Lowest Astronomical Tide (LAT) (LAT)

The lowest tidal level predicted to occur under average meteorological conditions from any combination of astronomical influences.

LAT is widely used as chart datum because predicted astronomical tides should only rarely fall below it. It is not the lowest sea level physically possible: wind, atmospheric pressure and other non-astronomical effects can lower observed water below LAT. Chart datum is defined by the producing hydrographic authority and should be checked on the chart rather than assumed always to be LAT. Predicted height of tide is measured above that datum, while charted depths and drying heights use the same reference.