🔍 Key Concepts

• How charted light heights are shown on nautical charts and in the Light List • The difference between mean high water, mean low water, and other tidal datums • Why mariners care about how high above the water a light appears for visibility and range calculations

💭 Think About

• Ask yourself: when you see a light’s height listed (in feet or meters) on a chart, from what standard water level would that height logically be measured so it is conservative and safe for all tidal stages? • Think about which tidal datum is commonly used on U.S. nautical charts for vertical clearances like bridges and lights. • Eliminate any answer choices that are not a clearly defined hydrographic or tidal datum actually used in charting.

✅ Before You Answer

• Verify which tidal datum is used for charted heights of lights and structures in the Coast Pilot or Light List introduction sections. • Check which of the options is a standard tidal datum versus a vague or non-standard term (e.g., “average water level,” “geographical sea level”). • Confirm that the chosen datum would give mariners a minimum guaranteed clearance/height under typical tidal conditions, not an optimistic one.