Chart datum
Chart datum turns up in 2 fields: hydrography, tides. Here is what it means to the people in each one.
Chart datum in hydrography
The low-water level from which charted depths are measured, set so actual depth is seldom much less than shown.
The vertical reference for depths and drying heights on a nautical chart. It is set low enough that the water seldom falls far below it, so a mariner adds the predicted tide to the charted depth and any error falls on the safe side. Many countries use Lowest Astronomical Tide; the United States uses Mean Lower Low Water. Survey depths reach chart datum through tide observations or an ellipsoid separation model.
"Depths are reduced to chart datum, so the 10.2 in the channel is exactly what the pilot sees on the ENC."
Chart datum in tides
The low-water zero that charted depths and tide table heights are measured from.
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."