# How close to the magnetic poles will a compass be accurate?

So the Earth has a giant magnetic field we use to determine direction. The field roughly looks like a magnetic dipole, but because the field is generated by a turbulent liquid dynamo near the Earth's core, it's not a perfect dipole.

This leads me to believe that as you approach the magnetic north or south poles, your compass reading will become increasingly vague, and that there's probably a finite region near the pole where the compass direction rather randomly points some direction other than the pole itself.

Additionally, as you near the pole of a perfect dipole, the component of the magnetic field tangent to the surface approaches a magnitude of zero. Because real-world compasses have friction, imperfect magnets, and so forth, there's surely some minimum field strength required to accurately move the compass magnet.

Finally, I know that the pole has daily cycles as well as long-term movements. My guess is the daily cycles can be predicted with some kind of almanac or equation, and that travelers near the pole could receive periodic updates as to the center of the daily cycles so they could adjust their local declination accordingly. Still, there would be some kind of precision error here that might be significant.

Between the three effects above (and possibly others I don't know about), I'm guessing there's a real-world limit on how close you can get to the exact magnetic pole before a real-world compass can no longer be trusted.

How close can we get to the magnetic pole and still use a compass to determine direction?

To narrow the scope of the question, I'm thinking of a standard compass I can pick up at Walmart for like \$10 (say, this one). However, answers regarding high-end compasses and very low-end compasses (like, a magnetized needle through a cork floating in water) would also be appreciated.

Additionally, my question relates to humans using compasses for navigation, so if the answer is less than a few hundred meters, that's close enough to "right on the pole" for my purposes. At that point, you could simply look at the building or other landmark you're trying to find. Conversely, if the answer is several hundred miles, that would make it impossible to use traditional compass-based navigation between settlements near the magnetic pole.

• I didn't think the daily changes were predictable. I have seen aviation charts of northern Canada that have warnings about this issue, and I think that may be the major source of error. In that case it would be hard to define a precise area where compasses can not be trusted. Aug 17, 2017 at 3:28
• No scientific answer, but I gave up using magnetic compasses about 1000km from the magnetic pole. Three different, high quality, compasses showed over 90 degrees difference, so we used the GPS instead. Solar compasses used to be popular in polar regions, but difficult to use in the field when it's cold and windy and dust and snow all over. However, 2000km or even a bit less from the magnetic pole, a magnetic field compass works rather good, but huge declination that is changing enough over the years so one need to update the bearings from e.g. old reports. Aug 22, 2017 at 3:34