Chart Datum (CD)

The local reference level from which charted depths and drying heights are measured.

Chart datum provides the common zero used for soundings on a nautical chart and for published tidal heights. In many areas it is based on Lowest Astronomical Tide, but the datum is not identical everywhere; the notes and symbols on the relevant chart or tide publication identify the reference in use. An estimated water depth can normally be found by adding the predicted or observed height of tide to the charted depth. Weather can make the actual water level higher or lower than the prediction, so prudent navigation also allows for atmospheric pressure, wind, waves, vessel movement and an appropriate under-keel clearance.