Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Bryn (SH 426 361) – 30–99m Sub-Twmpau reclassified to 30-99m Twmpau

There has been confirmation of a Hill Reclassification to the list of 30-99m Twmpau, 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 using 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 Bryn

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.

When the original Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill appeared in the accompanying Hills to be surveyed sub list as it did not meet the criteria then used for the main P30 list; however this sub list has now been standardised with drop values and interpolated heights also included in the main P30 and the accompanying sub list.

When this list was standardised and interpolated heights also included this hill was listed with an estimated c 28m of drop, based on the 36m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and an estimated bwlch height of c 8m based on interpolation of bwlch contouring between 0 – 10m. 

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

These values were re-evaluated when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online, and as this map has 5m contour intervals the bwlch contouring was reassessed.  However, as there is a rail line passing through this bwlch the contours on this map are not continuous and could in affect be between 0 – 5m or between 5m – 10m at the critical point of the bwlch, they were taken as being between 5m – 10m with an estimated c 7m height for the bwlch, giving this hill c 29m of drop.

Extract from the Ordnance Survey Interactive Coverage Map hosted on the Geograph website

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 slopes.

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.

                                                                  

LIDAR image for the summit area of Bryn

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)

Therefore, 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, with LIDAR analysis giving a 6.6m bwlch height positioned at SH 43103 36535, with these values giving this hill 30.0m of drop, which confirms its 30-99m Twmpau status.

Gathering data with the Trimble GeoXH 6000 at the summit of Bryn

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)

My thanks to Chris Pearson for suggesting this hill as a P30 and to Chris Crocker for initial LIDAR analysis

For details on the survey that confirmed the P30 status of this hill

Myrddyn Phillips (February 2019)