Depths on nautical charts are measured downward from what reference level?

• A. Mean low water
• B. Mean high water
• C. Mean sea level
• D. Chart datum

Answer: (A)

Depths on nautical charts are referenced to a fixed plane called chart datum. That zero point stays the same regardless of tides, so every depth value shows how far the seabed lies below that datum. In many charting systems this datum corresponds to mean low water, meaning a depth reading indicates how many meters below the mean low water plane the bottom sits. The actual water depth at any moment would then depend on the current tide relative to that datum. Mean high water or mean sea level would not serve as the stable baseline for chart depths, since they move with the tides. Chart datum is the official reference used, and in this context it aligns with mean low water.