Mapping Mountains – Hill Reclassifications – 100m Twmpau

Bryniau (SH 579 711) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a Hill Reclassification to a hill that is listed in the 100m Twmpau, with the summit height and its location, drop and status of the hill initially confirmed by LIDAR analysis and subsequently by a summit survey with the Trimble GeoXH 6000 which were conducted by Myrddyn Phillips, with the latter taking place on the 1^st October 2018.

LIDAR image of Bryniau

The criteria for the list that this hill reclassification applies to are:

100m Twmpau - Welsh hills at or above 100m and below 200m in height with 30m minimum drop, with an accompanying sub list entitled the 100m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 100m and below 200m 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 was included in the Hills to be surveyed sub list that accompanied the 100m height band of P30 hills as it did not at that stage meet the criteria used in this list.  When the sub list was standardised, and interpolated heights and drop values also included this hill was listed with an estimated c 30m of drop, based on the 117m spot height adjoined to a triangulation pillar positioned at SH 57894 71129 and an estimated bwlch height of c 87m based on interpolation of bwlch contouring on Ordnance Survey 1:25,000 Explorer maps. 

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

The drop value was re-assessed when the Ordnance Survey Interactive Coverage Map became available online via the Geograph website, this mapping has a 118m spot height positioned to the north-east of the trig pillar at SH 579 711, which resulted in the summit relocation of this hill from ground beside its triangulation pillar, and the drop value was again estimated to be c 30m, with a revised estimated bwlch height of c 88m.

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

However, it was not until LIDAR became available that these two positions could be compared and confirmation that the land where the 118m spot height appears is higher than the land beside the 117m map heighted trig pillar and that the hill had sufficient drop to be classified as a P30.

LIDAR image of Bryniau

The land where the summit of this hill is situated is a part of Bryniau and this was derived from local enquiry and substantiated from the Tithe map, and it is the name that this hill is now listed by.  The hill is adjoined to the Glyderau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and is positioned with the A4087 road to its west and minor roads to its south and east, and has the small city of Bangor towards its north.

If wanting to visit the hill permission to do so should be sought as the summit area is not a part of designated open access land, for those wishing to do so the easiest approach is via a track leading to the old farmhouse of Bryniau.

The confirmation of the reclassification of Bryniau to 100m Twmpau status is due to LIDAR analysis and a summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, resulting in a 117.6m summit height and an 87.4m bwlch height, with these values giving this hill 30.1m of drop which is sufficient for it to be classified as a 100m Twmpau.

 

The Trimble GeoXH 6000 gathering data at the summit of Bryniau

The full details for the hill are:

Group:  Glyderau

Name:  Bryniau

OS 1:50,000 map:  114, 115

Summit Height:  117.6m (converted to OSGM15)

Summit Grid Reference:  SH 57971 71196

Bwlch Height:  87.4m (LIDAR)

Bwlch Grid Reference:  SH 57748 70845 (LIDAR) 

Drop:  30.1m (Trimble summit and LIDAR bwlch)

For details on the summit survey of Bryniau

Myrddyn Phillips (February 2019)