1. A computer-implemented method, comprising: receiving a current reading from a magnetometer of a mobile device; identifying historical data of readings previously calibrated using the mobile device, wherein each previously-calibrated reading includes an associated bias offset; calculating multiple estimates of Earth's current magnetic field by subtracting bias offsets for the previously-calibrated readings from the current reading; determining a confidence of a match between at least one estimate of the multiple estimates of Earth's current magnetic field and historical data, where the confidence is determined at least in part on historical data including magnitudes of Earth's magnetic field for the previously-calibrated readings; and if the confidence of the match for the at least one estimate of Earth's current magnetic field exceeds a threshold confidence level, calibrating the magnetometer with historical calibration data; wherein the method is performed by one or more processors.