What is the Semicircle Test used for?

Master the Navigation, Seamanship, and Shiphandling (NSS) – Officer of the Deck (OOD) Phase 2 Exam with engaging flashcards and multiple choice questions, complete with hints and explanations. Prepare comprehensively for your test!

When drawing baselines along a coast, you sometimes need to decide whether a bay should be treated as part of the coastline or as open sea. The semicircle test provides a simple geometric check for that decision. Imagine a semicircle centered on the mouth of the bay. If the landward coastline that forms the bay fits inside that semicircle—meaning the indentation isn’t wide enough to push the sea away from the straight baseline—then the bay can be included in the baseline, effectively allowing a straight line to cross the bay’s mouth. If the bay’s indentation is broad enough that the semicircle would cut through water that faces the open sea, the bay is not included in the baseline and is treated as open coast. In short, the semicircle test is used to decide whether bays should be included or excluded from a nation’s maritime baseline.