Mapping Mountains – Hill Reclassifications – 200m Twmpau

Gribin (SH 675 168) – 200m Sub-Twmpau reclassified to 200m Twmpau

There has been confirmation of a reclassification to the 200m Twmpau list due to LIDAR analysis conducted by Myrddyn Phillips, with the criteria for this list being:

200m Twmpau - Welsh hills at and above 200m and below 300m in height with 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau with the criteria for this sub category being all Welsh hills at and above 200m and below 300m in height with 20m and more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

This hill was listed in the Hills to be surveyed sub list when the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website as it was thought not  to meet the criteria then used for the main P30 list; however this list has now been standardised and interpolated heights and drop values also included, resulting in this hill being listed with an estimated c 39m of drop, based on an estimated c 211m summit height and an estimated bwlch height of c 172m based on bwlch map contouring between 170m – 180m.  The details for this hill were later amended when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online and it was then listed with a 211m summit height based on the spot height appearing on this map.

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

The name of the hill is Gribin and it 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), it is positioned overlooking the A 493 road which is to its north and has the small community of Arthog towards the south-west and the town of Dolgellau towards the east.

The summit and upper southern section of the hill is a part of designated open access land and a public footpath leads toward this from the west south-west of the hill, with another option for an ascent following a public footpath from the north-east of the hill.

The confirmation of the reclassification of Gribin to 200m 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.

 

The 1m DTM LIDAR analysis gives the hill the following details:

Name:  Gribin

Summit Height:  209.3m

Summit Grid Reference:  SH 67559 16814

Bwlch Height:  169.7m

Bwlch Grid Reference:  SH 67588 16670

Drop:  39.6m

LIDAR image of Gribin

Therefore, the 209.3m LIDAR analysis for the summit position at SH 67559 16814 and the 169.7m LIDAR analysis for the bwlch position at SH 67588 16670 gives this hill 39.6m of drop, which confirms its reclassification to 200m Twmpau status.

The full details for the hill are:

Group:  Cadair Idris

Name:  Gribin

OS 1:50,000 map:  124

Summit Height:  209.3m (LIDAR)

Summit Grid Reference:  SH 67559 16814 (LIDAR)

Bwlch Height:  169.7m (LIDAR)

Bwlch Grid Reference:  SH 67588 16670 (LIDAR)

Drop:  39.6m (LIDAR)

Myrddyn Phillips (May 2018)