Sunday, 4 February 2018

Mapping Mountains – Significant Height Revisions – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Bryn y Gwynt (SH 599 449)

There has been a Significant Height Revision to a hill that now appears in the following lists; 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the hill’s height, drop, dominance and status confirmed by analysis conducted by Aled Williams of data produced via LIDAR.

The criteria for the two listings that this significant height revision applies to are:

30-99m Twmpau – All Welsh hills at and above 30m and below 100m in height that have a minimum 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.  Accompanying the main Twmpau list is a sub list entitled Sub-Twmpau with this hill being reclassified from the 30-99m Sub-Twmpau category.  The criteria for 30-99m Sub-Twmpau status are all Welsh hills at and above 30m and below 100m in height that have 20m or more and below 30m of drop.

Y Trechol – The Dominant Hills of Wales – All Welsh P30 hills whose prominence equal or exceed half that of their absolute height.

The name of the hill is Bryn y Gwynt and this was derived from local enquiry and historical mapping (see Significant Name Changes) and it is situated in the Moelwynion range of hills and is placed 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 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 analysis of data produced via LIDAR this hill was listed in the Hills to be surveyed sub list that accompanied the Welsh P30 lists published on Geoff Crowder’s website.  When these sub lists were standardised and also drop values added, this hill was listed with c 29m of drop based on the 56m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 27m based on contouring at 10m intervals and 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 not positioned at the critical bwlch.

Extract from the Ordnance Survey 1:25,000 Explorer map

The summit height produced by data via LIDAR analysis is 58.2m, this is not a dramatic height revision when compared to some revised heights, but it does come within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose Ordnance Survey or Harvey map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR, also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble or by LIDAR.  As heights on different scaled Ordnance Survey maps are not consistent the height given on the 1:25,000 Explorer map is being prioritised in favour of the 1:50,000 Landranger map for detailing these revisions.

Therefore, this hill’s new summit height is 58.2m and this was produced from data via LIDAR, this is 2.2m higher than its previously listed height of 56m which appears on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Moelwynion

Summit Height (New Height):  58.2m (data via LIDAR)

Name:  Bryn y Gwynt

OS 1:50,000 map:  124

Summit Grid Reference:  SH 59949 44915 
Drop:  30.2m (data via LIDAR)

Dominance:  51.89%

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

Myrddyn Phillips (February 2018)

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