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

Boncyn Nadroedd (SJ 027 540) – 390m Double Sub-Pedwar reinstatement

There has been a reinstatement of a hill 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 LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Boncyn Nadroedd (SJ 027 540)

The criteria for the list that this reinstatement 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 reinstated to the 390m Double Sub-Pedwar category.  The criteria for this sub category being all Welsh hills at or above 390m and below 400m 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 Boncyn Nadroedd and it is adjoined to the Mynydd Hiraethog group of hills which are situated in the north-eastern part of North Wales (Region C1, Sub-Region C1A), and it is positioned with minor roads to its north, west and east, and has the town of Rhuthun (Ruthin) towards the east north-east.

When the original 300m 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 it was considered not to meet the criteria then used for this sub category.

When 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 23m of drop, based on an estimated c 390m summit height and an estimated c 367m bwlch height, with each based on interpolation of 10m contouring that appears on the Ordnance Survey 1:25,000 Explorer map, and these were the details that it appeared under after the 390m sub categories were added to this list. 

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

The details for this hill were 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 estimated bwlch height for this hill being amended to c 368m, with this based on interpolation of 5m contouring between 365m – 370m. 

Extract from the Magic Maps website

The details for this hill were also re-assessed against Ordnance Survey data hosted on the Magic Maps website as well as the WalkLakes website, and what was thought to be a tiny uppermost 390m summit ring contour is given a 389m spot height on these two interactive maps, implying that what was taken as a small ring contour on the Ordnance Survey 1:25,000 Explorer map, is in fact a spot height missing its 389m height, resulting in the deletion of this hill from 390m Double Sub-Pedwar status. 

Extract from the WalkLakes website

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.

Therefore, the reinstatement of this hill to 390m Double Sub-Pedwar status is due to LIDAR analysis, resulting in a 390.0m summit height and a 368.4m bwlch height, with these values giving this hill 21.6m of drop, which is sufficient for it to be classified as a 390m Double Sub-Pedwar. 

 

The full details for the hill are:

Group:  Mynydd Hiraethog 

Name:  Boncyn Nadroedd 

OS 1:50,000 map:  116

Summit Height:  390.0m (LIDAR) 

Summit Grid Reference:  SJ 02735 54033 (LIDAR) 

Bwlch Height:  368.4m (LIDAR) 

Bwlch Grid Reference:  SJ 02555 54073 (LIDAR) 

Drop:  21.6m (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 (October 2026)