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, bwlch height and their location, the drop and status of the hill derived from a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis conducted by Myrddyn Phillips.

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.

100m Twmpau by Myrddyn Phillips

The hill is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it 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.

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.

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 the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer maps and which is positioned at SH 57894 71129.

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 57968 71186.

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.

LIDAR summit image of Bryniau

The summit height produced by the Trimble GeoXH 6000 survey is 117.6m, and its position in relation to that originally listed summit comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour either on Ordnance Survey maps or interactive mapping, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is considered a relatively recent man-made construct, 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.

The Trimble GeoXH 6000 gathering data at the summit of Bryniau

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

 

The full details for the hill are:

Group:  Glyder Fawr

Name:  Bryniau

OS 1:50,000 map:  114, 115

Summit Height:  117.6m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference (New Position):  SH 57971 71196 (Trimble GeoXH 6000)

Bwlch Height:  86.2m (LIDAR)

Bwlch Grid Reference:  SH 57961 70836 (LIDAR) 

Drop:  31.3m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (February 2019)