In message
, at
05:41:17 on Thu, 15 Mar 2012, 77002 remarked:
There doesn't seem to be much distance between the 2 bores. Surely they won't
be that close the entire route? Looks like it would collapse.

The tunnel is concrete-lined as the machine moves forward.

I'm more interested in them apparently using GPS 38m underground!

There distance between the tunnels may not be constant.

No doubt they do spread apart as they get further from the portal.

Does GPS penetrate that far? One is intrigued as to how this works.
ISTR earlier tunnels being aligned by laser.

Surveying works because light travels in straight lines (or so the
Physicists tell us). In the old days you peered through a theodolite at
poles held by your colleagues. Today, perhaps you shine a laser in the
other direction. There's very little difference in the precision (which
is all about measuring angles), even if modern methods may be less
labour intensive.
--
Roland Perry

Roland Perry:
Surveying works because light travels in straight lines (or so the
Physicists tell us).

No, actually, what they tell us is that it travels in straight
lines *unless* it's traveling through a non-uniform medium.
When the Channel Tunnel was being built, they had to correct the
laser-based alignment for the variations in air temperature
within the incomplete tunnel.

However, I doubt that this would be much of an issue for the
rather shorter distances between stations on Crossrail.
--
Mark Brader | "The speed of sound is considerably less than the
Toronto | speed of light -- that is why some people appear bright
| until you hear them talk."

My text in this article is in the public domain.