measuring the offset between its internal antenna and the ground below and then started what I thought would be a very long wait until the 0.1m accuracy level before data should be logged was attained, miraculously I only had to wait about five minutes and suddenly the magical 0.1m mark appeared on the screen and I pressed ‘Log’ and disappeared in to the foliage to wait for the allotted data to be stored.

Gathering data at the summit of Ynys Gyffylog

I let the Trimble gather 16 minutes of data and stood in the sun with hardly a view due to the trees, but I felt content and more than happy that the Trimble was performing well and had attained its accuracy level incredibly quickly considering where it was placed.  During the wait for data collection I stamped a path back to where I had popped out on the summit ridge and made sure I remembered where this place was as the last thing I wanted was an alternative route back down to safety below.

Once sixteen minutes of data were gathered I switched the equipment off, packed the Trimble away and retraced my route back down the stone steps and along the narrow path beside the discarded tyres and barrowed potatoes.

  

Survey Result:

Ynys Gyffylog

Summit Height:  23.1m (converted to OSGM15)

Summit Grid Reference:  SH 62964 13848

Bwlch Height:  2.4m (LIDAR)

Bwlch Grid Reference:  SH 63071 13758 (LIDAR)

Drop:  20.6m (Trimble summit and LIDAR bwlch) (Double Sub-Twmpau addition confirmed)

Dominance:  89.42%

For further details please consult the Trimble Survey Spreadsheet