Mapping Mountains – Hill Reclassifications – Y Pedwarau – The 400m Hills of Wales

Bryn Twyn (SH 815 241) – 400m Sub-Pedwar addition

There has been an addition to the listing of Y Pedwarau – The 400m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill confirmed by LIDAR analysis conducted by Myrddyn Phillips.

LIDAR image of Bryn Twyn (SH 815 241)

The criteria for the list that this addition applies to are:

Y Pedwarau – The 400m Hills of Wales.  Welsh hills at or above 400m and below 500m in height that have 30m minimum drop, accompanying the main Y Pedwarau list are five categories of sub hills, with this hill being added to the 400m Sub-Pedwar category.  The criteria for 400m Sub-Pedwar status being all Welsh hills at or above 400m and below 500m in height that have 20m or more and below 30m of drop.  The list is co-authored by Myrddyn Phillips and Aled Williams and is published on Mapping Mountains in Google Doc format.

The name the hill is listed by is Bryn Twyn, and it is adjoined to the Arenig group of hills, which are situated in the central part of North Wales (Region A, Sub-Region A3), and it is positioned with the A494 road to its south-east, and has the small community of Rhyd-y-main towards the south-west.

Prior to LIDAR analysis this hill was originally listed with an estimated c 16m of drop, based on an estimated c 443m summit height and an estimated c 427m bwlch height, with these heights derived from interpolation of 10m contouring on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey Vector Map Local hosted on the Geograph website and which is entitled the Interactive Coverage Map became available online.  This mapping has many spot heights not on other publicly available Ordnance Survey maps.

The details for this hill were also re-assessed when the OS Maps website became available online.  This is the replacement for OS Get-a-map and has contours at 5m intervals which are proving consistently more accurate compared to the 5m contours that sometimes appear on Ordnance Survey 1:25,000 Explorer maps and used to appear on the online Vector Map Local.  These re-assessments resulted in the hill being listed with an estimated c 19m of drop, with the altered drop value based on interpolation taken from the uppermost 445m summit contour on OS Maps.

Extract from OS Maps

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. 

Therefore, the addition of Bryn Twyn to 400m Sub-Pedwar status is due to LIDAR analysis, resulting in a 447.1m summit height and a 426.3m bwlch height, with these values giving this hill 20.8m of drop which is sufficient for it to be classified as a 400m Sub-Pedwar.

The full details for the hill are:

Group:  Arenig

 

Name:  Bryn Twyn

OS 1:50,000 map:  124, 125

Summit Height:  447.1m (LIDAR)

Summit Grid Reference:  SH 81545 24141 (LIDAR)

Bwlch Height:  426.3m (LIDAR)

Bwlch Grid Reference:  SH 81619 24316 (LIDAR)

Drop:  20.8m (LIDAR)

For the additions, reclassifications and deletions to Y Pedwarau – The 400m Hills of Wales reported on Mapping Mountains since the May 2013 publication of the list by Europeaklist please consult the following Change Registers:

Y Pedwarau

Y Pedwarau – 400m Sub-Pedwarau

Y Pedwarau – 390m Sub-Pedwarau

Y Pedwarau – 390m Double Sub-Pedwarau

Myrddyn Phillips and Aled Williams (January 2020)