All About Nautical Charts Tutorial: Glossary of Terms

acoustic backscatter: reflection of sound waves back toward the sonar source after hitting the seafloor or an underwater object. The strength of the reflected sound helps determine what the seafloor consists of, such as sand, mud, or rock.

aids to navigation: devices or markers, such as buoys, beacons, and lights, that are placed in the water or along coasts to help mariners determine their position and safely navigate by warning of hazards or guiding them along safe routes.

anchorage areas: areas where mariners can safely anchor vessels. Solid or dashed lines and labeling typically indicate these areas; anchor-like symbols may be used for anchor-prohibited zones on nautical charts. Anchorage areas may also have other information so vessel operators can determine whether the space is enough to anchor their vessels.

angle of difference: the measured angle between two directions, such as the difference between true north and magnetic north, also known as magnetic declination.

area: refers to a particular portion of the Earth's surface. An area can be as large as a country or as small as a neighborhood.  

bathymetry: the measurement and mapping of the depth and shape of the ocean floor, similar to how topography shows the features of land.

bathymetric maps: maps that show the depth and shape of the ocean floor using contour lines and colors to represent underwater features.

beacons: structures emitting light that are permanently fixed to the ocean floor or land. They can be as monumental as a lighthouse or as simple as a single pole.

bench marks: fixed reference pounds on land with known positions and elevations, used by scientists and surveyors to measure changes in water levels, tides, and land elevation accurately.

buoy: also known as navigational buoys, these are floating objects that are anchored to the ocean floor. They can be used to mark channels, hazards, and other important things in the ocean. On a nautical chart, their shape and color can vary according to the type of buoy they are. 

cartographer: responsible for formatting the hydrographic and topographic data, interpreting it, controlling its quality, and making sure that it is easy to understand as nautical charts are made ready for use.

channels: routes that navigators can safely follow into a marina, anchorage, or passageway. They are represented by symbols that indicate the channel's location and depth — as lines with depth soundings (numbers representing depth measurements) on either side.

What’s the difference between coastline and shoreline? These words both mean the boundary between water and land. Coastline is used to describe the approximate boundaries over a large area. Shoreline is used to describe the precise location of the boundary between land and water.

compass rose: A circle shown on nautical charts that displays direction, indicating north, south, east, and west, which helps navigators determine their heading.

daybeacon: signs posted on land or water; in water, these are usually used as channel or hazard markers. They are generally used only during daytime navigation, as they are without lights. 

dead reckoning: A method of estimating a vessel’s current position by using its previous location, speed, direction, and time traveled, without relying on external references like GPS or landmarks.

declination: the angle of difference between true north and magnetic north, used by navigators to correct compass readings for accurate direction.

depth curves: sometimes called fathom curves or depth contour lines, these are lines that connect points of equal depth.  

depth soundings: sometimes called soundings or water depth measurements, a series of numbers running along a contour line indicate the water depth at a specific point. Depths may be in feet, fathoms (1 fathom = 6 feet), or meters (1 meter = 3.28 feet). Sometimes depth is depicted by color as well: shades of blue can indicate shallow water and submerged obstructions; areas of deeper water without obstructions are typically shown as white. 

dumping or disposal areas: The U.S. Environmental Protection Agency sets aside and manages ocean dumping sites under the Marine Protection, Research and Sanctuaries Act (MPRSA). All but one of these sites are currently used for the disposal of MPRSA-authorized dredged material. (The U.S Army Corps of Engineers dredges sediment from some waterways to make it easier for vessels to pass through.) Many of these ocean disposal sites are located offshore of major U.S. ports and harbors. There is currently one ocean disposal site off American Samoa for fish processing wastes.

Eastern Hemisphere: the half of the Earth located east of the prime meridian and west of the 180th meridian, including most of Europe, Africa, Asia, and Australia.

echolocation: a method used by animals, such as dolphins and bats, to locate objects by producing sounds and listening for the echoes that bounce back to them after making contact with objects in their environment.

echosounder: an instrument that measures water depth by sending sound waves toward the seafloor and recording the time it takes for the echoes to return.

eddies: circular currents of water that spin off from larger ocean currents, often forming when water flows around obstacles or when currents meet, mixing nutrients and heat in the ocean.

Electronic Navigational Charts (ENCs): Digital versions of nautical charts that display detailed information about water depths, hazards, and navigation aids and can be used with GPS to provide real-time positioning for safer navigation.

elevation: the distance above sea level, usually measured in feet or meters.

erosion: the gradual wearing away and movement of soil, rock, or sand by natural forces such as wind, water, or waves, which can change coastlines and underwater landscapes over time.

equator: an imaginary line that circles the Earth halfway between the North and South Poles, dividing it into the Northern and Southern Hemispheres at 0 degrees latitude.

fathom: a nautical unit of measurement that is equal to six feet.

fog signal: a sound or light signal emitted in fog or mist by vessels to indicate their position and by lighthouses and buoys to indicate a shoreline or a channel, or to indicate an area of ocean with dangers in it. 

