Underwater contours are measured from which reference?

• A. Mean sea level.
• B. Normal surface elevation.
• C. Bottom depth.
• D. Atmospheric pressure.

Answer: (A)

Underwater contours, or bathymetric lines, map the depth of the seafloor relative to a fixed reference plane. That stable reference is mean sea level—the average height of the sea surface over time. Using mean sea level as the baseline allows depths to be compared consistently, despite tides and short-term fluctuations in the actual water surface. So a contour at a given value indicates the seafloor is that many meters below mean sea level.

The other ideas don’t fit as the reference for these contours: using the instantaneous surface elevation would vary with tides; using bottom depth would describe the measured depth itself rather than the reference plane, and atmospheric pressure doesn’t set a vertical reference for depth.