Mapping Mountains – Summit Relocations – 100m Twmpau

Bryniau (SH 579 711)

There has been a Summit Relocation 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.

Bryniau (SH 579 711)

The criteria for the list that this summit relocation 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.

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.

When the original Welsh 100m P30 list was published on Geoff Crowder’s v-g.me website the summit location of this hill was given to the 117m spot height adjoined to a triangulation pillar that appeared on contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and which is positioned at SH 578 711.

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

The details for this hill were re-examined when the Ordnance Survey Interactive Coverage Map hosted on the Geograph website became available online and this map gives a 118m spot height positioned to the north-east of the trig pillar at SH 579 711.

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 117 map heighted trig pillar.

LIDAR image of Bryniau

The summit height produced by the Trimble GeoXH 6000 survey is 117.6m, and its position in relation to that originally given comes 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 Bryniau

The summit height produced by the Trimble GeoXH 6000 is 117.6m and is positioned at SH 57971 71196, this position is approximately 80 metres north-east from where the summit was originally listed at this hill’s triangulation pillar.

 

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 (new position):  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)