An apparatus comprising: a processor; and a memory including computer program code, the memory and the computer program code configured to, with the processor, cause the apparatus to perform at least the following: determine whether or not recalibration is required of a magnetometer configured to compensate for hard-iron and soft-iron effects by determining whether a plurality of magnetometer readings received from the magnetometer is consistent with: a scaled pre-calibrated matrix describing the soft-iron effect for at least one scaling factor of the pre-calibrated matrix; and a pre-calibrated vector describing the hard-iron effect.