Tunnel boring to begin from the Royal Oak portal heading eastwards under
central London.

Pictorial:
http://www.bbc.co.uk/news/uk-17365934

Video:
http://www.bbc.co.uk/news/uk-england-london-17356912

So, it's finally really happening.

On Thu, 15 Mar 2012 10:53:04 +0000
Mizter T wrote:
Tunnel boring to begin from the Royal Oak portal heading eastwards under
central London.

Pictorial:
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.

B2003

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 more interested in them apparently using GPS 38m underground!
--
Roland Perry

On Mar 15, 11:11*am, Roland Perry wrote:
In message , at 10:55:45 on Thu, 15 Mar
2012, 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 more interested in them apparently using GPS 38m underground!


There distance between the tunnels may not be constant.

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

The Twenty-first Century is starting with this much needed and really
exciting rail project. When the economic recovery comes we see more
projects like this. London badly needs them.

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.

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

On 2012\03\15 21:03, Denis McMahon wrote:

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.

GPS is not accurate enough for laying a surface railway, never mind an
underground one.

In article , Basil Jet
scribeth thus
On 2012\03\15 21:03, Denis McMahon wrote:

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.

GPS is not accurate enough for laying a surface railway,



Really?.
never mind an
underground one.

How could you receive the GPS signals underground anyway?..
--
Tony Sayer

In message , at 00:48:37 on
Fri, 16 Mar 2012, Basil Jet remarked:
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.

GPS is not accurate enough for laying a surface railway, never mind an
underground one.

It's accurate to about 10cm (if you employ differential GPS) which
probably good enough for avoiding a 10m obstacle 40m underground.

Obviously, you don't use it to measure the distance between the rails
when you are laying the track.
--
Roland Perry