Reference Structure for a Tracking System

A method of determining a position of an instrument within a volume of patient with a reference member, comprising:

• connecting to the patient a set of axis electrodes;
• injecting a current into the patient with the set of axis electrodes;
• positioning a first reference electrode on the patient at a first reference position;
• evaluating an electrical property based on input of the first reference electrode including measuring a voltage drop of a patient-to-axis electrode interface;
• positioning an instrument with a position element into the patient;
• evaluating the electrical property within the patient based at least in part on sensing with the position element;
• and
• determining a position of the instrument based on the evaluated electrical property.

A method of determining the position of an instrument within a volume of a patient, comprising:

• positioning at least three sets of electrodes on the patient;
• driving the at least three sets of electrodes to inject a current along three substantially orthogonal axes into the volume of the patient;
• fixedly positioning a first reference electrode on a ventral surface of the patient at a first reference position;
• sensing a voltage based on the current injected into the volume of the patient;
• and
• automatically determining a reference electrical property at the first reference position.