1. A method for determining location using magnetic field readings, the method comprising: obtaining, by one or more computing devices, a reading of a magnetic field collected by a magnetometer; determining, by the one or more computing devices, a plurality of characteristics of a magnetic field based at least in part on the reading, wherein one or more of the plurality of characteristics describe a component of the magnetic field that exhibits a frequency at which electrical power is transmitted along one or more power lines; comparing, by the one or more computing devices, the plurality of characteristics to a plurality of expected values associated with each of a plurality of locations, wherein the plurality of expected values for each location are provided by a map of the magnetic field.