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

Wenallt (SN 967 784) 

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 LIDAR analysis initially conducted by Aled Williams and subsequently by Myrddyn Phillips. 

LIDAR image of Wenallt (SN 967 784)

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; 500m Sub-Pedwarau, 500m Double Sub-Pedwarau, 400m Sub-Pedwarau, 390m Sub-Pedwarau and the 390m Double Sub-Pedwarau.  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 Wenallt, 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 a minor road to its north-west and south, the A470 road to its west, and the B4518 road to its east, and has the town of Llanidloes towards the north.

When the original 400m height band of Welsh P30 hills was published on Geoff Crowder’s v-g.me website, this hill was included in the main P30 list with a 435m summit height.

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 40m of drop, based on the 435m summit spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and the 395m 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.  And these are the details the hill was listed by when the 1^st edition of the Y Pedwarau was published by Europeaklist in May 2013. 

Extract from the Ordnance Survey 1:25,000 Explorer 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 summit image of Wenallt (SN 967 784)

LIDAR analysis gives the highest ground on this hill as 433.0m positioned at SN 96743 78468, 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.

Therefore, the new listed summit height of this hill is 433.0m and this was derived from LIDAR analysis, this is 2.0m lower than the originally listed summit height, which was based on the 435m spot height that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Hirddywel 

Name:  Wenallt 

OS 1:50,000 map:  136, 147

Summit Height (New Height):  433.0m (LIDAR) 

Summit Grid Reference:  SN 96743 78468 (LIDAR) 

Bwlch Height:  394.0m (LIDAR) 

Bwlch Grid Reference:  SN 95366 78745 (LIDAR) 

Drop:  39.0m (LIDAR)

 

Myrddyn Phillips and Aled Williams (October 2025)