Sunday, 8 November 2015

Mapping Mountains – Trimble Surveys – Fforest Glud

09.10.15  Colva Hill (SO 194 553)  

Colva Hill (SO 194 553)

Colva Hill is positioned in the heart of Radnorshire where hills stand gentle and their heathered summits are eased with a number of vehicle tracks that pass between valleys and head toward their heights.  Today we wanted to visit Colva Hill which had been on our radar for a couple of years.  The hill is listed as having 146m of drop in the Marilyns list and c 147m in the 500m Twmpau list, this drop figure is close to the 150m required for Marilyn status, and with a map summit height of 532m and with the area of the bwlch having a 388m spot height on it that is not centralised, there was an outside chance that the hill has more prominence than currently listed.

We parked, having been given permission to do so, at the end of the narrow road that passes to the north and then east of Llyn Heilyn which is to the west of Llanfihangel Nant Melan.  It was good to be out with John and Graham again and we soon had our boots on and walking up the track and across fields following rights of way on old greened vehicle tracks.

John with level in hand on the way to Colva Hill

The weather was set fine for the day with blue sky and warmth for early October.  Cresting a small intervening pastoral ridge we continued down to cross the Gilwern Brook and then headed up the other side toward the expanse of this hill’s bwlch.

Heading toward the bwlch which is on the left of photo with trees close to it

Graham nearing the bwlch

The spot height on this bwlch appears at its eastern fringes, we started assessing this area from its west and spent quite some time laying out rows of flags slowly working our way east, with measurements taken to each five flagged row and noted.  As we progressed the land that looked as if it was rising toward our hill was in fact continuing to go downhill. 

With each reading taken by John with the level to the staff that Graham was positioning beside each flag, the area of the bwlch was mapped out and its hill to hill and valley to valley traverse slowly gave up its secrets.  I noted each height reading and after about 90 minutes we had pinpointed where the critical bwlch was positioned, this was in a boggy runnel on the eastern side of the area of the bwlch and proved to be approximately five metres away from where the 388m spot height appears on the ground.

John operating the level

Graham with the staff

As John and Graham assembled the Lerica GS15 for its hours’ worth of data collection I took a few photos with the heightened autumnal colour accentuated by the low afternoon sun.  As the GS15 gathered its allotted data we watched a farmer drive sheep down the opposite hillside, half an hour later he drove toward us and I sauntered over for a chat; Ray Rogers proved very friendly and gave out smiles in his thick Radnorshire accent, we talked about the hills, their names and his sheep.  A few minutes later and John and Graham came over and we all nattered away for ten minutes or so.

The Leica GS15 gathering data at the critical bwlch of Colva Hill

Graham and John with Ray Rogers

Graham and John beside the Leica GS15 at the bwlch of Colva Hill

After one hour of data were collected I positioned the Trimble on top of the GS15’s antenna and we gathered a further five minutes of data, having taken a 1.086m measurement offset between its internal antenna and the ground.  Once all the equipment was packed away we headed up the hill.

Gathering data with the Trimble at the critical bwlch of Colva Hill

Following one of the many green vehicle tracks that criss-cross these hills, we gained height and swung left (north) as we gained the south-west ridge of Colva Hill, this track led around Cwm Griffin and soon we were beside the trig pillar at the summit.

John approaching the summit of Colva Hill

This summit had already been surveyed with an Abney level and the high point found to be approximately seven metres north of the trig pillar.  As the afternoon sun sank ever deeper and a cooling chill enveloped the land we took a series of measurements with level and staff and found that the Abney measurement was spot on.  Soon afterward the Leica GS15 was set up and gathering its data.

Surveying for summit position on Colva Hill

Graham and John beside the Leica GS15 at the summit of Colva Hill

Once an hour of data were collected the Trimble GeoXH 6000 was set up on its draughts board and sat on top of the tripod that had previously supported the Leica equipment and gathered its five minutes of data.

The Trimble gathering data at the summit of Colva Hill

The Leica and Trimble set-up position at the summit of Colva Hill in relation to where the trig pillar is positioned

All that remained was to pack everything away and retrace our steps down the hill.  Passing over the bwlch we could see Ray and his son beside a large sheep pen, we stopped and chatted, they were just about to separate their sheep from any lingering souls who had joined from neighbouring herds and treat them against scab.

Heading down from the hill

As the sun sank behind a hill ridge to our west the last half hour back to the awaiting car proved a slightly chilly affair with autumnal clear skies heralding the onset of another cold winter.   

Survey Result:

Colva Hill

Summit Height:  532.2m (converted to OSGM15, Trimble GeoXH 6000)  

532.2m (converted to OSGM15, Leica GS15)

Summit Grid Reference:  SO 19449 55397

Bwlch Height:  387.8m (converted to OSGM15, Trimble GeoXH 6000)  

387.7m (converted to OSGM15, Leica GS15)

Bwlch Grid Reference:  SO 17968 55359

Drop:  144.4m (Trimble GeoXH 6000)  144.6m (Leica GS15) (Submarilyn 

status confirmed)

Dominance:  27.16% (Leica GS15)

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

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