Mapping Mountains – Significant Height Revisions – Y Pedwarau - The 400m Hills of Wales

Craig y Ganllwyd (SH 707 258)

There has been a Significant Height Revision 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 height revision 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.

However, it was not until the Trimble GeoXH 6000 survey that an accurate summit height could be given this hill.

The Trimble GeoXH 6000 gathering data at the summit of Craig y Ganllwyd

The Trimble GeoXH 6000 survey gives the highest ground on this hill as 430.9m positioned at SH 70775 25853, and this comes within the parameters of the Significant Height Revisions used within this page heading, these parameters are:

The term Significant Height Revisions applies to any listed hill whose interpolated height and Ordnance Survey, Harvey or other interactive map summit spot height has a 2m or more discrepancy when compared to the survey result produced by the Trimble GeoXH 6000 or analysis of data produced via LIDAR.  Also included are hills whose summit map data is missing an uppermost ring contour when compared with the data produced by the Trimble or by LIDAR analysis.

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

Another resource now available online is the interactive mapping hosted on the Welsh Government website and entitled the DataMapWales.  This mapping has 5m contours and its detail matches that produced from the OS Terrain 5 product, which compliments much of that produced from LIDAR.  And for this hill this mapping shows the 415m, 420m, 425m and the uppermost 430m ring contour that is missing from the contemporary Ordnance Survey 1:25,000 Explorer map.

Extract from the DataMapWales

Therefore, the new listed summit height of this hill is 430.9m and this was derived from a Trimble GeoXH 6000 survey, this is 20.9m higher than the uppermost 410m ring contour that appears on the Ordnance Survey 1:25,000 Explorer map and 7.9m higher than the previously listed summit height of 423m that appears as a spot height on Harvey maps.  With the caveat that the Ordnance Survey 1:50,000 Landranger map has an uppermost 430m contour ring which compliments detail on the 1:2,500 base map which has an imperial height of 1,411ft (430m).

The full details for the hill are:

Group:  Y Llethr

Name:  Craig y Ganllwyd

OS 1:50,000 map:  124

Summit Height (New 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 (August 2015)