Mapping Mountains – Hill Reclassifications – 100m Twmpau

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

There has been confirmation of a reclassification to the list of 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.

LIDAR image of Bryniau (SH 579 711)

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.

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.

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.

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

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 c 87m bwlch height, based on interpolation of contouring 5m contouring between 85m - 90m that appears on the Ordnance Survey 1:25,000 Explorer map.

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

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 57968 71186, resulting in the drop value remaining at an estimated c 30m with a revised estimated c 88m bwlch height and the summit relocation of this hill.

LIDAR summit image of Bryniau

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.

 

The Trimble GeoXH 6000 gathering data at the summit of Bryniau

Therefore, the confirmation of the reclassification of this hill from 100m Sub-Twmpau status is due to a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis, resulting in a 117.6m summit height and an 86.2m bwlch height, with these values giving this hill 31.3m of drop, which is sufficient for it to be classified as a 100m Twmpau.

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:  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)