How are geodetic and tidal datums related?
9. How are the differences between terrestrial and tidal datums linked? Back to top 10. How great a distance from a tide station can I move and still have a valid tidal/terrestrial datum relation? In the strict sense, a tidal datum is applicable only at the location of the tide station. This is because tides and therefore tidal datums are affected by the gradient of the geoid. In order to validly extend a tidal datum over some horizontal distance, the geoid gradient must be negligible compared to the projected horizontal distance. This requires knowledge of the local geoid gradient, which can be determined by gravity measurements or establishing a second tide station. For example, if a particular tidal datum is known to be accurate to within one centimeter, and it is desired to maintain that degree of accuracy, then the greatest horizontal distance a tidal datum can be projected would be the geoid gradient times the distance to equal one centimeter. A cautionary note – some coastal loc
