Which geographic or mapping term best matches the description: a plane to which depth measurements are referred; used in depth curves?

Which geographic or mapping term best matches the description: a plane to which depth measurements are referred; used in depth curves?

• A. Sounding datum
• B. Mean sea level
• C. Tide datum
• D. Cartographic datum

Answer: (A)

Depth measurements are referenced to a vertical datum, called a sounding datum, which serves as the fixed plane for zero depth. This provides a stable baseline so that depths measured at different times and places can be compared and plotted on depth curves. Mean sea level is used as a reference for elevations on land, not depths. Tide datum accounts for the variation in sea level due to tides, useful for tidal studies but not as the fixed plane for depths. Cartographic datum relates to horizontal position and map projection rather than vertical depth references. So the sounding datum is the plane to which depth measurements are referred and is the one used for depth curves.