In the preceding discussions of the tide-generating forces, the theoretical equilibrium tide produced, and factors causing variations, it has been emphasized that the tides actually observed differ appreciably from the idealized, equilibrium tide. Nevertheless, because the tides are produced essentially by astronomical forces of harmonic nature, a definite relationship exists between the tide-generating forces and the observed tides, and a factor of predictability is possible.
Because of the numerous uncertain and, in some cases, completely unknown factors of local control mentioned above, it is not feasible to predict tides purely from a knowledge of the positions and movements of the moon and sun obtained from astronomical tables. A partially empirical approach based upon actual observations of tides in many areas over an extended period of time is necessary. To achieve maximum accuracy in prediction, a series of tidal observations at one location ranging over at least a full 18.6-year tidal cycle is required. Within this period, all significant astronomical modifications of tides will occur.
Responsibility for computing and tabulating - for any day in the year - the times, heights, and ranges of the tides - as well as the movement of tidal currents - in various parts of the world is vested in appropriate governmental agencies which devote both theoretical and practical effort to this task. The resulting predictions are based in large part upon actual observations of tidal heights made throughout a network of selected observing stations.
The National Ocean Survey, a component of the National Oceanic and Atmospheric Administration of the U.S. Department of Commerce, maintains for this purpose a continuous control network of approximately 140 tide gages which are located along the coasts and within the major embayments of the United States, and it possessions, and the United Nations Trust Territories under its jurisdiction. Temporary secondary stations are also occupied in order to increase the effective coverage of the control network. Predictions of the times and heights of high and low water are prepared by the National Ocean Survey for a large number of stations in the United States and its possessions as well as foreign countries and United Nations Trust Territories.