Ocean Navigation: Laws, Terms and Tools

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Bridge of modern research vessel

The law of the sea is a body of customs, treaties, and international agreements by which governments maintain order, productivity, and peaceful relations on the sea. Notable in the development of the law of the sea are a number of international conventions signed in the latter half of the 20th century. The United Nations (UN) held its first Conference on the Law of the Sea (UNCLOS I) in 1956, which resulted in a 1958 Convention. The final conference, held in Montego Bay, Jamaica, in 1982, resulted in the 1982 Law of the Sea Convention (LOSC). The LOSC came into force in 1994 upon receiving the necessary number of UN signatories. [Source: NOAA]

While the United States ratified the 1958 Convention, as of late 2013, it had not become a party to the 1982 Convention. The United States recognizes that the 1982 Convention reflects customary international law and complies with its provisions.

NOAA is responsible for depicting on its nautical charts the limits of the 12 nautical mile Territorial Sea, 24 nautical mile Contiguous Zone, and 200 nautical mile Exclusive Economic Zone (EEZ). Each of these maritime zones is projected from what is called a “normal baseline,” which is derived from NOAA nautical charts. A “normal baseline” is defined under the Law of the Sea as the low-water line along the coast as marked on officially recognized, large-scale charts or the lowest charted datum, which is mean lower low water (MLLW) in the United States. The method of arriving at this baseline is described in the 1958 Convention and in the 1982 Convention. The U.S. normal baselines are ambulatory and subject to changes such as accretion (addition of land) and erosion. Unless the seaward boundary or zone is fixed, it will be subject to corresponding change.

The location of maritime zones and boundaries can have potentially far-reaching effects. As a result, NOAA works with other federal agencies, particularly the U.S. Department of State, to periodically update U.S. maritime zones and boundaries as depicted on NOAA navigational charts.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org


Navigation system on a merchant ship

The exclusive economic zone (EEZ) is the zone where coastal nations have jurisdiction over natural resources. The EEZ extends no more than 200 nautical miles (370 kilometers) from the territorial sea baseline and is adjacent to the 12 nautical mile (22.2 kilometers) territorial sea of the the country that has an EEZ. [Source: NOAA]

Within the EEZ, the the country has: 1) Sovereign rights for the purpose of exploring, exploiting, conserving and managing natural resources, whether living and nonliving, of the seabed and subsoil and the superjacent waters and with regard to other activities for the economic exploitation and exploration of the zone, such as the production of energy from the water, currents and winds; 2) Jurisdiction as provided for in international and domestic laws with regard to the establishment and use of artificial islands, installations, and structures, marine scientific research, and the protection and preservation of the marine environment; and 3) Other rights and duties provided for under international and domestic laws.


Seaspeak is the official language of navigators the world over. Whether on the high seas or at port, misunderstood communication can lead to serious and even dangerous situations. "Seaspeak", the official language of the seas, helps to prevent miscommunication. [Source: NOAA]

Have you ever wondered how crews on ships at sea communicate? Nowadays, crews consist of men and women from all over the world, and often, a single ship's crew comes from a number of different countries. But that was not always the case. Back in the 1960s, the United States and the United Kingdom dominated ocean traffic, and 80 percent of ships' crews were native English speakers. By the end of 1970s, however, the situation had reversed, and today, 80 percent of ship's crews do not speak English as a first language.

Sailboat terms: 1) mainsail; 2) staysail; 3) spinnaker; 4) hull; 5) keel; 6) rudder; 7) skeg; 8) mast; 9) Spreader; 10) shroud; 11) sheet ; 12) boom; 13) mast; 14) spinnaker pole; 15) backstay; 16) forestay; 17) boom vang

So what happens when a ship captain needs to communicate with his or her crew, to another ship, or to shore? Whether on the high seas or at port, misunderstood communication can lead to serious and even dangerous situations.

To avoid such confusion, in 1983 a group of linguists and shipping experts created a new system of communication called Seaspeak. English was chosen as the principle lexicon for Seaspeak because it was the most common language spoken on ships at that time, and, importantly, it was also the language of civil aviation. In 1988, the International Maritime Organization made Seaspeak the official language of the seas.

