Tunnel boring such as the Chunnel across the English Channel would be the one that amazes me....And if I remember correctly, they actually met on center very close...Someting like less than 2 feet, I believe.
What amazes me is it has a massive "sag" out in the deepest part of it....So "measuring" from both ends with that in the middle and about 32 miles....Mind bogging.
Quentin
NP Red I can somewhat understand how a line is surveyed over a mountain or perhaps a GPS is used to bore both ends of a tunnel so that the two boring machines meet exactly in the middle of the mountain. On longer tunnels they can bore a shaft down from above and lower machines so they can have four faces to work on instead of two. But can somebody explain how those workers at the bottom of the center shaft will have any idea which way to dig? How can they get an accurate bearing in the bottom of a hole? No GPS, Laser from where, compass won't work. Any ideas?
Tunnel surveying is an art and science developed in the 1800s for railways. You "turn the corner" with a transit. Tunnel surveying has some special problems such as distortions of light paths that have to be dealt with. It's demanding -- but not impossible.
Mud, add your wisdom here please. I've read the books but that's not nearly good enough.
RWM
NP RedHow can they get an accurate bearing in the bottom of a hole? No GPS, Laser from where, compass won't work. Any ideas?
I'm not able to answer your question, but I do know they have been doing this kind of construction long before the current GPS system was ever in place.
I believe one example would be: Cascade Tunnel...about 1929.
I can somewhat understand how a line is surveyed over a mountain or perhaps a GPS is used to bore both ends of a tunnel so that the two boring machines meet exactly in the middle of the mountain. On longer tunnels they can bore a shaft down from above and lower machines so they can have four faces to work on instead of two. But can somebody explain how those workers at the bottom of the center shaft will have any idea which way to dig? How can they get an accurate bearing in the bottom of a hole? No GPS, Laser from where, compass won't work. Any ideas?
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