The purpose of finding out the magnetic declination of a particular area is so that compass corrections can be made to ensure that the compass is pointing to true north. Magnetic north is what the average compass usually points to, but this is not the same as true north. True north does not change, but the magnetic north does, depending on location. Magnetic declination is the difference in angle between the magnetic field of your current location and true north.
Visit the National Oceanographic and Atmospheric Administration website. The NOAA, which provides information dealing with geophysical data, has a calculator to determine the magnetic declination of your location.
Click in the search box located in the upper-right corner of the website. Type "magnetic declination" in the box and click "Go." This will bring up the calculator.
Input your location's Postcode in the calculator box. Click "Get Location" to retrieve your magnetic declination. This computation will not only give you the declination, but also tell you how much the magnetic field has changed over the past few years.
Input the latitude and longitude of your location. If you do not have the Postcode of your current location, you can also calculate the declination by using the latitude and longitude, day, month and year.