Seaspeak defines the rules of how to talk on a ship's radio. The number of words is limited to ensure that messages and conversations are short and clear. Eight words, called message markers, precede each sentence. These words are Advice, Answer, Information, Instruction, Intention, Question, Request, and Warning.

An important rule of Seaspeak is that numbers made up of two or more digits are spoken in single digits. For example, the number 33 is spoken as "three three" and the time 9:33 a.m. is spoken as "zero nine three three." Coordinated Universal Time — the primary time standard or international time scale by which the world regulates clocks and time — is always used at sea.

Why Ships Use "Port" and "Starboard" Instead of "Left" and "Right?"

Since port and starboard never change, they are unambiguous references that are independent of a mariner’s orientation, and, thus, mariners use these nautical terms instead of left and right to avoid confusion. When looking forward, toward the bow of a ship, port and starboard refer to the left and right sides, respectively. [Source: NOAA]

In the early days of boating, before ships had rudders on their centerlines, boats were controlled using a steering oar. Most sailors were right handed, so the steering oar was placed over or through the right side of the stern. Sailors began calling the right side the steering side, which soon became "starboard" by combining two Old English words: stéor (meaning "steer") and bord (meaning "the side of a boat").

As the size of boats grew, so did the steering oar, making it much easier to tie a boat up to a dock on the side opposite the oar. This side became known as larboard, or "the loading side." Over time, larboard — too easily confused with starboard — was replaced with port. After all, this was the side that faced the port, allowing supplies to be ported aboard by porters.

Nautical Mile and a Knot?

The nautical mile measures distance and the knot measures speed. Nautical miles are used to measure the distance traveled through the water. A nautical mile is slightly longer than a mile on land, equaling 1.1508 land-measured (or statute) miles. The nautical mile is based on the Earth’s longitude and latitude coordinates, with one nautical mile equaling one minute of latitude.

But why use a different measurement system for marine navigation? Using latitude and longitude coordinates is more practical for long-distance travel, where the curvature of the Earth becomes a factor in accurate measurement. Nautical charts use latitude and longitude, so it’s far easier for mariners to measure distance with nautical miles. Air and space travel also use latitude and longitude for navigation and nautical miles to measure distance.

The word “mile” might leave you wondering if there’s a “nautical kilometer,” too. There’s not. The international nautical mile is used throughout the world. The measurement was officially set at exactly 1.852 kilometers in 1929 by what is now known as the International Hydrographic Organization. The U.S. and the United Kingdom both used slightly different measurements after that time, but the U.S. adopted the international nautical mile in 1954 and the U.K. in 1970.

One knot equals one nautical mile per hour, or roughly 1.15 statute mph. The term knot dates from the 17th century, when sailors measured the speed of their ship using a device called a “common log.” The common log was a rope with knots at regular intervals, attached to a piece of wood shaped like a slice of pie. Mariners would lower the wood piece into the water and allow it to float freely behind the ship for a specific amount of time (often measured with an hourglass). When the time was up, they would count the knots between the ship and the piece of wood, and that number estimated their speed.

High Seas Forecasts

Seafaring can be very dangerous for ships at sea. For centuries, little could be done to make shipping safer, due to the unpredictability of the weather, the vast size of the ocean, and a lack of international cooperation in the open ocean. However, the 1912 sinking of the "unsinkable" Titanic, with more than 1,500 casualties, sparked global concern about safety and standards for ships at sea. This tragedy would lead to new international conventions that established safety requirements and provided more accurate and standardized weather forecasts. [Source: NOAA]

navigation radar

Today, National Meteorological and Hydrological Services around the world provide real- or near-real-time weather forecasts and data that are transmitted globally. The World Meteorological Organization’s (WMO) Global Observing System relies on member countries, which use their facilities on land, at sea, and in space via satellites to forecast weather conditions. Together, these resources allow meteorological and other environmental observations on a global scale.

