Systems and techniques for tracking the geographic location. In one aspect, a method includes receiving timed magnetic field information descriptive of a magnetic field to which a magnetometer was subject, the magnetic field information having been collected while the magnetometer was associated with an animal, receiving timed longitude information descriptive of a longitude position of the animal, the longitude information having been collected while the magnetometer was associated with the animal, and matching the longitude information that is relevant to a first time to the magnetic field information that is relevant to the first time to determine a geographic location of the animal at the first time.