A method of correcting an inconsistent physical location of a single back patch, the method comprising: affixing front patches to the front of a patient and back patches to the back of the patient, each of the front patches and the back patches comprising magnetic field sensors configured to measure magnetic fields produced by external field generators and convey the magnetic field measurements as position-dependent signals; receiving first position-dependent signals from the front and the back patches at a first time; computing first location coordinates in a three-dimensional coordinate system comprising an X-axis, a Y-axis, and a Z-axis for the front and the back patches using the first position-dependent signals; identifying, using the first location coordinates of the back patches, first spatial relationships between the back patches as a first rigid body; receiving second position-dependent signals from the front and the back patches at a second time subsequent to the first time; computing second location coordinates in the coordinate system for the front and the back patches using the second position-dependent signals; identifying, using the second location coordinates of the back patches, second spatial relationships between the back patches as a second rigid body; detecting a discrepancy between the first and the second spatial relationships by detecting a change of location of a single back patch relative to the back patches other than the single back patch; wherein the method, upon detecting a discrepancy, further comprises: identifying third spatial relationships between the back and the front patches, using the first location coordinates of the front and the back patches, as a third rigid body; identifying fourth spatial relationships between the front and the back patches other than the single back patch, using the second location coordinates of the back patches other than the single back patch and the front patches, as a fourth rigid body; calculating, based on the third and the fourth spatial relationships, a location correction for the single back patch; and applying the calculated location correction to the second location coordinates of the single back patch, thereby determining corrected second location coordinates of the single back patch. affixing front patches to the front of a patient and back patches to the back of the patient, each of the front patches and the back patches comprising magnetic field sensors configured to measure magnetic fields produced by external field generators and convey the magnetic field measurements as position-dependent signals; receiving first position-dependent signals from the front and the back patches at a first time; computing first location coordinates in a three-dimensional coordinate system comprising an X-axis, a Y-axis, and a Z-axis for the front and the back patches using the first position-dependent signals; identifying, using the first location coordinates of the back patches, first spatial relationships between the back patches as a first rigid body; receiving second position-dependent signals from the front and the back patches at a second time subsequent to the first time; computing second location coordinates in the coordinate system for the front and the back patches using the second position-dependent signals; identifying, using the second location coordinates of the back patches, second spatial relationships between the back patches as a second rigid body; detecting a discrepancy between the first and the second spatial relationships by detecting a change of location of a single back patch relative to the back patches other than the single back patch; wherein the method, upon detecting a discrepancy, further comprises: identifying third spatial relationships between the back and the front patches, using the first location coordinates of the front and the back patches, as a third rigid body; identifying fourth spatial relationships between the front and the back patches other than the single back patch, using the second location coordinates of the back patches other than the single back patch and the front patches, as a fourth rigid body; calculating, based on the third and the fourth spatial relationships, a location correction for the single back patch; and applying the calculated location correction to the second location coordinates of the single back patch, thereby determining corrected second location coordinates of the single back patch. identifying third spatial relationships between the back and the front patches, using the first location coordinates of the front and the back patches, as a third rigid body; identifying fourth spatial relationships between the front and the back patches other than the single back patch, using the second location coordinates of the back patches other than the single back patch and the front patches, as a fourth rigid body; calculating, based on the third and the fourth spatial relationships, a location correction for the single back patch; and applying the calculated location correction to the second location coordinates of the single back patch, thereby determining corrected second location coordinates of the single back patch.