Friday, 26 December 2014

Mapping Mountains – Trimble Surveys – Mynydd Hiraethog



24.12.14  Mynydd Poeth (SH 953 513) and Llechwedd (SH 969 506)   

Llechwedd (SH 969 506)
With the potatoes set on a veerrrry low roast I met Mark in Cerrigydrudion just before 10.00am for festive activity on some Hiraethog Pedwar hills.  Mark had suggested visiting Mynydd Poeth, Llechwedd and Craig Hir (SH 960 544).

These hills are situated north of Cerrigydrudion with Mynydd Poeth and Llechwedd to the south of Llyn Brenig with each positioned either side of the B4501, whilst Craig Hir is further north and positioned between Llyn Brenig and the Alwen Reservoir.

Each hill was of numerical interest as Mynydd Poeth is listed as a Sub-Pedwar with c 29m of drop but a Tump with 30m of drop, and with higher hills to its west and east and no spot height on either connecting bwlch which are both between 380m – 390m, it meant that each bwlch would need surveying.  Llechwedd has a summit spot height of 450m which is embedded in forestry and it also has a 449m top outside of the conifer plantation, whist Craig Hir is immersed in mature forestry and would test the capability of the Trimble to attain its 0.1m logging accuracy and with a height of 453m based on a 1,485ft (452.6m) from an old Ordnance Survey 1:10,560 map, it would be interesting to compare this height with that of the Trimble’s if the mature conifers allowed data to be logged.

We parked just off of the road at the start of a forest track at SH 957 509, where one or two vehicles can be left.  As Mark donned his wellies I nipped over the adjacent fence and walked up the valley to valley direction from south to north on the west side of the road.  I’d examined this bwlch in a Google car the previous evening and judged that the direction of the bwlch went from one side of the road to the other and that the critical bwlch lay behind a small mound which is given a separate 390m contour ring on Ordnance Survey maps.

One of two positions that data were gathered at the easterly bwlch from Mynydd Poeth
When Mark joined me I had just completed the first data set taken from the easterly bwlch from Mynydd Poeth, Mark independently assessed the area of the bwlch and we decided that a second data set was required and the Trimble was positioned slightly south from its first placement.  As it gathered data Mark headed upto the summit, leaving me to scribble as much information relevant to the Trimble’s chosen position, as it gathered the last of its 300 points I happily took a few photos with the sun casting low light from the south.

As I packed the Trimble away I watched Mark become a small figure cresting the upper slopes of Mynydd Poeth.  Although the weather forecast for the day was good and the sun was casting long shadows, the wind gave a chill to proceedings and as I headed up the hill and joined Mark on the summit I put on a second pair of gloves as it was decidedly wintery on the top.

Mark heading toward the summit of Mynydd Poeth
The summit position was checked against a ten figure grid reference that Mark had input into his hand-held GPS, the Trimble was positioned with its internal antenna aligned with the top of a small embedded rock and once the 0.1m accuracy was attained I pressed ‘Log’ and scampered away from the equipment.

Gathering data at the summit of Mynydd Poeth
Once five minutes of data were collected we headed west down to the second option for the critical bwlch of Mynydd Poeth, this proved to be in a vast blanket bog.  As we walked into this morass I adopted a spread legged stationary surfing position with my body braced at the knee, as I flexed my knees the whole bog wobbled like a ginormous water bed.  Quite a remarkable place and probably the largest blanket bog that I’d had the pleasure to wobble on.

Mark exploring the blanket bog
Mark walked around in the middle of the bog trying to assess the rough position of its critical bwlch, whilst I gained height and dryness of foot to look down on it.  We both thought that a position close to a small patch of standing water was where the critical bwlch lay.  I re-joined the bog as Mark retired to the relatively dry land beside an adjacent fence and I stomped around a bit creating a small patch where the reed grass was flattened, by doing so I hoped to give the Trimble a better chance of picking up signals from orbiting satellites, as I stomped around and set the Trimble up the distinct smell of methane oozed from the bog.

Gathering data at the westerly bwlch from Mynydd Poeth
Once the allocated five minutes of data collector had ended I dived back into the bog to retrieve the Trimble and spent a few seconds wandering around trying to find it, all I could see was reed grass and puddles of water, eventually I stumbled across it, switched it off and we headed back over Mynydd Poeth toward the car and our next hill; Llechwedd.

