Magnetic declination is the angle between the local magnetic field and true geographic north. A compass points to magnetic north, which, unlike true north, is a constantly changing direction that varies at different points all over the globe. Knowing the magnetic declination of an area allows compass corrections so maps can be followed accurately.
The angle of declination varies all over the world and it constantly changes. This makes computerised resources the easiest and most accurate way to find the declination for a specific area.
The National Oceanographic and Atmospheric Administration (NOAA) has a declination calculator on its website. When you use the site to search for the declination, it will bring up the calculator. Input either the zip code or latitude and longitude of the area for which you need the declination. Click on the "Compute Declination" button to get your answer. This calculator will tell you not only the current declination but also by how much it is changing every year.
The lower left hand corner of USGS topographical maps has a diagram that shows the area's magnetic declination. The diagram consists of three lines: one with a star at the top, the other with the letters "MN" at the top and a third with the letters "GN" at the top. For finding the declination, you only need to concern yourself with the first two.
The line with the star on top represents true north. The line labelled MN shows the angle of magnetic north. Besides this line is a numeric set of degrees; this is the magnetic declination for the area. In other words if a compass is laid on top of a map with a 15 degree east declination, the needle will point 15 degrees east away from true geographic north.
An isogonic chart overlays the Earth's magnetic field onto outlines of a geographic area. Each of the lines representing the magnetic field is marked with its individual declinations. By finding an area using latitude and longitude on an isogonic chart you can find an approximate magnetic declination for that area. The USGS has these maps for download and printing at its website.
The diagram showing the magnetic declination on a topographic map also shows the year the declination was measured in. If the map isn't recent, make sure you get a new one. Or find out how much the declination has changed so you compensate in your navigation.