An apparatus for sensing, comprising: an array of patches fixed to a surface of a body of a subject, the patches including respective electrodes in contact with the surface, and at least one of the patches including a patch sensor configured to output a signal in response to a magnetic field applied to the body; a medical probe, wherein the distal end of the medical probe is configured to be inserted into a body cavity of the subject, and wherein the medical probe comprises an electrode positioned at the distal end of the medical probe; and a control console configured: at a first time, to process the signal so as to compute first field-based location coordinates of the at least one of the patches, and to compute first impedance-based location coordinates of the at least one of the patches by delivering an electrical current to the electrode inside the body and measuring an impedance between the at least one of the patches and the electrode inside the body, at a second time, subsequent to the first time, to process the signal so as to compute second field-based location coordinates of the at least one of the patches, and to compute second impedance-based location coordinates of the at least one of the patches by delivering the electrical current to the electrode inside the body and measuring the impedance between the at least one of the patches and the electrode inside the body, to compute a first relation between the first field-based location coordinates and the first impedance-based location coordinates, to compute a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, to compute a field-based location coordinate correction responsively to the difference, and to apply the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field. an array of patches fixed to a surface of a body of a subject, the patches including respective electrodes in contact with the surface, and at least one of the patches including a patch sensor configured to output a signal in response to a magnetic field applied to the body; a medical probe, wherein the distal end of the medical probe is configured to be inserted into a body cavity of the subject, and wherein the medical probe comprises an electrode positioned at the distal end of the medical probe; and a control console configured: at a first time, to process the signal so as to compute first field-based location coordinates of the at least one of the patches, and to compute first impedance-based location coordinates of the at least one of the patches by delivering an electrical current to the electrode inside the body and measuring an impedance between the at least one of the patches and the electrode inside the body, at a second time, subsequent to the first time, to process the signal so as to compute second field-based location coordinates of the at least one of the patches, and to compute second impedance-based location coordinates of the at least one of the patches by delivering the electrical current to the electrode inside the body and measuring the impedance between the at least one of the patches and the electrode inside the body, to compute a first relation between the first field-based location coordinates and the first impedance-based location coordinates, to compute a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, to compute a field-based location coordinate correction responsively to the difference, and to apply the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field. at a first time, to process the signal so as to compute first field-based location coordinates of the at least one of the patches, and to compute first impedance-based location coordinates of the at least one of the patches by delivering an electrical current to the electrode inside the body and measuring an impedance between the at least one of the patches and the electrode inside the body, at a second time, subsequent to the first time, to process the signal so as to compute second field-based location coordinates of the at least one of the patches, and to compute second impedance-based location coordinates of the at least one of the patches by delivering the electrical current to the electrode inside the body and measuring the impedance between the at least one of the patches and the electrode inside the body, to compute a first relation between the first field-based location coordinates and the first impedance-based location coordinates, to compute a second relation between the second field-based location coordinates and the second impedance-based location coordinates, when there is a difference between the second relation and the first relation, to compute a field-based location coordinate correction responsively to the difference, and to apply the field-based location coordinate correction in tracking a position of a magnetic tracking sensor inside the body, based on signals received from the magnetic tracking sensor in response to the applied magnetic field.