As we crested the summit of Mynydd Poeth, the view south-westward was dominated by Arennig Fawr which was a grey and sombre shape without the snow of recent days, all gone as the mild westerly’s battled with the cold of high winter air.

Arennig Fawr; grey and sombre
We chose the forest track to make our way up Llechwedd and opted for the open hillside for our descent.  Again, Mark had made a reference to a ten figure grid reference for where a forest ride left the track to head toward the summit of Llechwedd, as we approached this position the southern skies had turned grey and the first of the days showers fell with a mixture of light rain and sleet being the norm.

The forest track to the north of the summit of Llechwedd
We followed the forest ride through copious amounts of heather as it gained height toward the edge of the conifers, once here we followed the edge of the forest hoping to find another ride that headed north close to where the 450m spot height appears on the map.  We couldn’t find it and concluded that the ride must have been covered in new plantings after the last tree felling had taken place.  This meant a conifer bash!

The forest ride leading to open hillside before the conifer bash to the summit
With Mark leading the way with another ten figure grid reference directing us through pine needles we miraculously popped out into a small heather and brambled clearing, which has the remains of four metal stations for the Fire Tower which is indicated on the map and which at one time stood on the summit of the hill.

I positioned the Trimble on top of one of the small remaining stantions, measured the offset from internal antenna to ground level as 0.43m and hoped that it would obtain its 0.1m accuracy.  Considering where it was positioned this accuracy was obtained quickly and I pressed ‘Log’ and joined Mark for a cup of mint and honey tea – yummy, yummy.

Gathering data at the summit of Llechwedd
The route out of the forest was direct toward the nearest open hillside and proved fun with Mark leading the way on a compass bearing and me following.  I started taking photos part of the way through the conifers and watched Mark being engulfed by trees, seemingly disappearing and being swallowed whole.

Pedwar bagging
Once out from the conifers we headed west toward where the 449m spot height appears on the map, thankfully this high point was clear of conifers and as the sun sank lower in the western sky the Trimble gathered another data set.

Gathering data at the 449m map heighted top
We then headed down to the car and drove a couple of miles north and parked close to the Llyn Brenig visitor car park.  By now daylight hours were receding and the rich winter colours had been replaced by a dimmed light.  We left the car, walked a short distance up the road and headed on a footpath toward a forest track which bisected the mature trees which make up the forestry around Craig Hir.

Llyn Brenig
Thankfully the gaps between the trees are large and enables easy walking upto the high point of the hill, as this was neared late afternoon light flickered through the canopy highlighting the mossed remains of felled tree stumps.

Approaching the summit of Craig Hir
As the Trimble was placed on what we deemed to be the high point we settled in for a long wait, I hoped it would attain its required accuracy but its starting accuracy of 4.8m was not a good sign.  I checked this height on numerous occasions and its best was only 21cm.  However, this height had bounced back up on subsequent checks and after patiently waiting for 15 minutes I closed it down, packed it away and down we went to Mark’s car as another shower just skimmed us to the north.

Hoping that the roast potatoes had not turned to charcoal I headed toward Christmas in Nantlle and stopped off to visit Dewi and Linda in Porthmadog on the way.  Good to catch up with Dewi who I hadn’t seen for quite some time.  Next stop Nantlle, I just hoped that Santa had brought me a large bag of pick and mix!



Survey Result:


Mynydd Poeth

Summit Height:  419.3m (converted to OSGM15) (significant height revision)

Summit Grid Reference:  SH 95375 51301 (summit relocation confirmed)

Bwlch Height:  387.6m (converted to OSGM15)

Bwlch Grid Reference:  SH 95707 50953

Drop:  31.7m (400m Sub-Pedwar reclassified to Pedwar) 

Dominance:  7.57%




Llechwedd

Summit Height:  450.2m (converted to OSGM15)

Summit Grid Reference:  SH 96984 50697

Bwlch Height:  386.9m (converted to OSGM15)

Bwlch Grid Reference:  SH 95069 51315

Drop:  63.4m 

Dominance:  14.07% 



For further details please consult the Trimble survey spreadsheet click {here}

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