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