Mapping Mountains – Summit Relocations – Y Pedwarau – The 400m Hills of Wales

Mynydd Bwlch y Groes (SN 868 356) 

There has been a Summit Relocation 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 conducted by Myrddyn Phillips. 

Mynydd Bwlch y Groes (SN 868 356)

The criteria for the list that this summit relocation 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, 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 Bwlch y Groes and it is adjoined to the Mynydd Epynt group of hills, which are situated in the central part of South Wales (Region B, Sub-Region B1), and it is positioned with the A40 road to its south-west and a minor road to its immediate east, and has the village of Pontsenni (Sennybridge) towards the south-east. 

Summit extract from the Ordnance Survey 1:25,000 Explorer map

When the 1^st edition of the Y Pedwarau was published by Europeaklist in May 2013, this hill was listed with 140m of drop, based on the 442m summit spot height adjoined to a triangulation pillar positioned at SN 86857 35624 that appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and a 302m bwlch spot height that appears on the Ordnance Survey 1:25,000 Explorer map. 

Bwlch extract from the Ordnance Survey 1:25,000 Explorer map

The details for this hill were reassessed based on a Trimble GeoXH 6000 summit survey, resulting in a 441.7m height positioned at SN 86857 35625 and contour detail on the OS Maps website.  This mapping was the replacement for OS Get-a-map and until recent times had contours at 5m intervals which were proving consistently more accurate compared to the 5m contours that sometimes appear on the Ordnance Survey 1:25,000 Explorer map and used to appear on the Ordnance Survey Vector Map Local hosted on the Geograph website and which was entitled the Interactive Coverage Map.  This mapping had bwlch contouring between 305m – 310m, with interpolation placing the height of the bwlch as an estimated c 306m, resulting in the drop value of this hill being amended to an estimated c 136m.

The Trimble GeoXH 6000 gathering data beside the triangulation pillar on Mynydd Bwlch y Groes

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 Mynydd Bwlch y Groes (SN 868 356)

LIDAR close up summit image of Mynydd Bwlch y Groes (SN 868 356)

LIDAR analysis gives the highest ground on this hill as 441.9m positioned at SN 86898 35691, and matching that produced by the Trimble GeoXH 6000 survey with 441.7m positioned at SN 68656 35625 for ground beside the trig pillar, and this comes within the parameters of the Summit Relocations used within this page heading, these parameters are:

The term Summit Relocations applies when the high point of the hill is found to be positioned; in a different field, to a different feature such as in a conifer plantation,  within a different map contour, to a different point where a number of potential summit positions are within close proximity, when natural ground or the natural and intact summit of a hill is confirmed compared to a higher point such as a raised field boundary or covered reservoir that is judged to be a relatively recent man-made construct, or a relocation of approximately 100 metres or more in distance from either the position of a map spot height or from where the summit of the hill was previously thought to exist.

Therefore, the height produced by LIDAR analysis to the summit of this hill is 441.9m and is positioned at SN 86898 35691, this position is not given a spot height on the contemporary Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map, and is approximately 66 metres north north-eastward from where the previously listed summit is positioned and importantly the summit feature has altered from beside the triangulation pillar to featureless ground.

 

ills of Wales, and are reproduced below@

The full details for the hill are:

Group:  Mynydd Epynt

Name:  Mynydd Bwlch y Groes

OS 1:50,000 map:  160

Summit Height:  441.9m (LIDAR)

Summit Grid Reference (New Position):  SN 86898 35691 (LIDAR)  

Bwlch Height:  305.05m (LIDAR)

Bwlch Grid Reference:  SN 88817 38632 (LIDAR)

Drop:  136.9m (LIDAR)

 

Myrddyn Phillips and Aled Williams (June 2024)