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.]