Magnetic sensing to provide geometry information

A system comprising:

• an electromagnetic spatial measurement apparatus comprising:
• a moveable sensor to provide a sensor signal in response to an electromagnetic field, the measurement apparatus providing first position data representing multiple positions of the moveable sensor in a given coordinate system;
• a plurality of stationary sensors that each provides respective sensor signals in response to the electromagnetic field, the measurement apparatus providing second position data based on a position of each of the plurality of sensors in the given coordinate system;
• a processor configured to:
• compute internal geometry data based on the first position data, the internal geometry data representing a plurality of locations in a three-dimensional space distributed across an anatomical surface within a patient's body;
• compute electrode geometry data based on the second position data, the electrode geometry data representing the location of each of a plurality of electrodes on an outer surface of the patient's body.