10.7: Tide Prediction

As we have seen, the dynamic-tide theory must take into account many variables to predict tidal height at any given location at a specific time. The best predictions made by applying dynamic-tide theory closely approximate the actual tides. However, the predictions are not accurate enough for use by mariners, particularly in complex estuaries and bays. Therefore, tide predictions are made from tide observations. Tidal variations are continuously observed and recorded at many locations worldwide. Reasonably accurate tide predictions can be made from a tide record covering only a few months, but much longer records produce more accurate predictions.

Once the tide record is available, computers can perform harmonic analysis of the data to identify components of the complex wave. For example, harmonic analyses would distinguish the five component waves in Figure 9-13b from the complex wave record in Figure 9-13a. Harmonic analysis determines the wave heights and time lags (in relation to the moon’s orbital movement or other motions) of each of the many different tidal components (Table 10-1). More than 390 different tidal components have been identified. Once the timing, periodicity, and amplitude of each component have been determined for a particular location, computing the magnitude of each component for any future time is relatively easy. Simple addition of the components gives the tidal height at the future time and location.

Tide predictions made in this way are used extensively. They are the basis for the tide predictions published daily on many news and marine websites and the printed tide tables offered in bait-and-tackle stores, dive shops, and marinas. The NOAA Tides & Currents site also provides constantly updated, detailed tide information.

Satellite-based radar sensors accurate enough to measure tidal heights have allowed detailed mapping of the tides in the open-ocean areas where few tidal-height measurements had previously been made. Satellite-collected tidal-height data sets are steadily accumulating and will eventually enable extremely accurate computer models of the tides for most locations worldwide. 

Tidal currents vary locally, often in locations just a few tens or hundreds of meters apart, and predictions must be based on at least several months of current measurements at each specific location. Unfortunately, data for one location are inadequate to make predictions for other locations, especially within complex estuaries.