Monday, 4 May 2015

Mapping Mountains – Trimble Surveys – Cadair Idris



01.05.15  Gau Graig (SH 743 140) 

Gau Graig (SH 743 140)

Cadair Idris was the second mountain I visited and I can still remember the thrill of discovering the beauty of its sculptured high cwm with Llyn Cau nestled in the cirque of cliffs and crags.  I consider that we then ventured up Cadair completely unprepared, as we took a party tent with us and pitched it beside Llyn Cau and brought more cans of lager than water.  Thankfully the evening was blissfully tranquil; otherwise the tent would have been easily blown to shreds.  The lager was drunk, we all had a great time and ascended the stone shoot to the summit the following morning in a variety of footwear including Doc Mart shoes, pumps and plimsolls.

This visit made a lasting impression and I re-visited in a similar manner a year or so later.  Following these two visits I invested in proper walking boots, maps and waterproofs and haven’t looked back since.  I have so many memories of these hills from sleeping in the summit shelter on three occasions to being dropped off at the top of the A487 and walking the full ridge, descending to Abermaw (Barmouth) and hitching back home.

The hills radiating out from Cadair Idris still draw me close with many fond memories from countless visits.  These hills are positioned from west to east with the summit of Cadair Idris; Pen y Gadair, centrally situated.  At the eastern end of these hills is Gau Graig, a 683m map heighted hill that is pyramidal in shape when viewed from its east.  The hill is steeply honed on this eastern side, it appears in a number of different hill lists, but the one that interested us today was its Hewitt and Sim listing, as it just scrapes in with a listed c 30m of drop.  I’d looked at the bwlch contouring for this hill on a number of occasions and had given Gau Graig c 28m of drop and therefore a Sub-Twmpau in my Twmpau list to the Welsh P30s.

Gau Graig had been suggested by John for us to survey, as Graham was on holiday we decided to just take a level and staff with us and have the Trimble as back-up to produce an absolute height for the hill and check its height values against the more accurate ones produced by the line survey.

We parked in the large pull-in area to the south-west of the high point of the A487 at SH 753 135.  From this car park a foot stile gives access onto a narrow but good path that leads north, gaining height as it does so before curving around to the west and following a fence up toward the steepening bulk of Gau Graig.

John on the lower section of Gau Graig

On our way we came upon Ian Tether who had travelled from Anglesey, he was wrapped up and seated, waiting for the military aircraft that use this high pass as part of their Machynlleth loop.  Across the valley at the top of the Craig y Llam cliffs were other people doing likewise, I’d already driven past an almost full car park at Bwlch Oerddrws when on my way to meet John, and these people were also out on the lower hillside waiting for the low level fly past of the military aircraft.  We stopped and chatted with Ian for a few minutes before continuing up beside the fence.

Ian Tether wrapped up waiting for the military aircraft

The ascent of Gau Graig from this direction can be a bit of a lung buster as the higher one gets the ground gets steeper with the final part through a rock band before the ground levels off just before the summit.

The ground is just about to get a wee bit steep

Once at the top we put on extra layers of clothing as although the forecast was for sunshine shielded by high cloud and for the day to be dry, a chillingly blown easterly wind was sweeping across the hills.

Mynydd Moel (SH 727 136) from Gau Graig

We had come well prepared with ten figure grid references for two potential bwlch positions which had a small intervening bump between, the furthest of these positions from the summit of Gau Graig also had a 659m spot height on the Ordnance Survey enlarged mapping on the Geograph website.  If the position of this spot height was at the critical bwlch and if this and the summit spot height were accurate the hill would only have 24m of drop and not the c 30m given in the Hewitts and Sims listing.

Before starting the line survey we set up the level and took readings to two rocks beside the cairn that appears overlooking the northerly part of this hill’s summit area, and onto a prominent rock positioned much closer to the fence that we had followed upto the summit.  The latter proved higher, the rock is positioned approximately 40 metres from the fence as opposed to the cairn which is probably about 140 metres from the fence.

Having found the high point we gathered five minutes of data with the Trimble and walked down to the area of the bwlch.

Gathering data from the summit of Gau Graig with the cairned top in the background

The Trimble GeoXH 6000 at the high point of Gau Graig with Mynydd Moel in the background

John had input the two ten figure grid references for the potential bwlch position into his hand-help GPS and when down in the area of bog that constitutes the bwlch we found that the nearest one to the summit of Gau Graig was close to a small pool, whilst the other was beside a ladder stile.  We took readings to the two and found the one beside the pool to be approximately 0.5m lower than the one beside the ladder stile, therefore we could dispense with more accurate surveying at the farthest option and concentrate our efforts around the confines of the small pool, and as you can imagine these were generally a bit squelchy.

If the spot height on the Ordnance Survey enlarged Geograph map was correct it would mean that the critical bwlch was approximately 0.5m lower than 659m, therefore giving the hill only 24.5m of drop, and therefore we already knew that there was a probability that Gau Graig would be deleted from the Hewitt and Sim ranks.

Having found that the small pool option was where the critical bwlch lay, we positioned two lines of flags at 2 metre intervals running across the bwlch from two valley to valley directions and took readings at each flag, this told us that the direction of the hill to hill traverse intersected the fence and the critical bwlch lay beside the pool on its north-westerly side.

The first line of flags positioned across the bwlch on the valley to valley traverse

We left three coloured flags at this position and started line surveying up toward the summit.  John operated the level and noted the readings whilst I held the staff.  Nearing the summit and we both thought that the hill didn’t have the required 30m of drop and 75 minutes after starting the line survey we took the last reading onto the high point of the rock that we had already determined to be the summit of the hill.  As John did the sums I did a pan around with the video camera and went to John to film him giving the result.

The result is that Gau Graig has 25.7m of drop and if confirmed by Alan Dawson the hill will become a Sub-Hewitt and Sub-Sim.  We had considered doing a line survey back to the three flags at the bwlch to ascertain a closing error, but the margin of drop dictated that this wasn’t really necessary. 

Before leaving the summit I took a few photos of John with the staff at the high point of the hill, and we then walked back down to the bwlch where John gathered ten figure grid references with his hand-held GPS and I positioned the Trimble on its improvised tripod and gathered five minutes of data.

John at the summit of Gau Graig

Gathering data at the critical bwlch of Gau Graig

John noting the ten figure grid reference for the bwlch with his hand-held GPS

All we now had to do was reverse our inward route back through the steep rock band and down beside the fence toward the waiting car.

Reversing our inward route down through the rock band and the steep section

John with the cliffs of Craig y Llam in the background

The last steep section before the path levels off

It had been another excellent day on the hill and another result which will mean a status change for a hill in two lists, three if you include the Tumps.     


 Survey Result:


Gau Graig

Summit Height:  683.8m (converted to OSGM15)

Summit Grid Reference:  SH 74373 14023

Bwlch Height:  658.4m (converted to OSGM15)

Bwlch Grid Reference:  SH 73913 13777

Drop:  25.4m (Trimble geoXH 6000) 25.5m (Line Survey) (Hewitt reclassified to Sub-Hewitt, Simm reclassified to Subsimm and 600m Sub-Twmpau status confirmed)

Dominance:  3.71%



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


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