Suunto It appears the Suunto and Casio compass watch has a function for setting the magnetic variation but it's not automatic. So you would need to know your local variation setting. Pretty cool but nothing beats the potential for accuracy as a good old magnetic needle.
Magnetic variation, or declination is not only changing constantly, it varies many degrees from region to region. The National Geophysical Data Center has a web site where you can find the exact magnetic declination for different regions.
To Find Variation A needle pointing towards north is "Magnetic North". In order to use your compass for accurate locating, you need "True North". The difference between true north and magnetic north is the magnetic variation, which may vary to 25 degrees.
When you buy a pocket compass, many have a rotating needle rings that will allow one to set in the magnetic declination and therefore refer to a true north meridian, or true north. These compasses will work on either north or southern hemisphere. The leveling of the needle may be interrupted by local magnetic attraction (lode bearing bedrock is one source) but your compass has to be pretty precise to be adversely effected.
BTW....in a pinch you can determine your local magnetic declination using a shadow stick. More on that later.....