Compass errors

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For my sins I seem to spend quite a lot of time playing with other people's survey data as well as my own.

Over the weekend I have been having an interesting exchange about compass errors. We found a discrepancy between two surveys of the same cave, done thirty years apart, of up to 14 degrees. Now, it is probable that 4 degrees of that is down to magnetic deviation, but that might possibly leave a compass error of ten degrees.

This was no surprise to me, as much of the data I have used has had anything up to 12 degrees of correction needed, though much of that was Mag Dev, in Ireland where it's bigger than round here, but was found to be alarming by others.

So my question is: what sort of compass errors have people had to deal with?

Peter Burgess:
I have found places with anything up to 20 degrees variance, but these were not in comparison with other surveys, just errors that messed up loop closure, which were easily detected by comparing fore and back bearings. The cause was almost always hidden steelwork, or live HT cables close by! That said, I have found significant differences between two people measuring the same bearing, possibly caused by things like steel spectacle frames or steelwork in or around the helmet. That could explain why one of 'your' surveys is consistently at variance with another if one was made by someone with a steel plate in his head.

I'm not talking about unusual situations or blunders, Pete, but about systematic errors on a compass.

Ed W:

I use fore and backsights for a lot of my surveying, and this usually means callibrating two person/instrument combinations.  I don't have any data in front of me, but differences of 1-2 degrees between people with different instruments seem to be the norm.  I would guess that this is part person related and part instrument related.  I have seen cases of up to 3.5 degrees.  In most cases these differences are repeatable with the same person/instrument combination. 

I have never seen a systematic error of 10 degrees, though I guess it is possible.  However, if the difference in bearing is consistent with historical data, then I don't see why you shouldn't just rotate the old data to fit, if someone wants to argue with you they can always measure it themselves!

Hope this is useful,


Peter Burgess:
How can you tell? Anyway you asked "what sort of compass errors have people had to deal with?" and I provided an answer. I won't bother next time. I have seen two surveys, both seemingly consistent in layout but about 30 degrees out. I have no idea why this was so, but I suspect it was a draughting error not a surveying error.


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