"Chet" writes:

I guess I didn't make my question clear...
Yes, I understand how an above ground single tower would provide an accurate
reference point for both tunnel teams.
However, once you are several dozen yards under a river, how do you view
that tower?
A periscope sounds very difficult given the water seals that would
constantly be necessary, to say nothing of the top of the unit not being
sheared off by the incredibly heavy river traffic...
Soooooo.... any actual sand hogs out there??????


No, a periscope at shore. Build such so it is a perfect U;
in other words, light shined in one end comes back
in the plane/parallel to the source. Turn it on its side;
with one end at the dig depth, and the other looking at
the tower on the other shore. The two mirrors are in sync.

------- -laser in
|
|
|
|
| ______ ground level_________
|
|
|
------- laser out-- |more dirt here

Shine a laser in from the far side; it enters the
periscope, does a 180, and shines on the face of the dig.
That's where you start drilling/blasting/etc.

Now, they din't have lasers, and a 100 ft tube would be
a PITA, but I'm sure surveyers have a way I don't know
about of syncing the two 90 degree angles.



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