Lowest Astronomical Tide is the lowest level the tide reaches in predictions from astronomy alone, with weather held at average, found by computing predictions across at least one full nodal cycle of about 19 years. Many countries use it as chart datum because the tide almost never falls below it without help from the weather. Its counterpart, Highest Astronomical Tide, often marks the upper limit for vertical clearances.
Charts here are on LAT, so the tide table never shows a minus figure; you only get below datum with a strong offshore blow.
Also heard as
- lowest astronomical tide
Related terms
Chart datum
The low-water zero that charted depths and tide table heights are measured from.
MLLW
Mean Lower Low Water: the average height of the lower of each day's two low waters, used as chart datum in the United States.
Nodal cycle
The 18.6-year cycle in the tilt of the Moon's orbit that slowly raises and lowers tidal ranges.
Astronomical tide
The part of the water level caused only by the gravitational effects of the Moon and Sun.