Lesson 5: Mobile Mapping Fundamentals

Nicolas MalloyAuthor: Nicolas R. Malloy


5.5 Magnetic Declination


When using a compass you can compensate for this by setting the magnetic declination. Magnetic declination is the angular deviation between true north and magnetic north. Magnetic declination changes annually, so it is important to know the current declination and to make sure your compass is set correctly.

Isogonic map of the continental United States as of 2015. Red lines on the map indicate positive declination (east). Blue lines on the map indicate negative declination (west). The green line running through the Midwest is the agonic line, which has 0° declination. This is a screenshot taken from the NOAA Declination Map Viewer.

Isogonic map of the continental United States as of 2015. Red lines on the map indicate positive declination (east). Blue lines on the map indicate negative declination (west). The green line running through the Midwest is the agonic line, which has 0° declination. This is a screenshot taken from the NOAA Declination Map Viewer.

You can determine your local magnetic declination by using an isogonic map. An isogonic map shows a series of isogonic lines, lines indicating the magnetic declination along those lines as well as the rate of change over time. On an isogonic map, the a line where magnetic north and true north are the same, 0° declination, is called an agonic line.

You can determine your local magnetic declination by using an isogonic map. An isogonic map shows a series of isogonic lines, lines indicating the magnetic declination along those lines as well as the rate of change over time. On an isogonic map, the line where magnetic north and true north are the same, 0° declination, is called an agonic line.
You can also determine your local magnetic declination using a map for your region. The USGS Quadrangles have the declination printed near the bottom of the map in the form of a declination diagram.

If too much time has passed from this date, the declination on either the USGS quadrangle or the isogonic map will no longer be accurate. Luckily, NOAA provides an online calculator to determine your local magnetic declination.

You can look up your local declination using the NOAA Magnetic Field Calculator .

You can look up your local declination using the NOAA Magnetic Field Calculator .