Chart datum

Chart datum is the reference level to which charted depths (soundings) and drying heights are reduced on a nautical chart. Most hydrographic offices use the level of lowest astronomical tide (LAT), the lowest tide predictable under average meteorological conditions, so that the actual depth is almost always at least the charted figure. Tidal height is added to the charted depth to give the real depth at a moment, the basis of every under-keel clearance and tidal-window calculation. It differs from the land-survey vertical datum.