Mapping Mountains – Significant Height Revisions – Y Trichant - The 300m Hills of Wales

Pen y Bigil (SH 576 621)

There has been a Significant Height Revision to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips.

Pen y Bigil (SH 576 621)

The criteria for the list that this height revision applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Pen y Bigil and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A4244 road to its north-west and the A4086 road and Llyn Padarn to its south, and has the villages of Deiniolen towards the north and Llanberis towards the south.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list with a non-interpolated c 320m summit height as it was considered not to meet the criteria then used for the main P30 list.

When this list was standardised and interpolated heights and drop values also included, the details for this hill were reassessed and it was listed with an estimated c 33m of drop, based on an estimated c 326m summit height and an estimated c 293m bwlch height, with each based on interpolation of 5m contouring that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps became available online, the former has a 1081ft height and the latter has a 1080ft height given to the summit of this hill, these heights equate to 329m in metric, and when coupled with the c 293m estimated bwlch height gives this hill an estimated c 36m of drop.

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

Extract from the Ordnance Survey series of Six-Inch maps

However, it was not until the survey with the Timble GeoXH 6000 that the details for this hill could be accurately re-assessed  The 331.0m summit height produced by the Trimble GeoXH 6000 survey is not a dramatic increase in height compared to the 329m spot height, but it does come 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.

The Trimble GeoXH 6000 gathering data at the summit of Pen y Bigil

 

Therefore, the new listed summit height of this hill is 331.0m, and this was derived from a Trimble GeoXH 6000 survey.  This is 2.0m higher than its previously listed summit height of 329m, which was based on the imperial height that appeared on the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps, and 5.om higher than its previously estimated height of c 326m.

The full details for the hill are:

Group:  Glyder Fawr

Name:  Pen y Bigil

OS 1:50,000 map:  114, 115

Summit Height (New Height):  331.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 57600 62119 (Trimble GeoXH 6000)

Bwlch Height:  292.6m (LIDAR)

Bwlch Grid Reference:  SH 58014 62044 (LIDAR) 

Drop:  38.4m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (March 2019)

Mapping Mountains – Hill Reclassifications – Y Trichant - The 300m Hills of Wales

Pen y Bigil (SH 576 621) – Sub-Trichant reclassified to Trichant

There has been confirmation of a reclassification to a hill that is listed in the Y Trichant – The 300m Hills of Wales, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis and a Trimble GeoXH 6000 summit survey conducted by Myrddyn Phillips.

Pen y Bigil (SH 576 621)

The criteria for the list that this hill reclassification applies to are:

Y Trichant – The 300m Hills of Wales – Welsh hills at or above 300m and below 400m in height that have 30m minimum drop, with an accompanying sub list entitled the Sub-Trichant, with the criteria for this sub category being all Welsh hills at or above 300m and below 400m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips with the Introduction to the list and the renaming of it appearing on Mapping Mountains on the 13^th May 2017, and the Introduction to the Mapping Mountains publication of the list appearing on the 1^st January 2022. 

Y Trichant - The 300m Hills of Wales by Myrddyn Phillips

The name the hill is listed by is Pen y Bigil and it is adjoined to the Glyder Fawr group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A1), and it is positioned with the A4244 road to its north-west and the A4086 road and Llyn Padarn to its south, and has the villages of Deiniolen towards the north and Llanberis towards the south.

When the original 300m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was included in the Hills to be surveyed sub list as it was considered not to meet the criteria then used for the main P30 list.

When this list was standardised and interpolated heights and drop values also included, the details for this hill were reassessed and it was listed with an estimated c 33m of drop, based on an estimated c 326m summit height and an estimated c 293m bwlch height, with each based on interpolation of 5m contouring that appears on the contemporary Ordnance Survey 1:25,000 Explorer map.

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

The details for this hill were re-assessed when the Ordnance Survey 1:25,000 historical map and the series of Six-Inch maps became available online, the former has a 1081ft height and the latter has a 1080ft height given to the summit of this hill, these heights equate to 329m in metric, and when coupled with the c 293m estimated bwlch height gives this hill an estimated c 36m of drop.

