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