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)

  

Mapping Mountains – Trimble Surveys – Mynydd Epynt

30.12.19  Noethgrug (SN 837 373, not Trimbled) and Mynydd Bwlch y Groes (SN 868 356)  

Mynydd Bwlch y Groes (SN 868 356)

Visiting the hills of Mynydd Epynt between Christmas and New Year is becoming an annual tradition.  The land comprising the majority of this hill range is sadly out of bounds for much of the year due to military occupation.  However, all people deserve a holiday, and whilst the military are away this land becomes a quiet place, with just an occasional farmer spreading feed for grazing sheep and a rogue hillwalker or two.

Today we planned on visiting two Pedwar hills; Noethgrug and Mynydd Bwlch y Groes, with two 300m P30s as optional extras.  We were walking by 10.00am starting from the upper reaches of the Nant Gwennol valley where a forest track gained height up in to the misted realm of the conifer plantation.

The Nant Gwennol valley

Mist and conifers are well suited, neither are my favourites, but when combined they give an isolated solitude where darkened shapes of regimented trees merge.  Having gained height we left the main forest track and headed toward the summit on a narrow track which soon deposited us back on to the main forest track.

Leaving the main forest track

It was now time for a bite to eat as the next part of the walk involved tree bashing and following a ten figure grid reference toward the high point of Noethgrug with the aid of a hand-held GPS.

The outer trees gave some form of protection toward the inner, with the latter dark and dead with little greenery.  Patches of greenery only emerged the higher we got, with the summit beside an odd patch of green squeezed in amongst a mass of conifered branches.

The Trimble was set up beside the summit half way up a tree with the measurement offset noted between its internal antenna and the ground below.  There wasn’t much hope of it ever achieving the 0.1m accuracy level before data should be logged and after a few minutes I switched it off, packed it away and followed my two companions further in to the forest as we tried to find an old path back to the main forest track.

At the summit of Noethgrug

The old path was found, it had not been maintained and the trees soon enveloped us again.  Nearing the main forest track we came upon a few trees encrusted in moss.  Standing with branches sprayed out and greened with winter light adding to the background mist.  I stood and looked, a moment to savour before the next few miles on the continuation of the forest track.

Moss encrusted trees

We stopped where a narrower path led toward the summit of the first of the two optional 300m P30s.  We knew by now that time did not permit a visit as there was still a few miles to go to the top of our last hill; Mynydd Bwlch y Groes.

On the forest track to the north of Noethgrug

The forest track took us on to a paved road with the Epynt red flag flapping in the slight breeze, I took a few photos and we then continued.  A series of paved roads now took us over a slight ridge and up toward the public road that speeds its way beside the summit of Mynydd Bwlch y Groes.

One of the landmarks of the Epynt

Greeting us at the junction where the public road heads southward were signs for Dixie’s Corner; an unusual name so close to a Welsh summit.  The Epynt can be a surreal place to visit, with much of it out of bounds, but tranquillity can still be found, although the occasional passing vehicle as we marched up the road toward the triangulation pillar atop Mynydd Bwlch y Groes broke the silence and added to the surreal element as they quickly approached and disappeared again in to the mist.

An unusual name to find near the summit of a Welsh hill

Reaching the summit was welcome and gave me the opportunity to at least survey one summit or bwlch for my endeavours.  The Trimble was soon set up atop my rucksack gathering its allotted data.  During data collection I stood with my two companions and chatted and only stopped doing so after 11 minutes of data were gathered and stored.

Gathering data beside the trig pillar of Mynydd Bwlch y Groes

We left the summit and retraced our way back toward Dixie’s Corner and its near large barns and then headed westward down a wide track which shot downward in a military style straight line.  At its bottom there was now height to regain, again on the track, this was unwelcome at the end of the walk, but it had to be done.

Down from the hill

The track led us to an enclosed lane, passing a large old house on the way.  The old lane forever lost height fording the stream at its bottom before the last bit of torturous uphill back to the awaiting car.

LIDAR summit image of Mynydd Bwlch y Groes

LIDAR bwlch image of Mynydd Bwlch y Groes

LIDAR – Postscript

Since visiting these hills full LIDAR coverage is now available.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height and positional data that is now freely available for England and Wales.  Consequently the numerical details for Mynydd Bwlch y Groes have been analysed using this technique, resulting in LIDAR being prioritied in favour of the Trimble GeoXH 6000 summit result.

