Mean sea level is the reference datum used for which of the following features?

• A. Underwater contours.
• B. Depth curves.
• C. Coastal features.
• D. Tidal datums.

Answer: (A)

A fixed zero level is used so elevations and depths have a consistent reference. Mean sea level provides that long-term average surface of the ocean, serving as the baseline from which depth is measured. When mapping the ocean floor, depth values are plotted as how far below this mean sea level the seabed lies, so the underwater contours—lines of equal depth—are defined relative to mean sea level. That makes mean sea level the standard reference for these features.

Coastal features depend on the land–water boundary and can shift with tides and coastal processes, so they aren’t defined by this universal depth reference. Tidal datums are themselves specific measures tied to particular water level references (like mean lower low water or mean higher high water) built on, but not identical to, the general baseline provided by mean sea level for seabed contours.