So, just what could have caused a 13° degree error?  What can the backcountry traveler do?

Remember that the correct operation of the compass is dependent on the action of the magnetic needle to guide the hiker through the backcountry. A lot of items in a pack and clothing can affect the needle. Most understand that ferrous objects such as a belt buckle, and car keys will deflect the magnetic needle. Still, take a good look at what is in a pack. The batteries from the GPS receiver and a flash light may cause a compass needle to move too.

High tension power lines and a vehicle’s electrical system may also cause a magnetic needle to deflect. Moving a few steps from the vehicle should be sufficient. One may have to move over one hundred feet from the power lines to avoid deflection. (GPS Made Easy, Michael Ferguson.)

Some locations will have a high concentration of iron near the surface. This is known as “local attraction.”  Such concentrations will cause the needle to move too. Unlike declination, moving away from the immediate area may cause the deflection to stop. The local Bureau of Land Management or Forest Service Office should be able to identify areas affected by local attraction.

It is possible for the magnetic needle to lose its polarity. This is a function of time and manufacture.  With research, one can learn how to restore the magnetism. That said, with the modern liquid filled compass this is probably more trouble than it is worth.  Occasionally, check the alignment of the compass. In the small town where I live, residential streets are aligned true north and south.  Standing on the curb on such a street provides a quick verification of how the compass is working.  To me verification means that the compass direction will mirror that of the street; if the street tracks true north then the adjusted compass should provide a bearing to true north.