🔍 Key Concepts

• Chart datums and how they are defined for depth vs. height • Difference between soundings (depths) and elevations (heights) on nautical charts • Where Mean Lower Low Water (MLLW) is commonly used as a reference

💭 Think About

• Ask yourself: On a chart, what vertical reference is used so that mariners know the minimum expected depth? • Consider whether MLLW is more important for things sticking up out of the water (like lights and mountains) or for things under the water (like depths and channels). • Think about whether this reference would logically be used only on one coast of the U.S., or for more than one region.

✅ Before You Answer

• Verify which chart feature uses MLLW as its vertical datum: depths or heights. • Check whether lights and landmarks heights are referenced to Mean High Water (MHW) or Mean Lower Low Water (MLLW) in standard U.S. chart practice. • Confirm if the same depth reference is used on both U.S. coasts rather than only one.