Chart Datum

Chart datum is the reference level used on nautical charts for charted depths, drying heights, and tide heights. It is usually set near the lowest predictable tide level, such as Lowest Astronomical Tide or Mean Lower Low Water, so depths shown on a chart are conservative for navigation.

For boaters, chart datum determines how much water is actually under the keel at any moment. To check clearance, compare charted depth with tide height and your vessel’s Draught, allowing for local effects shown on a Chartplotter. It is especially important around bars, reefs, marina entrances, and during a Spring Tide.

PredictWind tide charts use the relevant local station datum, helping you estimate real water depth by adding predicted tide height to the charted depth. For Departure Planning, always allow a safety margin for atmospheric pressure, swell, sounder offset, and chart survey accuracy.