The tidal datum chosen for a nautical chart and its tide tables, set near the bottom of the tidal range so charted depths err on the safe side. In the United States it is MLLW, which roughly half of all lower lows dip below; many other countries use LAT, which the tide almost never goes under. Predicted heights in tide tables are measured from chart datum, so adding the predicted height to the charted depth gives the expected depth at that time. Negative predictions mean less water than charted.
Tide table's on chart datum, so the 2.1 metre high water adds straight onto the charted 8.
Also heard as
- CD
- sounding datum
Related terms
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.
LAT
Lowest Astronomical Tide: the lowest level that tide predictions reach when the weather is held at average.
Tidal prediction
A forecast of tide heights or currents computed from harmonic constants and astronomy, with no weather included.
Minus tide
A low tide predicted to fall below the chart datum zero, shown as a negative number in the tide table.