Chart Datum
The reference tidal level from which chart depths + drying heights are measured — typically Lowest Astronomical Tide (LAT) in modern charts.
Regulatory detail & full definition
Chart Datum is the reference level from which soundings on a nautical chart are measured — the level below which the water rarely falls. Modern IHO-compliant charts use Lowest Astronomical Tide (LAT) as the datum. Actual water depth at a given time = charted depth + height of tide above Chart Datum. Under-keel clearance calculation depends on correctly converting between chart datum + local tide tables + observed height.
Authoritative source: International Hydrographic Organization ↗
Related
Source: IHO S-4 chart specifications
High confidence
Editorial confidence: how we grade.