In maritime jurisdiction, at what point is the baseline established for measuring territorial seas?

• A. the highest astronomical tide
• B. mean sea level
• C. the average of all tides
• D. the lowest tidal mark

Answer: (D)

Baseline for measuring territorial seas is set from the low-water line along the coast—the line of lowest tide observed there. This provides the fixed starting point from which the sea area is measured outward, typically 12 nautical miles. While baselines can be drawn as straight lines between points along irregular coastlines, they must always be anchored to the low-water line shown on nautical charts. The other tolerances—highest astronomical tide, mean sea level, or the average of tides—do not provide a stable, landward reference for measuring the sea, so they aren’t used as the baseline. The lowest tide mark, i.e., the low-water line, is the reference that aligns with how baselines are established.