Acquiring ECG signals from electrodes positioned in a heart taken over a single heartbeat, selecting a morphology pattern within a window of interest around time of occurrence annotations for the signals; computing a weighted cross-correlation between each morphology pattern of the signals and a stored template morphology pattern, to generate a weighted correlation coefficient of a match between the morphology patterns of the acquired signals and the stored morphology pattern; iteratively changing a phase of the signals relative to the phase of the morphology pattern and repeating the step of generating the weighted correlation coefficient at each iteration; determining a maximum value of the weighted correlation coefficient based on the iterations; comparing the maximum value to a threshold; and when the maximum value exceeds the threshold, accepting the heartbeat as having been caused by the arrhythmia and incorporating a location of the arrhythmia into a local activation map.