Calculating magnetic declination from a GPS (some actually display it, but not all):

1. Use the GPS Setup tools to set the GPS to True North - as opposed to Magnetic North.

2. Use the GPS to get the True North bearning to any waypoint. For example, the bearing from home to the nearby park is 262 degrees with the GPS set to use True North.

3. Use the GPS Setup tools to change the GPS from True North to Magnetic North.

4. Use the GPS to get the Magnetic North bearing to the very same waypoint (make sure you don't move the GPS). For example, the bearing from my home to the nearby park is 265 with the GPS set to use Magnetic North.

5. Subtract the Magnetic North Bearing from the True North Bearing to get the magnetic decliantion. In equation form this is

MagDeclination = TrueNbearing - MagNbearing

For my example, (262 - 265) = -3.

6. If the magnetic declination is negative, this means the needle points to the west of true north. In my case the needle points 3 degrees west of true north.

If the magnetic declination is positive, this means the needle points to the east of true north.

In the U.S. the magnetic declination zero-line (actually called the agonic line) pretty much follows the Mississippi River.

EAST of the Mississippi River magnetic north is WEST of true north. This is NEGATIVE declination since magnetic north is -x degrees from true north. It is also called WEST declination.

WEST of the Mississippi River magnetic north is EAST of true north. This is POSITIVE declination since magnetic north is +x degrees from true north. It is also called EAST declination.

You may have heard of the phrase "West is Best and East is Least". The West/East direction refers to the direction of the declination, not the region of the U.S., and whether you add or subtract to make your non-declination-adjusted compass point to the right place. Best means you need to add and Least means you have to subtract.