Chart Navigation & Plotting · Chart Basics — Soundings and Datum
Charted soundings on a U.S. chart are referenced to:
- AMean Lower Low Water (MLLW), so actual depth is usually greater than charted✓ Correct
- BMean High Water, so actual depth is usually less
- CThe current sea level at all times
- DThe average of high and low water
Explanation
U.S. charted depths use the MLLW sounding datum, a conservative low-water reference. The actual depth is the charted sounding plus the height of tide, so depths are usually deeper than charted — but a strong negative tide can make them shallower than charted.
Authority: Chart No. 1; NOAA
Practice the full Chart Navigation & Plotting bank
Free spaced-repetition quizzing across 2550 USCG exam questions — it schedules your reviews so the ones you miss come back until they stick.
Related Chart Navigation & Plotting questions
- Chart Basics — Soundings and Datum
Charted soundings on a U.S. NOAA chart are referenced to which datum?
- Chart Basics
On a Mercator chart, one nautical mile is measured using the:
- Chart Basics
The principal advantage of the Mercator projection for navigation is that:
- Chart Basics
Charted depths (soundings) on a U.S. chart are referenced to which tidal datum?
- Chart Basics
Vertical clearances of bridges and overhead cables on a U.S. chart are normally referenced to:
- Distance, Speed & Time
A vessel makes good 12 knots. How far does it travel in 20 minutes?