Thursday, 25 January 2018

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

Bryn Tŵr (SH 604 441)

There has been a Significant Height Revision to a hill that is listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, and which was initiated by a survey with the Trimble GeoXH 6000 which took place on the 23rd December 2017 in wet conditions but with clear visibility.

The criteria for the two lists this height revision affects are:

30-99m Twmpau – Welsh hills at and above 30m and below 100m in height that have 30m minimum drop.  With the word Twmpau being an acronym standing for ‘thirty welsh metre prominences and upward’.

Y Trechol – The Dominant Hills of Wales - Welsh P30s whose prominence equal or exceed half that of their absolute height.

The name of the hill is Bryn Tŵr and it is placed in the Moelwynion range of hills which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and the hill is positioned overlooking the A 4085 road and the Nanmor to its east, with it being positioned between the village of Beddgelert to its north north-west and Porthmadog to its south south-west. 

As the summit of the hill is not on designated open access land permission to visit should be sought, however the hill has a well-used path leading to its summit indicating that people have visited this hill for many years.

Prior to the survey with the Trimble GeoXH 6000 this hill was listed with c 40m of drop and 76.92% Dominance based on the 52m spot height that appears on the Ordnance Survey Interactive Coverage Map hosted on the Geograph website and an estimated bwlch height of c 12m based on contouring between 10m – 20m.  The summit height produced by the Trimble survey is 54.5m (converted to OSGM15) with its position to the west of where the 52m spot height appears and outside of the uppermost 50m ring contour on Ordnance Survey maps, with this summit height coming 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 a previous estimated summit height based on interpolation of contours.  Also included are hills whose summit map data is missing an uppermost ring contour when compared to the data produced by the Trimble.  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 54.5m (converted to OSGM15) which is 2.5m higher than its previously listed height of 52m and 14.5m higher than the uppermost 40m ring contour on Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps.

The full details for the hill are:

Group:  Moelwynion

Summit Height (New Height):  54.5m (converted to OSGM15)

Name:  Bryn Tŵr

OS 1:50,000 map:  124

Summit Grid Reference:  SH 60453 44187

Drop:  c 43m

Dominance:  77.99%

Gathering data from the summit of Bryn Tŵr which resulted in this hill's significant height revision

Myrddyn Phillips (January 2018)

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