Monday, 31 July 2017

Mapping Mountains – Hill Reclassifications – Y Trichant

Cyrnau Mawr (SN 751 750) – Sub-Trichant addition

There has been an addition to the listing of Y Trichant due to analysis of LIDAR data by Aled Williams and subsequently confirmed via a survey with the Trimble GeoXH 6000.  Y Trichant is the title for the hills in the 300m height band of the Twmpau (thirty welsh metre prominences and upward) and takes in all Welsh hills at or above 300m and below 400m in height that have a minimum 30m of drop, with the introduction to the re-naming of this list appearing on Mapping Mountains on the 13th May 2017.

The hill did not appear in the sub category that accompanied the original Welsh P30 lists when published on Geoff Crowder’s website, this sub category has now been standardised and named the Sub-Trichant and comprises all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.

Prior to analysis of LIDAR data the hill was not classified but it was catalogued with c 16m of drop based on an estimated c 390m summit height and an estimated c 374m bwlch height, the latter height based on interpolation of bwlch contouring between 370m – 380m.

The hill is a part of the Elenydd range with its Cardinal Hill being Pen y Garn (SN 798 771) and is placed in the Region of Mid and West Wales (Region B, Sub-Region B2).  The hill is situated with the B 4574 to its north and the B 4343 to its west and is positioned between the small communities of Pontarfynach (Devil’s Bridge) to its north-west and Cwmystwyth to its east south-east. 

For those wishing to visit the summit the whole upper section of the hill is a part of designated open access land, and this has immediate access from the B 4343 to the west or from a public footpath from the same road that intersects with the open access lane.

The name of the hill is Cyrnau Mawr and its addition to Sub-Trichant status is due to the analysis of LIDAR data by Aled Williams.  LIDAR (Light Detection & Ranging) is highly accurate height data that is now freely available for much of England and Wales.

Aled’s analysis of LIDAR data gives the hill the following details:

Cyrnau Mawr

Summit Height:  391.4m

Summit Grid Reference:  SN 75155 75007

Bwlch Height:  369.8m

Bwlch Grid Reference:  SN 75343 74906

Drop:  21.6m

Therefore, the 391.4m LIDAR data produced for the summit position at SN 75155 75007 and the 369.8m LIDAR data produced for the bwlch position at SN 75343 74906 gives this hill 21.6m of drop which is sufficient for its inclusion as a Sub-Trichant, with the details from the Trimble GeoXH survey being 391.4m (converted to OSGM15) summit at SN 75154 75006 and 369.8m (converted to OSGM15) bwlch at SN 75342 24906, giving this hill 21.6m of drop. 

The full details for the hill are:

Cardinal Hill:  Pen y Garn

Summit Height:  391.4m (converted to OSGM15 Trimble, and LIDAR data)

Name:  Cyrnau Mawr

OS 1:50,000 map:  135, 147

Summit Grid Reference:  SN 75154 75006 
Drop:  21.6m (converted to OSGM15 Trimble, and LIDAR data)

Cyrnau Mawr (SN 751 750) confirmed as a Sub-Trichant

My thanks to Aled Williams for sending the details of this hill to me

Myrddyn Phillips (July 2017)

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