18 Applications: Magnetic Declination

Elizabeth Johnson

Learning Objectives

Students will be able to:

  • Describe why north on a compass (which points to magnetic north) does not necessarily align with geographic north.
  • Determine the magnetic declination and direction using a map key.

Earth’s Magnetic Field and Migrating Magnetic North

The video above provides an overview of Earth’s magnetic field, and how it changes through time.

Determining Magnetic Declination on a Map

If you are trying to navigate using a traditional topographic quadrangle map, it is necessary to set the declination of your compass.  The declination is the difference between the geographic north direction and the magnetic north direction at the time the map was made, expressed in degrees and a direction (east or west).

This figure shows the difference between geographic north (the rotational axis extends from geographic north to geographic south poles), and magnetic north, the direction a compass needle points. Topographic maps include arrows that show the direction to true north (marked with a star) and the direction to magnetic north (marked as MN). Depending upon the location, magnetic north could appear to be to the west of geographic north, as in example A, or to the east of geographic north, as in example B.

Examples

This magnetic declination information was taken from a topographic map in Tillamook, Oregon:

In this example, magnetic north is east of geographic north, by an angle of 15 degrees, 15 minutes.

The next magnetic declination was taken from a topographic map of Yonkers, NY:

What is the declination direction and angle at this location?

 

 

License

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Physical Geology Laboratory by Elizabeth Johnson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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