Global Positioning System (GPS): a satellite-based navigation system that provides precise location and time information anywhere on Earth, helping mariners and others determine their exact position.

gravitational force: the natural force of attraction between two objects with mass. Gravitational force is what causes the ocean’s tides as the moon and sun pull on the Earth’s water. 

hemisphere: one half of the Earth, divided either by the equator into the Northern and Southern Hemispheres or by the prime meridian into the Eastern and Western Hemispheres.

high tide: the point in the tidal cycle when the ocean’s water level is at its highest, caused by the gravitational pull of the moon and the sun on the Earth’s oceans.

Highest Observed Tide (max tide): the greatest height reached by the ocean’s surface during a rising tide, recorded at a specific location over a period of observation.

hydrography: the science of surveying and charting bodies of water — for example, the ocean.

Hydrographic Data Acquisition and Processing System (HDAPS): a NOAA-developed system used to collect and process sonar data from hydrographic survey ships, allowing scientists to map the seafloor and identify underwater features.

hydroplot system: the first fully automated system developed by NOAA in the 1960s for collecting and processing hydrographic survey data, used to create accurate nautical charts of the ocean floor.

landforms: features on the Earth’s surface, such as mountains, valleys, plains; coastal features, such as peninsulas or bays; and underwater features, such as ocean basins and mid-ocean ridges.

landmark: natural or man-made features on land that can be used as an aid for navigation because they are distinguishable in some way, often by height or color; for example, a peninsula, lighthouse, church, or water tower.

latitude: measures the distance north or south of the equator.

lead weight: a heavy weight, often attached to a marked line, that sailors once used to measure water depth by lowering it until it touched the seafloor.

lidar (Light Detection and Ranging): a technology that uses laser light to measure distances to create detailed maps of land and shallow water features by analyzing how long it takes light pulses to return after hitting a surface.

lighthouse: a building, usually a tower, with a beacon light to warn ships at sea. Lighthouses can be represented with different symbols depending on their type and how they operate. For example, a lighthouse that emits a continuous light is depicted with a solid triangle, while one that emits a flashing light may be depicted with a triangle with a broken line.

limits: a defined border or boundary of something.

longitude: the angular distance of a place east or west of the prime meridian at Greenwich, England, which is the earth’s zero of longitude.

low tide: the point in the tidal cycle when the ocean’s water level is at its lowest, exposing more of the shoreline and shallow areas.

Lowest Observed Tide (min tide): the lowest water level ever recorded at a specific location during a falling tide over a period of observation.

magnetic north: the direction a compass needle points, toward Earth’s magnetic field in the Northern Hemisphere, which differs slightly from true north and changes location over time.

manual plotting: the process of recording and charting survey or navigation data by hand, using tools like rules, compasses, and pencils before computer systems were available.

mariner: a person who is responsible for navigating the vessel and who likely earns a living (usually independently) at sea; a sailor is mostly responsible for the day-to-day tasks necessary to keep the vessel running smoothly.

Mean High Water: the average height of all high tides recorded at a specific location over a standard 19-year period, known as the National Tidal Datum Epoch.

Mean Higher High Water: the average height of the higher of the two daily high tides observed at a location over the National Tidal Datum Epoch.

Mean Low Water: the average height of all low tides recorded at a location over the National Tidal Datum Epoch, used as a reference point for measuring water depth and shoreline boundaries.

Mean Lower Low Water: the average height of the lower of the two daily low tides observed at a location over the National Tidal Datum Epoch, commonly used as the reference for depths shown on nautical charts.

Mean Range of Tide: the average difference in height between mean high water and mean low water at a specific location over the National Tidal Datum Epoch.

Mean Sea Level: the average height of the ocean’s surface, calculated from hourly water level measurements over the National Tidal Datum Epoch, and used as a reference for land and sea elevations.

meridians: imaginary lines of longitude that run from the North Pole to the South Pole and are used to measure distances east or west of the prime meridian.

multibeam echosounder: a sonar device that sends out multiple sound beams in a fan shape beneath a vessel to measure water depth and create detailed, three-dimensional maps of the seafloor.

National Tidal Datum Epoch (NTDE): a 19-year period used by NOAA to calculate average tidal measurements and establish consistent reference points, or datums, for comparing water levels over time.

nautical: relating to sailors, navigation, or ships.

navigate, navigation: the activity of planning a route from one place to another.

