Mapping Mountains – Summit Relocations – 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales

Bryn (SH 426 361)

There has been a Summit Relocation to a hill that is now listed in the 30-99m Twmpau and Y Trechol – The Dominant Hills of Wales, with the hill suggested as a P30 by Chris Pearson in February 2018 and its status confirmed by LIDAR analysis conducted by Chris Crocker.  The hill was subsequently analysed by LIDAR and surveyed with the Trimble GeoXH 6000 by Myrddyn Phillips, with the latter taking place on the 10^th September 2018.

LIDAR image of the summit of Bryn

The criteria for the two listings that this summit relocation 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.

The locally known name of the hill is Bryn, and this is the name it is now listed by, and it is adjoined to the Pen Llŷn group of hills, which are situated in the western part of North Wales (Region A, Sub-Region A1), and is positioned overlooking the sea to its south and the A 497 road to its north, and has the town of Pwllheli towards the west.

As the summit of this hill is not a part of designated open access land permission to visit should be sought, for those wishing to do so a public footpath approaches the hill from its north and access to the summit is relatively easy from here via its north-west which avoids the majority of gorse on its upper part.

When this hill was first listed in the Hills to be surveyed sub list that accompanied the 30-99m height band of hills in the original Welsh P30 list published on Geoff Crowder’s v-g.me website, it was listed with a 36m summit height which appears as a spot height on the Ordnance Survey 1:25,000 Explorer map and is positioned at SH 42664 36179.

Prior to the Trimble GeoXH 6000 survey this hill’s summit and bwlch contours were analysed with LIDAR, with the conclusion that the hill is not a natural P30 as LIDAR bwlch contouring implies that the natural bwlch would once have been over 7m in height.  This bwlch is now reduced in height due to a railway cutting. 

An appropriate name for the hill was also researched via the Tithe map, finding that the upper section of the hill which now comprises one field, used to comprise two, with the boundary between each showing as a slight elevation on LIDAR.  This boundary although old, is not ancient, and would be a man-made construction.  LIDAR also shows that the natural summit of this hill still exists, close to the old field boundary.  Therefore I wanted to take data sets from the slightly raised field boundary, and the natural summit with LIDAR giving two potential positions for this, with one adjacent to the old field boundary and the other a few metres distant from it.

Therefore, three data sets were taken with the Trimble GeoXH 6000 from the summit area of this hill, resulting in:

1^st survey:  36.750m at SH 42673 36171 (raised old field boundary)

2^nd survey:  36.5965m at SH 42668 36171 (average of two surveys)

3^rd survey:  36.675m at SH 42656 36173 (natural summit)

The height produced by the Trimble GeoXH 6000 survey to the natural summit of this hill is 36.7m and is positioned at SH 42656 36173, this is not a dramatic difference in position compared to where the 36m spot height appears on the Ordnance Survey 1:25,000 Explorer map, but it does come within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies to any listed hill whose summit meets the following criteria; where there are a number of potential summit positions within close proximity and the highest point is not where previously given, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist, or when the summit of the hill is in a different field compared to where previously given, or when the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary that is judged to be a relatively recent man-made construct.  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 relocations.

The Trimble GeoXH 6000 gathering data at the summit of Bryn

Therefore, the natural summit height produced by the Trimble GeoXH 6000 survey is 36.7m and is positioned at SH 42656 36173, this is relatively close to where the 36m spot height of this hill is positioned and is approximately 17 metres from where the raised old field boundary is situated, and as this is considered a relatively recent man-made construct it is not considered as a part of this hill’s height.

The full details for the hill are:

Group:  Pen Llŷn

Name:  Bryn

OS 1:50,000 map:  123

Summit Height:  36.7m (converted to OSGM15) (natural summit)

Summit Grid Reference:  SH 42656 36173 (natural summit)

Bwlch Height:  6.6m (LIDAR)

Bwlch Grid Reference:  SH 43103 36535 (LIDAR) 

Drop:  30.0m (Trimble summit and LIDAR bwlch)

Dominance:  81.88% (Trimble summit and LIDAR bwlch)

For details on the survey that relocated the summit position of this hill

Myrddyn Phillips (January 2019)