Abstract: There is provided a method, comprising: acquiring results of a set of Earth's magnetic field, EMF, measurements; acquiring motion data indicating the angular rotation that the mobile device experienced during each of a plurality of EMF measurement pairs, wherein each EMF measurement pair indicates a measured first and a second EMF value; determining a bias estimate of the magnetometer on the basis of the plurality of EMF measurement pairs and the corresponding motion data by finding the bias estimate that minimizes distances between the measured second EMF values and the measured first EMF values rotated about the bias estimate by the corresponding angular rotations; and determining a correction factor corresponding to the bias estimate and applying the correction factor to the magnetometer readings.