What is chart datum?

What is chart datum?

• A. The most recent tide height
• B. The reference level from which depths on a chart are measured.
• C. The surface of the water
• D. The magnetic north reference

Answer: (B)

Chart datum is a fixed reference level from which depths on a nautical chart are measured. It provides a consistent baseline so every depth shown on the chart represents how deep the water is above that imaginary plane, not the actual water surface at the moment. In practice, different regions use different datums (such as LAT or MLLW), so a charted depth remains constant even as tides rise and fall. To know the real depth at any time, you must account for the current tide or water level and add or subtract from the chart depth accordingly. The other options describe the water surface at a specific moment or a navigation reference unrelated to depth measurements, which is why they don’t fit as the chart datum.

Chart datum is a fixed reference level from which depths on a nautical chart are measured. It provides a consistent baseline so every depth shown on the chart represents how deep the water is above that imaginary plane, not the actual water surface at the moment. In practice, different regions use different datums (such as LAT or MLLW), so a charted depth remains constant even as tides rise and fall. To know the real depth at any time, you must account for the current tide or water level and add or subtract from the chart depth accordingly. The other options describe the water surface at a specific moment or a navigation reference unrelated to depth measurements, which is why they don’t fit as the chart datum.