Cardiac activation timing is mapped using a catheter-mounted roving electrode instead of a fixed (e.g., coronary sinus) electrode. The roving electrode is used to measure an initial electrophysiological signal at an initial cardiac location as a reference signal, which is defined as a reference signal. Local activation time(s) for other cardiac location(s), also measured using the catheter-mounted roving electrode, are determined relative to the reference signal. The stability of the reference signal can be monitored, such as by comparing activation rates or cycle lengths between an instantaneously-measured electrophysiological signal and the initial electrophysiological signal. Smaller differences between the two (e.g., less than about 5%) can be compensated for, while larger differences can result in redefining the reference signal.
