Friday, 19 May 2017

Mapping Mountains – Hill Reclassifications – 500m Twmpau

Craig Stwlan (SH 667 446) – 500m Sub-Twmpau deletion

There has been a deletion to the listing of Twmpau (thirty welsh metre prominences and upward) hills due to analysis of LIDAR data by Aled Williams.  The Twmpau is the title for the list of Welsh P30s and takes in all Welsh hills that have a minimum 30m of drop, with this deletion affecting the 500m height band of hills within the Twmpau.

Accompanying the main list of Twmpau hills is a sub category list of hills, with the criteria for this sub list in the 500m height band of hills being all Welsh hills at or above 500m and below 600m in height with 20m or more and below 30m of drop.  Details to this height band of the Twmpau were published on the Mapping Mountains site on 15th June 2015.

Prior to analysis of LIDAR data the hill had been listed with c 20m of drop based on the 568m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 548m based on interpolation of bwlch contouring between 540m – 550m.

The hill is situated in the Moelwynion group of hills with its Cardinal Hill being Moelwyn Mawr (SH 658 448) and is placed in the Region of North Wales (Region A, Sub-Region A1).  The hill is situated with the town of Blaenau Ffestiniog to the east north-east and the small village of Croesor to the west.

If wanting to visit the hill it can be easily accessed from the east north-east from the end of the paved public road which is situated above the small community of Tanygrisiau, beyond a locked gate the paved road continues up toward the reservoir of Llyn Stwlan, and Craig Stwlan is positioned just above the north-eastern part of this lake.

The name of the hill is Craig Stwlan and its deletion from 500m Sub-Twmpau 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:

Craig Stwlan

Summit Height:  568.2m

Summit Grid Reference:  SH 66735 44655

Bwlch Height:  548.5m

Bwlch Grid Reference:  SH 66693 44715

Drop:  19.7m

Therefore, the 568.2m LIDAR data produced for the summit position at SH 66735 44655 and the 548.5m LIDAR data produced for the bwlch position at SH 66693 44715 gives this hill 19.7m of drop which is insufficient for it to retain its status as a 500m Sub-Twmpau.

The full details for the hill are:

Cardinal Hill:  Moelwyn Mawr

Summit Height:  568.2m (LIDAR data)

Name:  Craig Stwlan

OS 1:50,000 map:  124

Summit Grid Reference:  SH 66735 44655

Drop:  19.7m (LIDAR data)

The overall total for the 500m Twmpau remains at 225 hills with the overall total for the 500m Sub-Twmpau decreasing by one to 96 hills.  The list of the 500m Twmpau was published on 15th June 2015 with all subsequent changes having been detailed on Mapping Mountains

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

Myrddyn Phillips (May 2017)

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