They were not that accurate. The centre points of the tunnels were 358mm horizontally and 58mm vertically out of line with each other. Mind you, I would expect two 13 mile long masonry drills bit to wander a little as you use them:-)
Adam
Did they ever find any artifacts/old coins ect,when they dug out the inland headers out? I bet ya the archeologist where there all the way sifting through the dirt. :-)
Problem is that on many holes you want a slight downwards slope toward the outside to ensure that water can not be tracked through.
Yep. Agree with that. Drill slightly downwards towards France.
Thought I noticed a bump in the middle ;-)
In the early 70s my Dad had to make a calculation about joining two coal mines (Dodworth and Redbrook). He had to do it using triangulation with no lazers, GPS etc. The biggest problems were the plumblines that were dropped down the mine shafts. These only worked with the mine ventilators turned off and it needed 2 days for the draughts to stop. The work/calculations were done over Christmas and he managed to get the shafts to line up to within 12 inches of each other over a 600 yard distance that crossed a fault line.
I found his old work papers in his loft this Christmas and he has said I can keep them. They will have a good home. The plumb dropped down the shaft was done by a machine but took 3 men to lift it onto the machine! The concrete post that the NCB installed for triangular recording is still in the field full of cows some 35 years later.
Adam
I'm sure William Scoresby proposed just such a system in 1820.
-- JGH
I remember a friend's mum doing this when I was about 13. She was fitting a new radiator and didn't have a drill bit long enough to reach from the 18th century bit of the house to the modern bit, so she carefully measured it round 8' or so through two doorways and got it bang on. A 20mm hole and the 15mm pipe slid through perfectly.
Yes,but he wasn't interested in drilling through walls accuratly.
I wouldn't be so sure with some of the Neodymium magnets that you can get these days. One of them and a sensitive indicator (compass) could well work.
Place magnet at wanted exit point with a pole firing into wall. Other side of wall move compass horizontally along wall until need is at 90 deg to wall mark that. Move compass vertically through previous mark untill need is at 90 deg again, mark. Both horizontal and vertical alignments.
I'm not sure what effect metal ties, pipes or near by radiators would have on the accuracy of the above method. It might be worth doing three horizontal and vertical "scans" to see if the three marks make a straight(ish) line.
I don't have a Neodymium magnet to play with.
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