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 is now listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the summit height, bwlch height, their locations, the drop, dominance 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 two listings that this significant height revision 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.

Y Trechol - The Dominant Hills of Wales - Welsh P30 hills whose prominence  equal or exceed half that of their absolute height.  With the criteria for Lesser Dominant status being those additional Welsh P30 hills whose prominence is between one third and half that of their absolute height, with the Introduction to the Mapping Mountains publication of this list appearing on the 3^rd December 2015, and the list is now available in its entirety on Mapping Mountains in Google Doc format.

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 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.

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

The summit height produced by LIDAR analysis is 58.4m and is positioned at SH 59949 44915, but as the summit has now been surveyed with the Trimble GeoXH 6000 giving 59.9m positioned at SH 59949 44919, it is this result that is being prioritised for listing purposes.  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.

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

Therefore, the new listed summit height of this hill is 59.9m and this was derived from a Trimble GeoXH 6000 survey, this is 3.9m higher than its previously listed height of 56m which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map.

The full details for the hill are:

Group:  Moelwynion

Name:  Bryn y Gwynt

OS 1:50,000 map:  124

Summit Height (New 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)

Dominance:  53.21% (Trimble GeoXH 6000 summit and LIDAR bwlch)  

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

Myrddyn Phillips (February 2018)