Thursday, 15 February 2018

Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Rhiw Tir (SH 601 451) – 30-99m Sub-Twmpau addition

There has been an addition to the listing of the 30-99m Twmpau (thirty welsh metre prominences and upward) due to analysis conducted by Aled Williams of data produced via LIDAR.  The 30-99m Twmpau is the title for the list of Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop.

Accompanying the main Twmpau list is a sub list entitled the Sub-Twmpau, with this hill being added to the 30-99m Sub-Twmpau category.  The criteria for 30-99m Sub-Twmpau status being all Welsh hills at and above 30m and below 100m in height with 20m or more and below 30m of drop.

Prior to analysis of data produced via LIDAR this hill was not listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s website.  When these lists were re-evaluated after they were standardised and also drop values added, this hill was listed with c 19m of drop based on the 54m summit spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 35m based on 10m contour intervals between 30m – 40m.

The name of the hill is Rhiw Tir and it is situated in the Moelwynion range of hills and is placed in the Region of North Wales (Region A, Sub-Region A1).  The hill is positioned with the Welsh Highland Railway and the A 498 road to its west and the A 4085 road to its north and east, and has the village of Beddgelert to its north north-west and the town of Porthmadog to its south south-west.

As the hill is not a part of designated open access land permission to visit should be sought, however the hill has a well-used path leading through the deciduous wood that takes in its summit indicating that people have visited this area for many years.

The addition of Rhiw Tir to 30-99m Sub-Twmpau status is due to analysis conducted by Aled Williams of data produced via LIDAR.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

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

Rhiw Tir

Summit Height:  54.4m

Summit Grid Reference:  SH 60112 45108

Bwlch Height:  31.3m

Bwlch Grid Reference:  SH 60060 44996

Drop:  23.1m

Therefore, the 54.4m data produced via LIDAR for the summit position at SH 60112 45108 and the 31.3m data produced via LIDAR for the bwlch position at SH 60060 44996 gives this hill 23.1m of drop, which is sufficient for its inclusion as a 30-99m Sub-Twmpau.

The full details for the hill are:

Group:  Moelwynion

Summit Height:  54.4m (data via LIDAR)

Name:  Rhiw Tir

OS 1:50,000 map:  115

Summit Grid Reference:  SH 60112 45108 
Drop:  23.1m (data via LIDAR)

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

Myrddyn Phillips (February 2018)

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