Which nautical term refers to the reference level for depths on charts?

• A. Chart datum
• B. Contour line
• C. Tide datum
• D. Sounding datum

Answer: (A)

Depths on nautical charts are reduced to a fixed zero level known as chart datum. This serves as the reference from which all soundings are measured, so a depth value on the chart indicates how far the seabed is above that zero point. The exact datum used can differ by chart region (often LAT or MLLW), and the chart legend will specify which one applies.

A contour line, by contrast, marks points of equal depth on the map, not the zero reference level. A tide datum relates to the water level used for predicting tides, not the chart’s depth zero. Sounding datum would refer to the depth measurements themselves, but the standard nautical term for the zero reference on charts is chart datum.