Extract from the Ordnance Survey series of Six-Inch maps

Extract from the Ordnance Survey 1:25,000 historical 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 image of Pen y Bigil (SH 576 621) 

The road cutting splicing through the area of the bwlch

Therefore, the confirmation of the reclassification of this hill from Sub-Trichant status is due to a Trimble GeoXH 6000 summit survey and LIDAR bwlch analysis, resulting in a 331.0m summit height and a 292.6m bwlch height, with these values giving this hill 38.4m of drop, which is sufficient for it to be classified as a Trichant. 

 

The full details for the hill are:

Group:  Glyder Fawr

Name:  Pen y Bigil

OS 1:50,000 map:  114, 115

Summit Height:  331.0m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 57600 62119 (Trimble GeoXH 6000)

Bwlch Height:  292.6m (LIDAR)

Bwlch Grid Reference:  SH 58014 62044 (LIDAR) 

Drop:  38.4m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Myrddyn Phillips (March 2019)

Mapping Mountains – Trimble Surveys – Glyder Fawr

06.10.18 & 10.10.18  Foel Gron (SH 583 621) and Pen y Bigil (SH 576 621)  

Pen y Bigil (SH 576 621)

These two hills overlook Llyn Padarn and the Yr Wyddfa range and form a part of the western extension of Elidir Fawr as it descends to the lowlands of the Glyderau.  I’d visited the lower part of Pen y Bigil many years ago when along with my brother we investigated numerous rock outcrops for scrambling possibilities; otherwise the two hills had stared back at me for many years rising in their bouldered confines above the small community of Deiniolen.

I approached from the south walking up a public footpath enclosed by a stone wall and wired fence on one side and upright slate sides on the other.  The footpath crested the southern ridge of Foel Gron and it was here that I headed on to the hill as a faint path is marked on Ordnance Survey maps and I hoped this would lead me toward the summit bi-passing the profusion of stone walls that mark this hill.

The way to the hill

Yr Wyddfa dominating the southern view

My hope for an easy ascent was dashed as a stone wall soon barred further progress, a cumbersome climb then followed.  Once on the other side the summit was within reach. 

By now the sun had made an appearance, a welcome addition as the skies in the main had remained grey over the last few days.  The play of light interspersed with shadow gave perspective to the land with Yr Wyddfa dominating the southern view.

I took two data sets from the summit area of Foel Gron, the first on a small grassed knoll beside a stone wall where the 353m spot height appears on the map.  This entailed wedging my rucksack on top of the wall and with the Trimble perched on my rucksack it now cleared the barb wired fence which also adorned the stone wall.  Once the offset between its internal antenna and the ground below was measured I stood back as it gathered its allotted data and looked out toward the high peaks of Eryri.

Gathering the first data set on Foel Gron

The second data set was taken from ground to the south of the grassy knoll, this is where 2m DTM LIDAR gives the summit to be positioned, and visually this looked higher than the grassy knoll, although eyesight alone is not ideal for such interpolation.  I also noted ground further to the west that looked of similar height, which I later visited but did not take a third data set from.

Gathering the second data set on Foel Gron

Once data were stored I packed the Trimble away and headed down toward the minor road between the two hills.  I found the descent relatively easy and used gaps in a number of walls to gain access to the road.

Descending Foel Gron toward Pen y Bigil

It was only a short distance from where I emerged on the road to the access track that leads up to the high mast beside the summit of Pen y Bigil.  On my way up the track I chatted to a woman in her car who had retired and found the house of her dreams, this looked out on Yr Wyddfa and nestled in to the side of Pen y Bigil, she said that it was a beautiful view, although it was prone to get a bit of weather!

The summit of Pen y Bigil is beyond the mast and comprises a rock outcrop, once the Trimble was balanced aligned with the high point I sat below it and looked down on Llyn Padarn and over its glistening waters to the pointed top of Yr Wyddfa, a stunning place to be in the early afternoon.

Gathering data at the summit of Pen y Bigil

Once data were stored I retraced my inward route down the track and took a data set from where LIDAR implies the natural bwlch of Pen y Bigil to be situated.  Just below this position the minor road has carved through this part of the hill forming a cutting with the road bwlch now lower on the hill to hill traverse.  The remaining natural bwlch was beside the track which is slightly elevated above the grazing fields on either side.

