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 measures are tied to a reference surface that moves with the water itself, while underwater contours are tied to a fixed datum. The normal surface elevation—the actual water surface at a given moment—changes with tides, waves, and weather, so depths calculated from that surface will vary over time. In contrast, mean sea level is a stable reference used for describing the seabed’s location relative to a constant datum, so underwater contours map the bottom with respect to that fixed level. Because of this difference in reference, the same location can show different depth values depending on the tide when using the surface as the reference, while the underwater contour lines reflect depth relative to mean sea level. This distinction is why depth curves and underwater contours are not interchangeable.

Depth measures are tied to a reference surface that moves with the water itself, while underwater contours are tied to a fixed datum. The normal surface elevation—the actual water surface at a given moment—changes with tides, waves, and weather, so depths calculated from that surface will vary over time. In contrast, mean sea level is a stable reference used for describing the seabed’s location relative to a constant datum, so underwater contours map the bottom with respect to that fixed level. Because of this difference in reference, the same location can show different depth values depending on the tide when using the surface as the reference, while the underwater contour lines reflect depth relative to mean sea level. This distinction is why depth curves and underwater contours are not interchangeable.