The 3DM-GX3 is calibrated at the factory. The user is also provided with Hard and Soft Iron Calibration software to field calibrate the GX3 in situ. This video further describes the function: http://www.microstrain.com/video/hard-soft-iron-calibration
Important note: Hard and soft iron calibration is not required for the 3DM-GX3-15 and the 3DM-GX315 OEM. These units do not contain magnetometers.
From time to time the 3DM-GX3 may need recalibration. For example, as a result of coming in contact with magnetic influences (magnets, motors, etc.), residual magnetism may be picked up by the on-board components which will alter the calibration. Another example: the 3DM-GX3 receives a severe shock, slightly altering the position of the circuit boards in relation to the enclosure, again altering the calibration. In these cases the unit should be returned to the factory for recalibration.