Myrddyn Phillips - Mapping Mountains: Mapping Mountains – Trimble Surveys

Sunday, 31 December 2017

Mapping Mountains – Trimble Surveys – Pumlumon

29.10.17 Ffridd (SN 793 991 [only bwlch surveyed SN 790 993]), Copa Shon (SN 781 993) and Pen y Graig Fawr (SN 773 996)

Pen y Graig Fawr (SN 773 996)

Occasionally I think of the early list compilers and the resource, or lack of, that they worked with, these being the Ordnance Survey One-Inch maps or the Bartholomew’s maps of the day, or if they were lucky access to the Six-Inch series of maps. Nowadays the resource is almost unlimited with a plethora of online maps available, both contemporary and historical, these include the multitude of spot heights on the Interactive Coverage Map hosted on the Geograph website and the 5m contouring on OS Maps which is the relatively recent update to Get-a-map, these are complimented by independent surveyors producing heights that are more accurate than the mass of data available via Ordnance Survey and by analysis of LIDAR data which is causing havoc with the status of many a hill. If your thing is hill data, we are lucky to be living in such an age. With these various items at a list compiler’s disposal it is relatively easy to analyse a hill’s drop, however interpolation of contours is still needed, and with one of today’s hill’s I wanted to see if it’s status as a Subhump listed with 99m of drop was correct, as Ordnance Survey bwlch contouring suggested that its drop value was nearer to c 90m which is the minimum required for Subhump status. This hill is Pen y Graig Fawr, and to add further interest it also has an equal map heighted 217m summit named Copa Shon adjacent to it.

These two summits form a part of the land to the south-east of Machynlleth and I devised a route also taking in a lower P30 whose summit is immersed in conifer plantation, this route would also take me to the critical bwlch of whatever summit proved higher, thus saving me a drive to survey this one point.

As I walked up the narrow lane toward the farm of Croeslyn which is positioned close to the critical bwlch of the higher of these two equal map summited hills, a slow moving drizzled shower crept up the valley as light grey cloud pushed southward. Having previously analysed this bwlch from the confines of a Google car I knew the lay of land and where a gate gave access to the field where the bwlch is positioned, however when I arrived at the gate giving this access I heard and saw the local farmer in his yard at Croeslyn, if I could see him, he could see me, which wasn’t ideal for the ten minutes or so that I wanted to spend in his field. I slithered my way over the slippery gate and quietly crept across the field adjacent to a hedge which would at least give a semblance of cover. Thankfully the critical point lay close to the hedge and therefore I didn’t have to wander in to the centre of the field, and within a few moments the Trimble was set up gathering data. Once 300 datum points were stored, I closed the equipment off, took a few photos, packed it away and quietly slithered back over the slippery gate.

Gathering data at the critical bwlch of Copa Shon

I gathered data from the critical bwlch of Copa Shon just in time as the cow nearest the hedge is close to where the Trimble was set up just a few minutes earlier

I then headed up the narrow lane to where it ends and turns in to a track and then footpath across the rain soaked fields toward the conifered summit of Ffridd, as I did so the morning’s stillness was interrupted by what sounded like a rutting stag, it was a cow, but a cow that was obviously a little non-plussed that only a few minutes earlier a Trimble had been in its field. I’d noted the herd of cows in the adjacent field when gathering data, and as access between each field was through a large gap in the intervening hedge I was wary of them heading my way, they did so shortly after I left, and in a manner that sounded as if they were reclaiming their territory.

The forested summit of Ffridd (SN 793 991)

A forest ride marked as a public footpath gave access between the conifers and an unmarked ride then led up towards the high point of Ffridd. This was relatively easily found and involved hardly any tree bashing as higher ground always indicated the way forward and two further unmarked rides eased passage, when at what I considered the summit I stood on a tree stump and took a few photos.

The summit of Ffridd

It was only a short distance out of the forest once backtracking to the public footpath, immerging out of conifers however brief a visit to their murky depths is always a welcome experience and as I did so an occasional patch of blue and a glimmer of sunlight penetrated an otherwise light grey and white sky. Across the valley the shapely profile of the Tarren hills were on grand display with the higher Cadair Idris dominating the horizon.

The next point to survey was the connecting and critical bwlch of Ffridd which proved to be beside a drainage channel next to the continuation of the same conifer plantation that embeds its summit. I assessed the lay of land from various directions and repeated this process until deciding on where to place the Trimble and stood back as it gathered its customary five minutes of data.

Gathering data at the critical bwlch of Ffridd

I was now on open hillside, although this did involve a number of barb wired fences and an occasion gate easing access. It felt good to be out on hills that I had not previously visited giving different perspective upon higher hills whose summits I had visited on many an occasion.

Tarren y Gesail from the approach to Copa Shon

I followed the forest boundary for a few fields before the bulk of Copa Shon rose ahead, this hill’s summit consists of grass a couple of metres from a fence and whose highest point is easy to distinguish. As the Trimble gathered data I stood and looked north toward the Tarennydd and their extended ridge and across to two smaller hills whose summits I hoped to visit later in the day.

Gathering data at the summit of Copa Shon

Ffridd from the summit of Copa Shon

It was only a short walk down to the connecting bwlch and its critical point was easy to pinpoint, this was beside a gate on a wet bit of mud beside grazed and closely cropped grass. As five minutes of data were gathered I stood beside the fence leading toward the gate and scribbled down all necessary details of the survey; time started, name of hill, feature, measurement offset, datum points taken, overall time of survey, number of satellites logged and measurement uncertainty of placement. All of this is repeated for every survey and helps in passing the waiting game.

Gathering data at the critical bwlch of Pen y Graig Fawr

Packing the Trimble away it was only a short walk to the high point of Pen y Graig Fawr which is adorned by a trig pillar with its base next to what looks like a small outcrop of flattened and embedded rock, however the high point of this rock has a suspicious looking bit of concrete on it, so these rocks may have been placed at the summit when the trig was first positioned on the hill, if so, this reduces the hill’s natural summit height as I took data aligned with the highest rock. As data were gathered the hill’s to the south glimmered in silvery light with delicate greens and autumnal shadows casting out across the land.

Gathering data at the summit of Pen y Graig Fawr

Delicate colour upon the hills

Once the allotted five minutes of data were stored I packed the equipment away and headed down to the fence connecting with the gate where the Trimble had gathered the last bwlch data, and continued down on a track leading back to Penegoes where my car was parked. It had been a good four hours on the hill and I now wanted to investigate the summit, and later the bwlch, of Coed Pant y Glo (SH 767 014); another P30 which I had not previously visited.

Survey Result:

Ffridd (significant name change)

Bwlch Height: 172.2m (converted to OSGM15)

Bwlch Grid Reference: SN 79089 99337

Drop: 41m

Dominance: 19.16%

Copa Shon

Summit Height: 217.3m (converted to OSGM15, and higher summit confirmed)

Summit Grid Reference: SN 78108 99347

Bwlch Height: 127.9m (converted to OSGM15)

Bwlch Grid Reference: SN 80033 98740 (bwlch position with Pen y Graig Fawr swapped)

Drop: 89.5m (Subhump status recommended to be swapped from Pen y Graig Fawr and then deleted)