🔍 Key Concepts

• How water levels on rivers and lakes are commonly referenced on navigation charts • What "mean sea level" represents in hydrographic surveying • Whether local structures or variable features (like groundwater levels) are reliable vertical datums

💭 Think About

• Ask yourself: when river or lake elevations are given in feet on charts or in lock/dam data, what fixed reference point are they usually tied to so they are consistent everywhere? • Consider which option would be a standardized, long‑term reference rather than something that could change with local conditions. • Think about what surveyors and chart makers use as a common vertical reference across different regions.

✅ Before You Answer

• Eliminate any answers that depend on local, changeable conditions rather than a permanent reference. • Check which choice represents an official, standardized vertical datum used for water surface elevations. • Verify that the option you choose would still be valid even if bridges are rebuilt or groundwater levels change.