Charted Depth

The vertical distance from chart datum to the seabed or the top of a charted underwater feature.

Soundings printed on a nautical chart are referenced to chart datum. Adding the predicted or observed height of tide gives an estimate of the water depth at that time. The navigator then compares the result with the vessel’s draught and allows an appropriate under-keel clearance.

A charted depth is not a guarantee of the least depth that will always be available. Water levels can fall below predictions because of weather, and seabed levels can change through silting, scour or shifting sand and shingle. Survey age, chart notes, current corrections and local warnings should therefore be considered when planning passage through shallow water.