A computer software program is described that maps the electrical activity of a patient's heart. The software program utilizes inputs from electrodes contained within a heart chamber. Inputs from the electrodes cause the program to calculate the heart chamber volume, and then to determine the position of the electrodes within the heart chamber. The program then utilizes inputs from the electrodes to calculate the three-dimensional volumetric electrical field distribution of said heart chamber. This calculation is accomplished using a spherical harmonic series expression. Finally, the computer program displays the electrical field distribution of the heart chamber.