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

 

Mynydd Perthi (SN 953 727) – 400m Sub-Pedwar addition

There has been an addition which is retrospective 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 derived from a Trimble GeoXH 6000 survey and LIDAR analysis conducted by Myrddyn Phillips and Aled Williams. 

LIDAR image of Mynydd Perthi (SN 953 727)

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 list are five categories of sub hills, with this hill being included in 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 Mynydd Perthi and it 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 A470 road to its south-west and a minor road to its south-east, and has the town of Rhaeadr Gwy (Rhayader) towards the south south-east.

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 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.

When the 1^st edition of the Y Pedwarau co-authored by Myrddyn Phillips and Aled Williams was published by Europeaklist in May 2013, the details for this hill were reassessed and it was listed with 26m of drop, based on the 458m summit spot height that appears on the Ordnance Survey 1:25,000 Explorer map and the 432m 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

However, it was not until the survey with the Trimble GeoXH 6000 and subsequent LIDAR analysis 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 Trimble GeoXH 6000 gathering data at the summit of Mynydd Perthi

Therefore, the confirmation of the addition of this hill to 400m Sub-Pedwar status is due to a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis, resulting in a 457.0m summit height and a 430.5m bwlch height, with these values giving this hill 26.5m of drop, which is sufficient for it to be classified as a 400m Sub-Pedwar.                                         

 

The full details for the hill are:

Group:  Hirddywel

Name:  Mynydd Perthi

OS 1:50,000 map:  136, 147

Summit Height:  457.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SN 95321 72719 (Trimble GeoXH 6000)

Bwlch Height:  430.5m (LIDAR)

Bwlch Grid Reference:  SN 95572 72577 (LIDAR)   

Drop:  26.5m (Trimble GeoXH 6000 summit and LIDAR bwlch) 

 

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 (November 2025)