Know Your Limits!


The invisible lines on the water that affect your rights underway  (published March 2012)
Mariners who sail both U.S. inland and coastal waters are required to know one set of boundary limits—the ones that mark the jurisdiction of the U.S. Inland Navigation Rules. These boundaries are called Demarcation Lines; they are specified at the back of the USCG printing of the Navigation Rules. On the inland side of these lines, the U.S. Inland Rules apply; seaward of these lines, the COLREGs (International Rules) apply. Lights, sounds and maneuvering rules are different in these two regions. Sailing farther offshore, and in particular when approaching a foreign country, we are obligated to know about other limits—those generally referred to as marking the territorial sea of the country we approach. They are defined by the United Nations Convention on the Law of the Sea ( as a belt of coastal waters extending 12 nm from the baseline of a coastal state, which separates the territorial sea from the nation’s internal waters. The territorial sea is regarded as the sovereign territory of the state, although foreign ships (military and civilian) are allowed innocent passage through it. When adjacent countries have overlapping territorial waters, the usual convention is to draw the boundary halfway between their baselines.

Innocent passage means—besides the obvious prohibitions on military or commercial activities of any kind—that you traverse the sea without entering the nation’s internal waters, and that no cargo or personnel leaves or enters your vessel, and no fishing is allowed. The broad catchall is Article 19, Part 2 (l): “…nor any other activity not having a direct bearing on passage.”

The baselines used (by the U.S. and other nations) are not defined the same way as the U.S. Demarcation Lines, which are usually all coastal waters and inland of a line drawn point to point along the headlands marking entrances to what are called the Inland Waters. Baselines, in contrast, mark the boundary between the territorial sea and the internal waters of a country. Baselines are typically defined as the low-water line along the coast with extensions across headlands or sometimes following the tide lines into large bays, but there are variations. In fact, the U.S. at one point used a fixed 3-nm limit as marking its baseline, but technically this is no longer used, though some state and other legal jurisdictions still use that definition and consequently it is often referred to on nautical charts. The UN has on record a list of the waypoints that each country uses to establish its baseline.

There is also an important document at UN-LOS link called “Suspicion of Innocent Passage,” which states for each country the restrictions they have placed on the transit of foreign vessels into their territorial waters. These are typically temporary restrictions to account for military exercises, but they can also be notices of claims in disputed waterways. In any event, once across the baseline there is no innocent passage—you are in the country. According to the US Coast Pilot: “The United States has full sovereignty over its internal waters and ports as if they were part of its land territory,” so other countries would likely have a similar interpretation.

Since we may not be able to check the primary UN sources, it is important for international sailing to check the International Notice to Mariners and other warnings at the NGA link at (the primary link changes often so we archive it here). These international notices are to be distinguished from the better known Local Notices to Mariners for U.S. waters presented online at the USCG Nav Center (

This fundamental 12-nm limit for territorial seas is in the minds of many international sailors, but on closer look to the Law of the Sea that interpretation might not be conservative enough in some circumstances. According to the Law of the Sea, each country has the right to extend their 12-nm territorial limit by another 12 miles with a contiguous zone. The special restrictions and rights a country can claim for their contiguous waters are specified in the Law, but we can use what the US claims as a guideline. The U.S. contiguous zone was declared in a presidential proclamation in 1999, which in part stated: “Within the extended contiguous zone, the CG may now board and search a foreign vessel suspected of smuggling drugs, carrying illegal immigrants, polluting the ocean, or tampering with sunken ships or other underwater artifacts, without first obtaining permission from the country where the vessel is registered. Previously, such action could be taken only within 12 miles of the coast.”

Such a declaration in all effect extends the 12-nm limit to 24-nm, and since we can do it, there is no reason to think other coastal nations might not interpret their optional contiguous zone the same way. A check of the UN website for specific countries would answer that. Thus when planning routes past a foreign country you do not wish to enter, it could be prudent to keep this extended zone in mind with logged GPS positions. Some island nations use Archipelagic baselines, which effectively draws their baseline encircling their islands.

Other international limits exist such as the 200-nm Exclusive Economic Zones, wherein a coastal nation has control of all economic resources, including fishing, mining, oil exploration, and any pollution of those resources, but these do not affect navigation. The claiming nation cannot prohibit passage or loitering above, on, or under the surface of the sea outside of its territorial waters. And there are special cases of these types of limits such as the US 9-nm Natural Resource Boundary off the Gulf coast of Florida, Texas and Puerto Rico, which involves laws just as complex as those referring to the U.S. 3-nm limit—both of more interest to oil companies, shoreline management and conservation matters than to navigators. The 3-nm limit pertains mostly to states’ rights, which are defined in this document:

A related topic that does apply to navigation is a Restricted Zone marked on a nautical chart. These are often defined right on the chart, or there will be a number cited that directs you to the corresponding U.S. Code. Their activation is usually noted in the Local Notice to Mariners. Territorial seas, contiguous zones, and baselines should also be marked on nautical charts, but they may not be updated on old charts. U.S. charts do not show the baseline, but do show the 3-nm limit, which can at least be used for a quick location reference line!

Looking ahead, one area of international boundaries that will come up in the news in the next year or so is the navigation rights to the Canadian Northwest Passage. Due to global warming, this will be adequately clear for shipping in the near future, though previously only routinely passable by icebreakers. Canada claims this route as their internal waters, whereas the U.S. and all other maritime nations do not agree and claim rights of passage. Though as yet unresolved, I think we can be confident it will be solved in a friendly manner, to the best interests of all involved.

At the time of writing, the Law of the Sea is also in the news as Iran threatens to close the Strait of Hormuz, in direct conflict with the Law. The territorial seas of Iran and Oman are divided by the mid point line through the Strait, with established Traffic Separation Lanes on the Oman side. Nevertheless, the entire Strait is legally (Article 38. Right of Transit Passage) open to navigation, and any attempt to block it is a violation of the Law. The Law is a treaty that Iran has not signed, but nevertheless, it would be an act that would gain immediate international response. One cannot be so confident that the outcome will be solved in a friendly manner—not to mention, that the financial markets have already used this as an excuse to drive up the price of oil.

David Burch is the director of Starpath School of Navigation, which offers online courses in marine navigation and weather at He has written eight books on navigation and received the Institute of Navigation’s Superior Achievement Award for outstanding performance as a practicing navigator.