In Hydrographic Surveying, soundings are reduced to a chart datum established at a reference gauge station from a long period of tidal observations. Unfortunately, due to the variations in tidal characteristics from place to place, soundings can only be reduced to the chart datum within the vicinity of the gauge station. As we move away from the gauge station, it becomes necessary to obtain new information on the tidal characteristics and apply necessary corrections to the chart datum to obtain an appropriate sounding datum for reducing the soundings. To reduce soundings means to subtract the heights of ride, at the sounding locations and at the times of soundings, from the depths sounded to obtain the depths referenced to the chosen datum. Manual reduct6ion of soundings is a tedious aspect of the field hydrographer’s list of chores. There have been some attempts to automate the tidal reductions using digitized cotidal charts. The objective of this work has been to develop alternative approaches to automated tidal reductions, namely, using analytical cotidal models. The range ratio and time lag fields have been approaximated by surfaces described by two dimensional algebraic polynomials (Pn(ɸ, λ)). The observed time series at a reference station has been approximated by one dimensional trigonometric polynomial. With the coefficients of these Polynomials stored in the computer, the range ratio and the time lag at any point (ɸ[subscript i], λ[subscript i]) in the area can readily be predicted and the height of tide at the point and at time I can be predicted from the predicted height of ride at the reference station. Test computation, using data from the ‘Canadian Tides and Current Tables. 1978’ for the Bay of Fundy have been done. It has been shown that the water level (h) at a location (ɸ[subscript i], λ[subscript i]) can be produced with a standard deviation ([superscript 15] h [subscript i]) of 0.5 m or better.
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