A calibration/surgical tool includes an electromagnetic sensor array of two or more electromagnetic sensors in a known geometrical configuration. Electromagnetic tracking errors are characterized by a mapping of pre-operative absolute and relative errors based on a movement of a calibrated calibration/surgical tool through a pre-operative electromagnetic field. Using statistical mapping, a desired absolute error field is measured either in the clinic as the part of daily quality control checks, or before the patient comes in or in vivo. A resulting error field may be displayed to the physician to provide clear visual feedback about measurement confidence or reliability of localization estimates of the absolute errors in electromagnetic tracking.