Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Bryn y Gwynt (SH 599 449) – 30-99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been a reclassification to a hill that is listed in the 30-99m Twmpau, with the summit height, bwlch height, their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Aled Williams and a subsequent Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and Aled Williams.

LIDAR image of Bryn y Gwynt (SH 599 449)

The criteria for the list that this reclassification applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, with an accompanying sub list 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, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name the hill is listed by is Bryn y Gwynt and this was derived from local enquiry and the series of Ordnance Survey Six-Inch maps and it is situated in the Moelwynion range of hills in the Region of North Wales (Region A, Sub-Region A1), and is positioned with the A 4085 road to its north-east and has the Welsh Highland Railway to its immediate west and overlooks the Afon Glaslyn also to its west, and has the village of Beddgelert to its north north-west and the town of Porthmadog to its south south-west.

The hill is a part of Coed Hafod y Llyn, and as this name implies its summit is placed in woodland which is deciduous, and as the summit of the hill is not a part of designated open access land permission to visit should be sought, however paths are indicated on the map to the north, east and south of this hill’s summit and therefore gaining access to its lower slopes has probably been accepted for many years.

Prior to LIDAR analysis this hill was listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s v-g.me website.  When these sub lists were standardised and also drop values added, this hill was listed with an estimated c 29m of drop based on the 56m spot height that appears on the Ordnance Survey 1:25,000 Explorer map at SH 59881 44828 and an estimated bwlch height of c 27m based on contouring at 10m intervals between 20m – 30m, with the caveat that the 27m spot height that appears on a road at SH 60168 45293 on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website is judged not to be positioned at the critical bwlch.

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

LIDAR analysis confirms this hill to be a 30-99m Twmpau, and as the summit has now been surveyed with the Trimble GeoXH 6000, it is this result that is being prioritised for listing purposes.

The Trimble GeoXH 6000 gathering data at the summit of Bryn y Gwynt

Therefore, the reclassification of this hill from 30-99m Sub-Twmpau status is due to LIDAR analysis and a subsequent Trimble GeoXH 6000 survey, resulting in a 59.9m summit height and a 28.0m bwlch height, with these values giving this hill 31.9m of drop, which is sufficient for it to be classified as a 30-99m Twmpau.

The full details for the hill are:

Group:  Moelwynion

Name:  Bryn y Gwynt

OS 1:50,000 map:  124

Summit Height:  59.9m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 59949 44919 (Trimble GeoXH 6000) 

  

Bwlch Height:  28.0m (LIDAR)

Bwlch Grid Reference:  SH 60224 45248 & SH 60226 45258 & SH 60227 45259 (LIDAR)

Drop:  31.9m (Trimble GeoXH 6000 summit and LIDAR bwlch)

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

Myrddyn Phillips (January 2018)