Thursday, 19 November 2015

Mapping Mountains – Trimble Surveys – Fforest Glud

12.11.15  Gilwern Hill (SO 098 582) and Pt. 409.9m (SO 083 579)

Gilwern Hill (SO 098 582)

Two hills that are listed as Pedwarau with Gilwern Hill also being listed as a Submarilyn, this higher prominence hill has an interesting summit height as it is currently listed as 439m high in the Marilyns list and as 441m high in the Pedwar list.  The Marilyn height is based on the current Ordnance Survey map spot height whilst the Pedwar height is based on the 441.045m flush bracket height adjoined to the trig pillar that is situated on the summit area of the hill.  And what better way to sort the height of Gilwern Hill out than with two of the three hill list authors who put their names to the Marilyns and Pedwarau lists.

Alan had stayed the night in Welshpool on his way to the Bagger Rambles meet at Plas Tan y Bwlch in Maentwrog, and although the weather forecast was not good we decided that a survey needed to take place, with an option being to choose a hill that appeared in both listings and one where the data did not correspond and Gilwern Hill fitted the bill perfectly.

As I drove south and Alan navigated me toward these hills the grey, murky cloud descended and drizzly rain enveloped the land.  Thankfully the track approaching these hills from the south-west is driveable and we parked adjacent to a track junction at SO 087 580.

As we set off the cloud was down and an autumnal dampness pervaded the land, we’d come well prepared with three lots of surveying equipment, my Trimble GeoXH 6000, Alan’s Leica RX1250 and another attractive little yellow number, looking all coy and relatively new, this was Alan’s new toy and it matched mine perfectly.  We’d also come well prepared as wellies were de rigueur for the day, these are marvellous inventions and are essential pieces of hill walking kit for lower heighted autumnal and winter hills.

Somewhere in the gloom is the summit of Gilwern Hill

We soon left the track and followed an indistinct path across fields to the murky heights of Gilwern Hill’s summit, this is crowned by an ancient cairn that is now grassed over and which has a trig pillar perched on top of it.  The perimeter of the cairn can be relatively easily distinguished but all now merges into one, with field and cairn and what constitutes a hill’s summit blending and merging, and we both marched to the top of this grassed ancient construction and assessed where the high point lay.  This was a patch of grass adjacent to a number of small rocks all of which moved with none being embedded.

Once the high point had been determined Alan set his Leica up and we stood below the trig on the leeward side of the grassed cairn until 30 minutes of data were collected.  During this Alan and then I took data with our Trimbles from the top of the trig to compare against the flush bracket height given in the OS Trig Database with a measured 0.89m offset from top of the flush bracket to the top of the trig.

The Leica RX1250 set-up position at the summit of Gilwern Hill

After the Leica had been dismantled Alan took a data set with his Trimble from the high point and then I followed with a five minute data set with my Trimble.  Happy that we’d got sufficient data we packed everything away and headed back toward the car and our next summit which was conveniently positioned no more than five minutes from where the car was parked.

Gathering data with my Trimble GeoXH 6000 from the summit of Gilwern Hill

By now it was obvious that the wet conditions had set in for the day, but the mistiness that had skimmed the summit of our next hill had risen slightly, and although everything around was damp the rain was not sufficiently heavy to be uncomfortable.

I quite like symmetry

Alan beside the Leica RX1250 and the two Trimble GeoXH 6000's

The summit area of our next hill proved relatively flat and therefore we set the three pieces of equipment up within a metre or so of one another and had them all gathering data at the same time, which made me smile.  Alan's Leica and Trimble were set up at approximately the same height so he could compare their data, whilst each was positioned approximately 2-3cm below where I had positioned my Trimble.  During this a vehicle drove down the track from the direction of Upper Gilwern which is the farm between these two hills, I felt like waving and trying to stop it and running down to ask about the names of the hills as the one we were currently gathering data on is listed using the point (Pt.) notation in the current edition of Y Pedwarau.

Gathering data with the three pieces of equipment from the summit of Pt. 409.9m

Once data were collected we packed everything away, walked back to the car and drove to the area of the bwlch for Gilwern Hill.  This is positioned close to the A44 and I had zoomed around on this road during the morning in a Google Car and found a parking place close to a gate which gave access to a field and the area of the bwlch.

By the time we had parked and walked back up the road toward the gate the murky afternoon was quickly turning dimmer as dusk set in.  We found a gate and bashed through hawthorns and soon were heading down toward the bwlch, this proved relatively easy to find its critical point, and once we had assessed the lay of land from a number of directions Alan set the Leica and his Trimble to gather data.

The Leica RX1250 set up at the critical bwlch of Gilwern Hill

As data were gathered we chatted away as afternoon murk turned to dimmed dusk and by the time I set my Trimble up to gather its customary five minutes of data it was turning decidedly darkish. 

After all data were gathered we sloshed our way back up the field with Alan first and me scampering behind, losing one another in the darkening dimness and re-joining each other back at the car.  All that remained was the drive back home, a quick change, wash, snack, followed by Montgomery Film Club and an excellent Locke and a yummy Ginger Chicken in Spice UK afterwards…. Good times.

Survey Result:

Gilwern Hill

Summit Height:  440.7m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys)  440.7m (converted to OSGM15, Leica RX1250)

Summit Grid Reference:  SO 09895 58290

Bwlch Height:  294.0m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys)  293.9m (converted to OSGM15, Leica RX1250)

Bwlch Grid Reference:  SO 14378 60966

Drop:  146.6m (average of two Trimble GeoXH 6000 surveys)  146.8m (Leica RX1250) (Submarilyn status confirmed)

Dominance:  33.30% (using Leica RX1250 results) (non Lesser Dominant status confirmed)

Pt. 409.9m

Summit Height:  409.8m (converted to OSGM15, and average of two Trimble GeoXH 6000 surveys)  409.9m (converted to OSGM15, Leica RX1250)

Summit Grid Reference:  SO 08331 57995

Drop:  c 32m

Dominance:  7.81%

The comparisons between the results produced by the two Trimbles are as follows:

Gilwern Hill (top of Trig) = 440.742m (AD)    440.825m (MP) (all heights converted to OSGM15)

Gilwern Hill (summit) = 440.617m (AD)    440.691m (MP) (all heights converted to OSGM15)

Gilwern Hill (bwlch) =  294.048m (AD)    294.033m (MP) (all heights converted to OSGM15)

Pt. 409.8m  SO 083 579 (summit) =  409.879m (AD)    409.748m (MP) (all heights converted to OSGM15)


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

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