A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: David Fleming
Date: 2015 Jun 30, 21:07 -0700
Magnetic Fields are horizontal virtually nowhere on the earths surface. They have dip, ie point also in a vertical direction as well as horizontally.
When you speak of near power lines do you mean seeing interferrence from AC or DC power lines. It is likely that needle compasses do not have high frequency response so they would not be sensitive to 60 cycle- this is speculation needs to be verified.
Flux gate compasses use 10s of KHz signals to sense the magnetic field. Obviously their designers are aware of this so I would expect they filter and diesign with 60 cycle interference in mind, so who knows.