On Thu, 15 Mar 2012 11:11:40 +0000, Roland Perry wrote:
In message , at 10:55:45 on Thu, 15 Mar
2012, d remarked:
http://www.bbc.co.uk/news/uk-17365934
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 fairly sure the tunnel segments are up to the job, they're probably
similar to the chunnel ones, or even the HS1 ones.
I'm more interested in them apparently using GPS 38m underground!
I expect that's a simple throw away line that means rather less than it
sounds. I expect that "navigation" is going to be driven by laser
sightings out of the rear of the machines, along the tunnel bores using
intermediate survey points, to a datum point outside the tunnel bore.
The datum point might be fixed by gps, although for a fixed point where
absolute accuracy is needed, I suspect that it will be physically
surveyed. Although the co-ordinates of various surveying points will be
known, the fact that gps systems can use / display such co-ordinates
doesn't mean that other systems that use such co-ordinates are gps, which
is what I suspect that someone failed to understand when writing that.
I wouldn't trust a gps derived position underground even if I could
receive the signals - you don't know how much bouncing about it's done
getting through the soil, pipes, rocks of various types, cables etc above
you, and every signal bounce is a loss of accuracy.
Rgds
Denis McMahon