What is "chart datum"

n. (context nautical English) the level below which depths are indicated and above which heights of the tides are expressed; usually mean level of low water at a spring tide

A chart datum is the level of water that charted depths displayed on a nautical chart are measured from. A chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.

A chart datum is a vertical datum and must not be confused with the horizontal datum for the chart.