Mapping Mountains – Hill Reclassifications – 100m Twmpau

Bryn Mawr (SH 765 766) – 100m Sub-Twmpau reclassified to 100m Twmpau

There has been confirmation of a reclassification to a hill that is listed in the 100m Twmpau, with the summit height and its location, the drop and status of the hill confirmed by LIDAR analysis, and a subsequent summit survey with the Trimble GeoXH 6000, both conducted by Myrddyn Phillips with the latter taking place on the 15^th October 2018.

Bryn Mawr (SH 765 766)

The criteria for the list that this 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.

The name of the hill is Bryn Mawr, and it is adjoined to the Carneddau group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is encircled by minor roads with the B5106 and the Afon Conwy (River Conwy) to its east, and has the town of Conwy towards its north-east.

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 28m of drop, based on an estimated summit height of c 143m and an estimated bwlch height of c 115m, with both based on interpolation of 10m contouring on the Ordnance Survey 1:25,000 Explorer map.

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

The drop value for this hill was re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website became available online; this mapping is entitled the Interactive Coverage Map.  Although this mapping still has no spot heights for either summit or bwlch it does have 5m contouring which enabled a better interpolation of height, and with an estimated c 143m summit and c 113m bwlch it gave this hill an estimated c 30m of drop.

Extract from the Ordnance Survey Vector Map Local hosted on the Geograph website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales. 

LIDAR image of Bryn Mawr (SH 765 766)

The confirmation of the reclassification of Bryn Mawr to 100m Twmpau status is due to LIDAR analysis and a subsequent summit survey with the Trimble GeoXH 6000 conducted by Myrddyn Phillips, resulting in a 114.0m summit height and a 110.7m bwlch height, with these values giving this hill 33.3m of drop which is sufficient for it to be classified as a 100m Twmpau.

The full details for the hill are:

Group:  Carneddau

Name:  Bryn Mawr

OS 1:50,000 map:  115

Summit Height:  144.0m (converted to OSGM15)

Summit Grid Reference:  SH 76562 76669

Bwlch Height:  110.7m (LIDAR)

Bwlch Grid Reference:  SH 76263 76359 (LIDAR)

Drop:  33.3m (Trimble summit and LIDAR bwlch)

For details on the summit survey of this hill

Myrddyn Phillips (April 2019)