On navigation charts, what do the numbers in the soundings section represent?

On navigation charts, what do the numbers in the soundings section represent?

• A. Contour lines (fathom curves) connect points of roughly equal depth for bottom profile
• B. Measurements of the depth of the water at average low tide
• C. Fathom 6 feet
• D. Darker blue - shallow water

Answer: (B)

Soundings show the water depth at the chart datum—the fixed reference level used for the chart (typically mean lower low water). Each number tells you how deep the water is from that reference to the seabed at that location. They’re not contour lines, not a single tide-specific statement, and not just a color cue for shallow water. To know actual depth at a given moment, you adjust the charted depth by the current tide height.

Soundings show the water depth at the chart datum—the fixed reference level used for the chart (typically mean lower low water). Each number tells you how deep the water is from that reference to the seabed at that location. They’re not contour lines, not a single tide-specific statement, and not just a color cue for shallow water. To know actual depth at a given moment, you adjust the charted depth by the current tide height.