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

Disgwylfa (SN 982 663) – 400m Sub-Pedwar deletion

There has been a deletion from the listing of the Y Pedwarau – The 400m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips.

Disgwylfa (SN 982 663) is now deleted from the 400m Sub-Pedwar category

The criteria for the list that this deletion 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 list are five categories of sub hills, with this hill being deleted from the 400m Sub-Pedwar category.  The criteria for this sub category 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.

Y Pedwarau - The 400m Hills of Wales by Myrddyn Phillips and Aled Williams

 

The hill is adjoined to the Hirddywel group of hills, which are situated in the northern part of South Wales (Region B, Sub-Region B2), and it is positioned with the A44 road to its north-east and the A470 road to its south-west, and has the town of Rhaeadr Gwy (Rhayader) towards the north-west.

When the original 400m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was not included in the Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category. 

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

After the sub list was standardised, and interpolated heights and drop values also included the details for this hill were re-evaluated and it was listed with 20m of drop, based on the 477m summit spot height and the 457m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on Geograph website and which was entitled the Interactive Coverage Map.

 

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 Disgwylfa (SN 982 663)

Therefore, the deletion of this hill from 400m Sub-Pedwar status is due to LIDAR analysis, resulting in a 476.0m summit height and a 456.2m bwlch height, with these values giving this hill 19.8m of drop, which is insufficient for it to be classified as a 400m Sub-Pedwar. 

The full details for the hill are:

Group:  Hirddywel

Name:  Disgwylfa

OS 1:50,000 map:  136, 147

Summit Height:  476.0m (LIDAR)

Summit Grid Reference:  SN 98206 66311 (LIDAR)

Bwlch Height:  456.2m (LIDAR)

Bwlch Grid Reference:  SN 98416 66295 (LIDAR)

Drop:  19.8m (LIDAR)

For the additions, reclassifications and deletions to Y Pedwarau 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 (July 2017)