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)