Complexities of using Discrete Tidal Zoning for High Resolution Surveys in Shallow Water

Discrete tidal zoning is a methodology used by the National Ocean Service (NOS) to provide tide reducers for hydrographic surveys. Analyses of historical tide data, models, and other research are used to describe the tidal characteristics of a given survey area to generate co-tidal charts of co-range and co-phase lines. The number of zones for a particular survey depends upon the complexity of the tide in the area. Each zone is described by a range ratio and a time correction to a water level station in operation during the survey. Tide reducers are compiled by applying the appropriate time and range corrections to the sounding data relative to chart datum which is Mean Lower Low Water (MLLW). Shallow water areas typically have the most complex tidal characteristics where the amplitude of the tide reducers can be a significant percentage of the overall sounding depth and can dominate the overall error budget. In high resolution surveys using multi-beam technology, discrete tidal zoning introduces unwanted jumps when soundings cross from one zone to another. NOS is pursuing research into the application of smooth continuous functions for tide reducers to replace discrete tidal zoning. NOS is developing tools such as Tidal Constituent and Residual Interpolation (TCARI) and Kinematic Global Positioning System (GPS) for operational vertical control. However, these tools still require water level station installations prior to the survey for the development of tidal datum ties to geodetic datum using GPS and to map MLLW relative to the ellipsoid.