LAT (Lowest Astronomical Tide)

Lowest Astronomical Tide is the lowest tidal level that can be predicted to occur under average meteorological conditions and any combination of astronomical conditions, computed from the harmonic tidal constituents over a long period. The IHO adopted LAT as the standard chart datum so that the depth of water almost never falls below the charted sounding, giving a conservative margin for under-keel clearance. Negative surges driven by high barometric pressure or offshore wind can still take the real level below LAT, which passage plans must allow for.