Aliasing

Aliasing is the misrepresentation of a high-frequency signal as a lower frequency because the sampling rate is too low to resolve it. The Nyquist criterion requires sampling at more than twice the highest frequency present; below that, energy folds into spurious low frequencies that cannot be separated after the fact. In tidal analysis, sparse or short records alias semidiurnal and diurnal constituents into each other and into long-period terms, corrupting harmonic constants; satellite altimeters with multi-day repeat cycles alias the tides into apparent low-frequency sea-level change. The fix is to sample fast enough or to design the record length so the constituents of interest separate cleanly.