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

In message , at 10:25:02 on Fri, 16 Mar
2012, Roland Perry remarked:
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.

With an implied "apart from the fact it doesn't work under ground", of
course.

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

--
Roland Perry

Basil Jet wrote:

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.


Differential GPS is. This isn't your car's satnav, nor your hand-held
Garmin that you take when hiking, it is an extremely precise method of
establishing position with great accuracy using highly sophisticated
corrections of GPS satellite transmissions.

It also takes time to do; your car satnav or hand held GPS receiver
gives you a near instant fix to within a few metres, but differential
GPS takes hours* to give an accuracy of millimetres.

But GPS isn't much use down a tunnel. You can establish precise
positions at the shafts using GPS, but you need traditional methods
from then on in.


[*When I was last personally involved in the late 1990s, it took a
couple of days. I am assuming it has gotten quicker in the last
fourteen or fifteen years.]

On 16/03/12 12:22, Bruce wrote:
It also takes time to do; your car satnav or hand held GPS receiver
gives you a near instant fix to within a few metres, but differential
GPS takes hours* to give an accuracy of millimetres.

That's not differential GPS. DGPS is just as fast as normal GPS and uses
a secondary transmission of local GPS error derived from a GPS receiver
at a known position. It's good to about 10cm.

Higher degrees of accuracy come from techniques like long term averaging
or carrier phase tracking.

Ian