Depth curves should not be confused with underwater contours because they are measured from different references.

Depth curves should not be confused with underwater contours because they are measured from different references.

• A. Depth curves use the normal surface elevation, while underwater contours use mean sea level.
• B. Depth curves use mean sea level, while underwater contours use normal surface elevation.
• C. Both use the same reference datum.
• D. Depth curves are unrelated to depths.

Answer: (A)

Depth measurements depend on the reference level used to measure from. Depth curves are taken from the instantaneous water surface—the normal surface elevation—so the depth value changes with tides, waves, and other short-term water-level variations. Underwater contours, or bathymetric contours, are drawn using mean sea level as the reference datum, providing a stable baseline for seabed elevations. Because one is tied to the fluctuating water surface and the other to a fixed mean level, they reflect different references and need not match. That’s why the correct statement is that depth curves use the normal surface elevation, while underwater contours use mean sea level.