Friday, 21 August 2015

1824 - The Loch Foyle Baseline

Whilst in Northern Ireland last week my family and I were taken up to Gotmore, a wonderful viewpoint overlooking Loch Foyle and the area of Binevenagh.

As usual I spotted what looked like a trig and wandered off to find it.  Sadly, it was only an AA direction finder but on the way down I found this enticing sign.
15 08 16 Magilliagan Plain (10)
I bounced back to the family.  "Hey, there's a baseline down there," I wittered pointing at the Magilligan plain in front of us.
15 08 16 Magilliagan Plain (13)
I got the OSNI map out and the penny dropped.  I had seen these trigs when I was reading the map the night before and had wondered why they'd put them on the plain.
15 08 16 Magilliagan Plain (18)
In fact, now I paid more attention to the labelling of the trig triangles,  I realised I was looking at the North Base Tower (at the very top of the image) and the Minearny Base (centre of image).

I had to go and find them.  Roger, my husband, was game (he's put up with this sort of thing for years) but the girls were less supportive, rolled their eyes and hopped into their uncle's car for a lift home whilst Rog and I hunted.

I had no idea what we were looking for.  OK, a trig shaped object would have been my first choice but I didn't expect to find something with a fence around.  This is the North Tower.
15 08 16 Magilliagan Plain (15)
15 08 16 Magilliagan Plain (16)
We were unable to get close due to crops in the field, so headed to what I thought was the other end of the baseline, Minearny.
15 08 16 Magilliagan Plain (7)
Again, this was on private land so we could only view from a distance.
15 08 16 Magilliagan Plain (6)
Once home I read up on the baseline.  It was, at nearly 8 miles, the longest in its day.  If you read the Trig Ireland webpage you'll see that it wasn't truly necessary, as the Ordnance Survey of Ireland could have just extended the network from the OS GB stations across the water.  However, Lt Col Colby being a thorough surveyor (there are lots of us around) opted for the belt and braces approach.  It took two years to measure the baseline.

When it was resurveyed in 1960 it was found to be out by about an inch.   I make that an accuracy of 2ppm (parts per million) which shows the expertise of the survey team.  Amazing.

If I had any brains about me I would have realised that I'd been to see the North and Minearny Towers.  It didn't occur to me until I saw the diagram on the webpage that there would be a South tower.  Golly. I really am thick at times.  Here it is in Ballykelly.
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Screen Shot 2015-08-21 at 23.16.34
And even more annoyingly, the pictures on the web indicate it's a piece of cake to visit. OK, so I'd only be able to see the top of the concrete covering the cross hairs defining the actual point, but it would have been fun.  One to see when I'm back there and zero points for common sense from this Chartered Surveyor for not considering the North would have a South.

Saturday, 15 August 2015

The Walls of Derry - the Benchmark Circuit

I visited Derry today and walked the wonderfully intact walls.
2015-08-15 13.18.44 HDR
As ever, my focus was on that bit of the wall 10cm above the ground and I was soon rewarded.  I haven't seen an OSNI benchmark for a while.
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I kept my eyes peeled for more.
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And there was another one.
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And more.
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This part of the wall faces towards a Loyalist area (read the sign).
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Every so often you find a hairy one.  [I'm referring to benchmarks here].
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By this stage my sister-in-law noted that I'd even got her looking for these random cuts in walls.  I am a benchmarking missionary.
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I found seven in all.
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I bet there are more.  There's a part of me that wants to rewalk the walls and see if I've missed one.
2015-08-15 14.24.20

Geo Reykjavik - Small but Perfectly Formed

I didn't find many geo trophies in Reykjavik.  I tried my luck with the Parliament Building thinking there may be a benchmark or some measurement paraphernalia.
But I didn't find much: just a little benchmark to keep me keen.
I knew I'd have more luck at Hallgrimskirkja Church.
This is the only trig pillar I could source on the internet.  It's Danish but has no date on it. Presumably it dates to pre independence (so pre 1944).
The metal dial twizzled.  Hours of fun for geodesists.

Thursday, 13 August 2015

America and Eurasia - We Never Were Going to Get Close

Whilst in Iceland last week we twice visited the divergence zone of the North American and Eurasia continental plates.  They are zooming apart at up to 25mm per annum.
We visited the wonderful spot, Þhingvellir where there are amazing cascades of water from America to Eurasia - over a 40m drop.  This is the best spot on land to see the spreading mid Atlantic ridge.

Below you can see, on the right, the high and mighty N American plate, and to the left, the humble - yeh, right - Eurasian one. My girls were standing on new shiny 9000 year old ground (and slightly newer board walk) here.
A few days later we visited another site of this divergence zone: a good map is at the USGS site.  We were in the far south west of Iceland at Reykjanesbær and visited the bridge at Sandvík.  This bridge was built recently in 2002 as a symbol between America and Europe (here right and left).
It was windy in the gap.
Do you think we'll ever meet and make up?