Compass problem

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Walrus

Guest
Recently I tried to do a survey underground using my simple Sylva compass. It kept pointing the wrong way; it was following the line of buried iron rails.

Is there a cheap way of shielding the compass to prevent or minimalise the magnetic interference? Or do I have to spring for a specially shielded one?
 
graham

graham

New member
If you shield a compass from the magnetic field surrounding it, then how is it going to point north?  :confused:

Your problem is not your compass, which I suspect is working perfectly well, but the fact that the buried iron is affecting the magnetic field, the thing that the compass is reading.

The only answer if the problem is that bad is to carry out a non-magnetic survey, using a theodolite. These are very expensive.

 
D

darkplaces

Guest
er or move away from the rails yet still pointing the required direction, if that can be done.
 
W

Walrus

Guest
Tried - the passage was too narrow. Theodolite is waaay beyond the budget so I'll have to go for compass reading where it will work & a bit of educated guesswork.
 
Les W

Les W

Active member
Walrus said:
Tried - the passage was too narrow. Theodolite is waaay beyond the budget so I'll have to go for compass reading where it will work & a bit of educated guesswork.
Click to expand...

It doesn't have to be a theodolite, it depends on the desired accuracy, you could sight angles with a protractor for the legs where the iron is.

Or try back sights away from the rails if the section is short. Or a combination of techniques.
 
D

darkplaces

Guest
Les W said:
...back sights away from the rails if the section is short. Or a combination of techniques.
Click to expand...
We seam to have managed to get the data we wanted  using back-bearings and a couple of people taking readings from different compasses from different points and took the mean. Cracking little survey to be released once we have finished  :halo:
 
P

Peter Burgess

New member
Glad you fixed the problem. If you can get measurements into the affected passage from a number of other points where the problem doesn't exist, and can close round a loop onto all the other points, then the affected passage can be added by just joining up the dots. Not easy to explain without a piece of paper and a pencil, but as you have sorted your problem out, it doesn't matter really.

 
gus horsley

gus horsley

New member
I've surveyed loads of mines with bits of ironmongery in them - rails, compressor pipes, skips, etc.  I never had a problem with my old Silva compass being deflected.  The only thing that baffled me was that, despite many turns in the passage, the survey showed a straight line.
 
shotlighter

shotlighter

Active member
gus horsley said:
I've surveyed loads of mines with bits of ironmongery in them - rails, compressor pipes, skips, etc.  I never had a problem with my old Silva compass being deflected.  The only thing that baffled me was that, despite many turns in the passage, the survey showed a straight line.
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Not just me then! Some years ago we dug a cave in the peak called Lumb Hole, which seemed to twist & turn all over the place. When ex Crewie (now SWCC) Liam Keely surveyed it for us, you guessed it - a straight line!
Daftest compass problem though must be the survey I have of a Peak mine done in the 50's. It's suburb apart from one minor problem, the guy (who will remain nameless) got North & South reversed!!!
 
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