As a member of WMO, NOAA's National Weather Service (NWS) provides High Seas Forecasts for large areas of the U.S. Atlantic and Pacific oceans. The NWS also provides marine forecasts for U.S. coastal areas and the Great Lakes, offshore waters, and part of the Arctic Ocean. The NWS’s Unified Surface Analysis of world weather also provides data on a complex set of surface observations. These forecasts from NOAA, as well as predictions from other National Meteorological and Hydrological Services around the world, are available to anyone, and, most importantly, to all ships at sea.

Nautical Charts

A nautical chart is one of the most fundamental tools available to the mariner. It is a map that depicts the configuration of the shoreline and seafloor and represents hydrographic data,. It provides water depths, tide predictions, obstructions to navigation such as rocks and shipwrecks, navigational aids, anchorages, and other features. It differs from a “map,” which emphasizes landforms and encompasses various geographic and cartographic products. The nautical chart is essential for safe navigation. Mariners use charts to plan voyages and navigate ships safely and economically. Federal regulations require most commercial vessels to carry electronic or paper nautical charts while they transit U.S. waters. [Source: NOAA]

A chart is used by mariners to plot courses through open bodies of water as well as in highly trafficked areas. Because of its critical importance in promoting safe navigation, the nautical chart has a certain level of legal standing and authority. A map, on the other hand, is a reference guide showing predetermined routes like roads and highways.

Numbers on a nautical chart are depth measurements. Water depths are measured by soundings usually acquired by hydrographic surveys. The depths may be in either feet or fathoms. A fathom is a nautical unit of measurement and is equal to six feet. On a chart, water depths may be connected with a line known as a depth contour, similar to the topographic lines or surface features that you see on a map. Depth contours present a picture of the bottom to the mariner.In addition to showing water depths, a nautical chart also tells mariners about dangers to navigation, aids to navigation, anchorages, and other features. U.S. Chart No. 1 explains all the numbers and symbols found on both paper and electronic nautical charts.

Lighthouses and Totten Beacons

lighthouse in Norway

Though numerous lighthouses still serve seafarers, modern electronic aids to navigation play a larger role in maritime safety in the 21st century. Lighthouses and beacons are towers with bright lights and fog horns located at important or dangerous locations. They can be found on rocky cliffs or sandy shoals on land, on wave-swept reefs in the sea, and at entrances to harbors and bays. They serve to warn mariners of dangerous shallows and perilous rocky coasts, and they help guide vessels safely into and out of harbors. The messages of these long-trusted aids to navigation are simple: either STAY AWAY, DANGER, BEWARE! or COME THIS WAY! [Source: NOAA]

While lighthouses still guide seafarers, nowadays, the Global Positioning System (GPS), NOAA’s nautical charts, lighted navigational aids, buoys, radar beacons, and other aids to navigation effectively warn mariners of dangerous areas and guide them to safe harbors. Some 48,000 federal buoys, beacons, and electronic aids of the marine transportation system mark more than 25,000 miles of waterways, harbor channels, and inland, intracoastal and coastal waterways, and serve more than 300 ports.

By 1852, Lieutenant James Totten, the U.S. Army's assistant to the Coast Survey, had installed 15 wooden signal poles in the reefs to create more accurate charts of the Florida Keys. Local mariners quickly recognized that the poles themselves helped them safely navigate the reefs, and by 1855, Totten and his team installed a second generation of 16 poles using a more permanent material — iron. The "beacons" each displayed a letter, starting with "A" and ending with "P." Today, remnants of Totten Beacons are protected as historical resources by the Florida Keys National Marine Sanctuary (FKNMS).

Today, GPS and sophisticated electronic navigation systems along with lighted navigational aids, buoys, radar beacons, and other visual aids maintained by the U.S. Coast Guard help warn mariners of dangerous coral reefs and shoals. FKNMS installs and maintains more than 700 buoys for resource management purposes. Mooring buoys, which are round with a blue stripe, make it possible for a vessel to linger on a reef without using an anchor and potentially damaging fragile marine resources. Round yellow regulatory buoys in the sanctuary mark areas which have specific regulations, while cylindrical white buoys delineate special zones.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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