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

Cerrig Llwydion (SN 909 731) – 500m Sub-Pedwar addition

There has been an addition to 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 confirmed by LIDAR analysis conducted by Aled Williams.

LIDAR image of Cerrig Llwydion (SN 909 731)

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 500m Sub-Pedwar category.  The criteria for 500m Sub-Pedwar status being all Welsh hills at or above 500m and below 510m in height that have 30m minimum 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 Cerrig Llwydion and it is adjoined to the Elenydd group of hills, which are situated in the northern part of Mid and West Wales (Region B, Sub-Region B2), and is positioned with the Afon Gwy (River Wye) and the A470 road to its north-east, and has the village of Llangurig towards the north.

When the original 400m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website this hill was not included in the Hills to be surveyed sub list that accompanied the main P30 list, as only 400m sub hills were then a part of this list.

When the 390m subs and latterly the 500m subs were added to this list this hill was not included, as it was listed with a c 510m summit height based on an estimation of its height in comparison to the 508m map heighted summit of Sychnentydd (SN 910 724).

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 had 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.  This re-assessment resulted in the hill remaining listed with an estimated c 510m summit height.

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 Cerrig Llwydion and Sychnentydd

Therefore, the addition of this hill to 500m Sub-Pedwar status is due to LIDAR summit analysis, resulting in a 509.4m summit height and when coupled with the 462.0m bwlch height, these values give this hill 47.4m of drop which is sufficient for it to be classified as a 500m Sub-Pedwar.

The full details for the hill are:

Group:  Elenydd 

Name:  Cerrig Llwydion

OS 1:50,000 map:  136, 147

Summit Height:  509.4m (LIDAR)

Summit Grid Reference:  SN 90965 73141 (LIDAR)

Bwlch Height:  462.0m (LIDAR)

Bwlch Grid Reference:  SN 92807 72279 (LIDAR)

Drop:  47.4m (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 (February 2020)