Yr Wyddfa from Pen y Bigil

Gathering data at the remaining natural bwlch of Pen y Bigil

After the fourth data set of the walk it was only a short walk on the minor road back to my car, and just one small hill remained to visit and Trimble before visiting my brother in Ysbyty Gwynedd, and that hill was Dinas Dinorwig (SH 549 652).

Postscript:

I later checked the details for Foel Gron with the 1m DTM LIDAR model and discovered that land to the west of where I took the second summit data set is given as higher.  I thought drat, or other such words!

Foel Gron (SH 583 621)

I could easily forgo such things as 1m DTM LIDAR only gives this westerly point as being a few millimetres higher than where I took the second summit data set from, but this niggled in the back of my mind, and therefore I decided to re-visit and complete the task.

The opportunity to do so came four days after my initial visit, as I’d been out visiting and surveying a number of P30s in the north-eastern Carneddau and Foel Gron was only a short diversion from my route back toward Nantlle.

Although the forecast for the day predicted high winds, the north-eastern Carneddau had been well sheltered with only a semblance of light breeze during the many hours that I’d been wandering the hills earlier in the day, but upon arrival at the base of Foel Gron the wind whipped the land in a frenzy compared to the preceding hours of tranquility.

I was soon at the top of Foel Gron having ascended direct from the road between it and Pen y Bigil, this was my descent route on my first visit and proved an easy way off the hill.

Yr Wyddfa from Foel Gron

The Trimble directed me to the 1m DTM LIDAR summit which comprises ground I had looked out on and later visited after the two summit surveys of this hill previously conducted.

Once at this summit I remodelled a small cairn to wedge the Trimble in place and safeguard it from the strong wind and measured a 0.13m offset between its internal antenna and the ground at its base and set it to gather data.

Gathering data at the summit of Foel Gron

As it did its stuff I stood below it facing in to the wind and watched as the sun cast wonderful colour on the hill, looking up the valley Yr Wyddfa dominated the view, a great greened bulk with shadowed plunging cliff rimmed sides; a marvelous sight.

I let the Trimble gather 15 minutes of data as I was in no rush and stood happily as the wind span across the hill.  On occasion it feels the most natural thing to stand and face elemental forces whilst they are at play, and today it was a delight as the wind and its ferocity and uncontrolled noise hurtled past, this is especially so on a moorland hill as if blown over it would give a relatively soft landing on heather.

I wanted to stay on this hill until the sun sank in the western sea, but the colour enriched hillsides drew me down, and once data were stored and the Trimble packed away I slowly made my way back down the hill.

Old and new in Deiniolen

As I drove the short distance to where the road widened beside a church I stopped to change out of my boots and hill walking gear, and spent a number of minutes watching the oranged glow as the sun forever sank deeper in the sky.

Sunset from Deiniolen

It had been a day of contrasts starting with delicate mist as it played on the banks of Llyn Gwynant, followed by the silent flight of a barn owl, discovering a cannabis plant on a hill, meeting a farmer and being invited in for a refreshing drink and shortbread, meeting the Gorse Hill Caravan Park owner, having a number of hill names confirmed, my legs ripped by copious amounts of gorse and now watching the sun slowly set.  A wonderful way to spend a day. 

    

Survey Result:

Foel Gron

Summit Height:  351.6m (converted to OSGM15, Trimble GeoXH 6000)

Summit Grid Reference:  SH 58359 62170 (Trimble GeoXH 6000) (summit relocation)

Bwlch Height:  319.4m (LIDAR)

Bwlch Grid Reference:  SH 58679 62182 (LIDAR)

Drop:  32.2m (Trimble GeoXH 6000 summit and LIDAR bwlch)

Dominance:  9.16% (Trimble GeoXH 6000 summit and LIDAR bwlch)

Pen y Bigil

Summit Height:  331.0m (converted to OSGM15, Trimble GeoXH 6000) (significant height revision)

Summit Grid Reference:  SH 57600 62119 (Trimble GeoXH 6000)

Bwlch Height:  292.6m (LIDAR)

Bwlch Grid Reference:  SH 58014 62044 (LIDAR)

Drop:  38.4m (Trimble GeoXH 6000 summit and LIDAR bwlch) (Sub-Trichant reclassified to Trichant)

Dominance:  11.59% (Trimble GeoXH 6000 summit and LIDAR bwlch)

For further details please consult the Trimble Survey Spreadsheet