A system comprising: multiple sets of current injecting electrodes including a minimum of four current injecting electrodes which are configured for being arranged in a non-planar arrangement, the current injecting electrodes being configured to be secured to one or more stable locations relative to an organ in a patient's body, the multiple sets of current injecting electrodes comprising electrodes selected from the group consisting of current injecting electrodes that are mounted on a first catheter placed inside the body and body-surface electrodes; a second catheter that includes one or more measuring electrodes; a tracking system for tracking the positions of the measuring electrodes of the second catheter, wherein the tracking system is configured to use one of magnetic signals and injected currents for tracking; a third catheter that includes one or more additional electrodes that are configured to be positioned within the organ in the patient's body; and an electronic control system configured to: drive the current injecting electrodes to generate a field in an organ of a patient and to measure the field at each of the additional electrodes; a processing system configured to: model the field within the organ, wherein modeling the field includes determining expected signal measurements of the field inside the organ using a non-interpolation based calculation, the expected signal measurements being based on signals generated by the current injecting electrodes that are mounted on the first catheter placed inside the body or body-surface electrodes and measured by the measuring electrodes on the second catheter, the expected signal measurements of the field further being based on the positions of the measuring electrodes tracked by the tracking system; and determine a position of each of the one or more additional electrodes on the third catheter in the organ based on measurements of the field made by the additional electrodes and the determined expected signal measurements of the field. multiple sets of current injecting electrodes including a minimum of four current injecting electrodes which are configured for being arranged in a non-planar arrangement, the current injecting electrodes being configured to be secured to one or more stable locations relative to an organ in a patient's body, the multiple sets of current injecting electrodes comprising electrodes selected from the group consisting of current injecting electrodes that are mounted on a first catheter placed inside the body and body-surface electrodes; a second catheter that includes one or more measuring electrodes; a tracking system for tracking the positions of the measuring electrodes of the second catheter, wherein the tracking system is configured to use one of magnetic signals and injected currents for tracking; a third catheter that includes one or more additional electrodes that are configured to be positioned within the organ in the patient's body; and an electronic control system configured to: drive the current injecting electrodes to generate a field in an organ of a patient and to measure the field at each of the additional electrodes; a processing system configured to: model the field within the organ, wherein modeling the field includes determining expected signal measurements of the field inside the organ using a non-interpolation based calculation, the expected signal measurements being based on signals generated by the current injecting electrodes that are mounted on the first catheter placed inside the body or body-surface electrodes and measured by the measuring electrodes on the second catheter, the expected signal measurements of the field further being based on the positions of the measuring electrodes tracked by the tracking system; and determine a position of each of the one or more additional electrodes on the third catheter in the organ based on measurements of the field made by the additional electrodes and the determined expected signal measurements of the field. model the field within the organ, wherein modeling the field includes determining expected signal measurements of the field inside the organ using a non-interpolation based calculation, the expected signal measurements being based on signals generated by the current injecting electrodes that are mounted on the first catheter placed inside the body or body-surface electrodes and measured by the measuring electrodes on the second catheter, the expected signal measurements of the field further being based on the positions of the measuring electrodes tracked by the tracking system; and determine a position of each of the one or more additional electrodes on the third catheter in the organ based on measurements of the field made by the additional electrodes and the determined expected signal measurements of the field.