Tidal Definitions and Datum

Chart Datum is the zero from which charted depths and predicted tidal heights are measured. In areas with an appreciable ocean tide it is usually Lowest Astronomical Tide or a closely related low-water datum, but the chart title block is the authority. A sounding lies below Chart Datum, so add the predicted height of tide to estimate water depth.

A drying height is the height of a feature above Chart Datum. Subtract it from the height of tide to estimate water over the feature. If the drying height exceeds the predicted tide, the result is a height exposed above the water rather than a negative water depth.

Overhead clearances use a separate high-water reference. Current IHO guidance prefers Highest Astronomical Tide or a closely equivalent datum where tides are appreciable, while national alternatives are permitted and older charts may use another stated level such as MHWS. HAT and LAT are astronomical levels, not guaranteed observed limits: weather and surge can take water beyond them.