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

Craig y Ganllwyd (SH 707 258) - 400m Sub-Pedwar reclassified to Pedwar

There has been a reclassification to a hill that is listed in 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 a Trimble GeoXH 6000 survey conducted by Myrddyn Phillips and LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips.

Craig y Ganllwyd (SH 707 258)

The criteria for the list that this reclassification 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 reclassified 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 name the hill is listed by is Craig y Ganllwyd, and it is adjoined to the Y Llethr group of hills, which are situated in the western part of North Wales (Region C1, Sub-Region C1A), and it is positioned with a minor road to its south-east, and the A470 road to its east, and has the village of Llanelltyd towards the south.

When the original 400m height band of Welsh P30 hills compiled by Myrddyn Phillips was published on Geoff Crowder’s v-g.me website, this hill was included in the accompanying Hills to be surveyed sub list, with a non-interpolated c 420 summit height, with an accompanying note stating:  1:50000 and 1:25000 maps differ.

Extract from the Ordnance Survey 1:50,000 Landranger 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 an estimated c 21m of drop, based on an estimated c 418m summit height and the 397m bwlch spot height that appeared on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.

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

The details for this hill were re-evaluated when the 1^st edition of the Y Pedwarau was published by Europeaklist in May 2013 and it was subsequently listed with 26m of drop, based on the 423m summit spot height that appears on Harvey maps and the aforementioned 397m bwlch spot height, with a brief history of its map summit height appearing below.

Ordnance Survey 1:2,500 map from 1888 – 1889:  1,411ft (430m)

Ordnance Survey 1:50,000 map:  c 430m uppermost ring contour

Ordnance Survey 1:25,000 map:  c 410m uppermost ring contour

Harvey Maps:  423m spot height

However, it was not until the Trimble GeoXH 6000 survey and subsequently when 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.

The bwlch of Craig y Ganllwyd

The Trimble GeoXH 6000 survey gives the highest ground on this hill as 430.9m positioned at SH 70775 25853, with LIDAR bwlch analysis being prioritised over that produced by the Trimble GeoXH 6000.

LIDAR bwlch image of Craig y Ganllwyd

Therefore, the confirmation of the reclassification of this hill from 400m Sub-Pedwar status is due to a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis, resulting in a 430.9m summit height and a 398.0m bwlch height, with these values giving this hill 32.9m of drop, which is sufficient for it to be classified as a Pedwar. 

The full details for the hill are:

Group:  Y Llethr

Name:  Craig y Ganllwyd

OS 1:50,000 map:  124

Summit Height:  430.9m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 70775 25853 (Trimble GeoXH 6000)

  

Bwlch Height:  398.0m (LIDAR)

Bwlch Grid Reference:  SH 70467 25923 (LIDAR

Drop:  32.9m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips and Aled Williams (May 2014)