Northern Hemisphere: the half of the Earth located north of the equator, containing North America, Europe, most of Asia, and parts of Africa and South America.

obstructions: hazards that mariners need to be aware of. Examples of obstructions include rocks, shoals, wrecks, and hulks.

oceanography: the scientific study of the ocean, including its physical, chemical, biological, and geological properties, and how it interacts with the atmosphere, land, and marine life.

paper punch tape: an early data storage method that recorded information as a series of holes punched into a continuous strip of paper, once used in hydrographic survey systems before modern computers.

parallels: imaginary lines of latitude that circle the Earth from east to west, running parallel to one another and never intersecting, used to measure distances north or south of the equator.

poles: the two points on Earth where its axis of rotation meets the surface — the North Pole and the South Pole — representing the northernmost and southernmost locations on the planet.

prime meridian: the line of longitude at 0 degrees that runs through Greenwich, England, dividing the Earth into the Eastern and Western Hemispheres.

reef: ridges of jagged rock, coral, or sand just above or below the sea’s surface

reference point: a fixed location used as a basis for measuring or determining position, elevation, or distance, such as a bench mark or a point on a map or chart.

refract: to bend a wave, such as light or sound, as it passes through materials with different densities. For example, when sonar sound waves change direction in water layers of varying temperature or salinity.

relief: the differences in height from place to place on the land's surface, affected by geology.

route: a way taken in getting from the starting point to a destination; in navigation, it’s a planned path or course that a vessel will take to reach its destination.

run aground: when a vessel accidentally touches or becomes stuck on the seafloor or shore because the water is too shallow.

salinity: the measure of how much salt is dissolved in water, which affects the water’s density and the way sound waves and ocean currents move.

schooner: a type of sailing ship with two or more masts, commonly used in the 1800s for surveying and coastal navigation.

shallow-water multibeam sonar: a sonar system designed to map seafloor areas in relatively shallow coastal waters by sending out multiple sound beams at once to create detailed, high-resolution images of underwater features.

Shipboard Data System (SDS III): a data acquisition and processing system developed by NOAA in the early 1980s to modernize hydrographic surveying.

shoal: Natural underwater sand ridges, banks, or bars that make the water depth shallower than in surrounding areas.

side-scan sonar: a sonar system that sends sound waves outward from both sides of a towed or mounted device to produce detailed images of the seafloor and identify objects like rocks or shipwrecks.

single-beam sonar: a sonar system that sends one narrow sound beam straight down from a vessel to measure water depth directly beneath it.

Sonar (Sound Navigation and Ranging): a technology that uses sound waves to detect and locate objects underwater by sending out sound pulses and measuring the echoes that bounce back.

sound velocity profiler: an instrument that measures how fast sound travels through water by recording changes in temperature, salinity, and pressure at different depths, helping improve the accuracy of sonar data.

Southern Hemisphere: the half of the Earth located south of the equator, containing most of South America, one-third of Africa, Australia, Antarctica, and many Pacific islands.

three-point fix: a navigation method used to determine a vessel’s exact position by measuring angles to three known reference points on shore or at sea.

tide: the regular rise and fall of ocean water levels caused by the gravitational pull of the moon and the sun and the rotation of the Earth.

tide gauges: instruments placed along coasts that measure and record changes in sea level over time to monitor tides and provide data for tidal datums and nautical charts.

tidal cycle: the repeating pattern of rising and falling sea levels that occurs about every 24 hours and 50 minutes, including two high tides and two low tides in most coastal areas.

tidal datums: standard reference points based on average tidal heights over a 19-year period, used to measure and compare water levels for mapping, navigation, and coastal management.

topographical, topography: an accurate representation, in a realistic and detailed manner, of an area’s natural and human-made features, such as towns, buildings, and physical features. 

topographic laser scanners: laser-based instruments mounted on ships that measure features above the water’s surface, such as rocks and small islands, to help map coastlines and identify near-shore hazards.

track: a narrow path or road with a rough, uneven surface, especially one made by people or animals frequently moving through the same place; on a nautical chart, a straight line drawn in the direction of its course.

transducer: a device in a sonar system that converts electrical energy into sound waves to send into the water and then back into electrical signals when the echoes return, allowing depth and object detection.

transmitter: a component of a sonar system that sends out sound waves through the water to detect objects and measure distance.

trigonometric survey: a way of making maps by measuring angles and using math to figure out where places are and how far apart they are.

true north: the direction along Earth’s surface that points toward the geographic North Pole, used as a fixed reference for navigation.

real-time positioning: the ability to determine and display a vessel’s exact location instantly, often using GPS, to help mariners navigate safely and avoid hazards.

receiver: a component of a sonar system that detects and records returning sound waves after they bounce off the seafloor or other objects.

unexploded ordnance: explosive weapons such as bombs, bullets, shells, grenades, mines, etc., that should have exploded when they were fired, dropped, launched, or projected but failed to do so and still pose a risk of detonation.

Western Hemisphere: the half of the Earth located west of the prime meridian and east of the 180th meridian, including the Americas, parts of western Europe, and portions of Africa.

wrecks and hulks: a (ship)wreck is the sunken or beached remains left after a disaster at sea; a hulk is a ship that is afloat but not capable of going to sea; it may be an abandoned shell, or it may be an old ship that has had its equipment removed. These symbols indicate the location of sunken vessels and are often shown as an "X" or other distinctive shapes depicted with different symbols depending on their color and shape.