Survey Result:

Noethgrug 

Summit Height:  412m (spot height)

Summit Grid Reference:  SN 83711 37384 (hand-held GPS via DoBIH)

Bwlch Height:  c 293m (interpolation)

Bwlch Grid Reference:  SN 84875 39048 (interpolation)

Drop:  c 119m (spot height summit and interpolation bwlch)

Dominance:  28.88% (spot height summit and interpolation bwlch)

Mynydd Bwlch y Groes

Summit Height:  441.9m (LIDAR)

Summit Grid Reference:  SN 86898 35691 (LIDAR) (summit relocation)

Bwlch Height:  305.05m (LIDAR)

Bwlch Grid Reference:  SN 88817 38632 (LIDAR)

Drop:  136.9m (LIDAR) (Submarilyn deletion)

Dominance:  30.97% (LIDAR)

For further details please consult the Trimble Survey Spreadsheet

Mapping Mountains – Hill Reclassifications – Marilyns

Mynydd Bwlch-y-Groes (SN 868 356) – Submarilyn deletion

This is one in a series of retrospective Hill Reclassification posts that detail hills whose status has altered in the listing of the Welsh Marilyns.  These reclassification posts will give details of hills where I have had direct association with their change of status, and they will tie in with a forthcoming Change Register giving detail to this list and its alterations since publication in the TACit Tables booklet

The listing of Welsh Marilyns was published in booklet format by TACit Tables in February 1997 and entitled The Hewitts and Marilyns of Wales.  The Marilyns were originally listed as an all British compilation and were published by Cicerone Press in 1992 in the book entitled The Relative Hills of Britain.  The list compiler for the Marilyns is Alan Dawson.

When the Welsh Marilyns list was published in booklet format there were 156 qualifying hills with a further 27 Welsh Submarilyns also included.  The criteria for Marilyn status being any hill that has a drop of at least 150m, irrespective of their height, with the criteria for Submarilyn being any hill that has 140m or more and below 150m of drop.

The Hewitts and Marilyns of Wales by Alan Dawson

The details for this deletion appear below:

The name of the hill is Mynydd Bwlch-y-Groes, and it is situated in the Mynydd Epynt group of hills in the southern part of mid Wales, and is positioned with the A483 road to its north-west and the A40 road to its south, and has the town of Llanymddyfri (Llandovery) towards the west.

Prior to the deletion of this hill from Submarilyn status it was listed with 140m of drop, based on the 442m summit height which is adjoined to a triangulation pillar and which appears on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map and a bwlch height of 302m based on the spot height that appears on the Ordnance Survey 1:25,000 Explorer map.

Extract from the Ordnance Survey 1:25,000 Explorer map for the bwlch area of this hill

The details for this hill were examined during a re-assessment of the Y Pedwarau – The 400m Hills of Wales list and forwarded to the list author of the Marilyns; Alan Dawson.  The area of this hill’s bwlch was assessed using mapping on the OS Maps website.  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 mapping has bwlch contours between 305m – 310m with the position of the 302m spot height that appears on the Ordnance Survey 1:25,000 Explorer map below the 305m contour line, indicating that it is not positioned at the critical point of this bwlch.

Extract from the OS Maps website for the bwlch area of this hill

These details were forwarded by Myrddyn Phillips via email to the list author of the Marilyns on the 8^th October 2019, and the notification that this hill had been deleted from the status of Submarilyn was received via email from Alan Dawson on the 16^th October 2019.

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

The hill was subsequently analysed by Myrddyn Phillips using latest available LIDAR, resulting in a minor summit relocation with a 441.9m summit height positioned at SN 86898 35691 and a 305.05m bwlch height positioned at SN 88817 38632, with these values confirming its deletion with 136.9m of drop. 

 

The full details for the hill are: 

Name:  Mynydd Bwlch-y-Groes 

OS 1:50,000 map:  160

OS 1:25,000 map:  12, 187

Summit Height:  441.9m (LIDAR) 

Summit Grid Reference:  SN 86898 35691 (LIDAR) 

Bwlch Height:  305.05m (LIDAR) 

Bwlch Grid Reference:  SN 88817 38632 (LIDAR) 

Drop:  136.9m (LIDAR) 

 

Myrddyn Phillips (March 2020)