A method implemented by a computing device for localizing an activation site of origin to one of a plurality of segments of the heart comprising: recording a plurality of simultaneously recorded electrocardiographic signals; comparing a body surface potential map (BSPM) of interest to each BSPM template of a pre-determined set of BSPM templates, the set comprising one BSPM template for each pre-defined segment of the heart, wherein the BSPM of interest is calculated from the plurality of simultaneously recorded ECG signals and wherein comparing comprises, for each BSPM template of the set of BSPM templates: retrieving a BSPM template from memory; and calculating one or more comparison metrics for the BSPM template retrieved as compared to the BSPM of interest, wherein each of the BSPM templates was generated by averaging BSPMs of all pacing sites within the associated segment of the heart from a collection of BSPMs stored in memory; identifying the BSPM template that most closely resembles the BSPM of interest using the one or more comparison metrics calculated in order to identify the segment of the heart in which the activation site of origin is likely to be located; wherein each BSPM template of the set of BSPM templates represents a pre-determined number of ECG signals, wherein the BSPMs of interest was calculated from a different number of ECG signals than the pre-determined number and further wherein calculation of the BSPM of interest comprises interpolating between the different number of ECG signals by application of regression coefficients pre-derived from a collection of BSPMs each representing the pre-determined number of ECG signals. recording a plurality of simultaneously recorded electrocardiographic signals; comparing a body surface potential map (BSPM) of interest to each BSPM template of a pre-determined set of BSPM templates, the set comprising one BSPM template for each pre-defined segment of the heart, wherein the BSPM of interest is calculated from the plurality of simultaneously recorded ECG signals and wherein comparing comprises, for each BSPM template of the set of BSPM templates: retrieving a BSPM template from memory; and calculating one or more comparison metrics for the BSPM template retrieved as compared to the BSPM of interest, retrieving a BSPM template from memory; and calculating one or more comparison metrics for the BSPM template retrieved as compared to the BSPM of interest, wherein each of the BSPM templates was generated by averaging BSPMs of all pacing sites within the associated segment of the heart from a collection of BSPMs stored in memory; identifying the BSPM template that most closely resembles the BSPM of interest using the one or more comparison metrics calculated in order to identify the segment of the heart in which the activation site of origin is likely to be located; wherein each BSPM template of the set of BSPM templates represents a pre-determined number of ECG signals, wherein the BSPMs of interest was calculated from a different number of ECG signals than the pre-determined number and further wherein calculation of the BSPM of interest comprises interpolating between the different number of ECG signals by application of regression coefficients pre-derived from a collection of BSPMs each representing the pre-determined number of ECG signals.