A system comprising: an electromagnetic spatial measurement apparatus comprising: a moveable sensor to provide a sensor signal in response to an electromagnetic field, the electromagnetic spatial measurement apparatus providing first position data representing multiple positions of the moveable sensor in a given coordinate system; a plurality of stationary sensors configured to be positioned on an outer surface of a patient's body that each provides respective sensor signals in response to the electromagnetic field, the electromagnetic spatial measurement apparatus providing second position data based on a position of each of the plurality of stationary sensors in the given coordinate system; a plurality of electrodes configured to be positioned on the outer surface of the patient's body to sense electrical activity from locations on the outer surface of the patient's body; 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 the patient's body; compute electrode geometry data based on the second position data, the electrode geometry data representing the location of each of the plurality of electrodes on the outer surface of the patient's body; and reconstruct cardiac electrical activity onto a cardiac envelope based on the sensed electrical activity from the outer surface of the patient's body, the internal geometry data and the electrode geometry data. an electromagnetic spatial measurement apparatus comprising: a moveable sensor to provide a sensor signal in response to an electromagnetic field, the electromagnetic spatial measurement apparatus providing first position data representing multiple positions of the moveable sensor in a given coordinate system; a plurality of stationary sensors configured to be positioned on an outer surface of a patient's body that each provides respective sensor signals in response to the electromagnetic field, the electromagnetic spatial measurement apparatus providing second position data based on a position of each of the plurality of stationary sensors in the given coordinate system; a plurality of electrodes configured to be positioned on the outer surface of the patient's body to sense electrical activity from locations on the outer surface of the patient's body; a moveable sensor to provide a sensor signal in response to an electromagnetic field, the electromagnetic spatial measurement apparatus providing first position data representing multiple positions of the moveable sensor in a given coordinate system; a plurality of stationary sensors configured to be positioned on an outer surface of a patient's body that each provides respective sensor signals in response to the electromagnetic field, the electromagnetic spatial measurement apparatus providing second position data based on a position of each of the plurality of stationary sensors in the given coordinate system; a plurality of electrodes configured to be positioned on the outer surface of the patient's body to sense electrical activity from locations on the outer surface of the patient's body; 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 the patient's body; compute electrode geometry data based on the second position data, the electrode geometry data representing the location of each of the plurality of electrodes on the outer surface of the patient's body; and reconstruct cardiac electrical activity onto a cardiac envelope based on the sensed electrical activity from the outer surface of the patient's body, the internal geometry data and the electrode geometry data. 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 the patient's body; compute electrode geometry data based on the second position data, the electrode geometry data representing the location of each of the plurality of electrodes on the outer surface of the patient's body; and reconstruct cardiac electrical activity onto a cardiac envelope based on the sensed electrical activity from the outer surface of the patient's body, the internal geometry data and the electrode geometry data.