A nautical chart or hydrographic chart is a graphic representation of a

sea A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...

region or

water body A body of water or waterbody is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetlands, or more rare ...

and adjacent

coast A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, su ...

s or

banks A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. As banks ...

. Depending on the scale of the chart, it may show depths of water (

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of wate ...

) and heights of land (

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

), natural features of the seabed, details of the coastline,

navigational hazard A navigational hazard or hazard to navigation is: Hazard to navigation means an obstruction, usually sunken, that presents sufficient danger to navigation so as to require expeditious, affirmative action such as marking, removal, or redefinition o ...

s, locations of natural and human-made aids to

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...

, information on

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

s and

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

s, local details of the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

, and human-made structures such as

harbours A harbor (American English), or harbour (Commonwealth English; see spelling differences), is a sheltered body of water where ships, boats, and barges can be moored. The term ''harbor'' is often used interchangeably with ''port'', which is ...

, buildings, and bridges. Nautical charts are essential tools for

marine navigation Marine navigation is the art and science of steering a ship from a starting point (sailing) to a destination, efficiently and responsibly. It is an art because of the skill that the navigator must have to avoid the dangers of navigation, and it ...

; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper (raster navigational charts) or computerized

electronic navigational chart An electronic navigational chart (ENC) is an official database created by a national hydrographic office for use with an Electronic Chart Display and Information System (ECDIS). ECDIS and ENCs are the primary means of electronic navigation on c ...

s. Recent technologies have made available paper charts which are printed "on demand" with cartographic data that has been downloaded to the commercial printing company as recently as the night before printing. With each daily download, critical data such as Local Notices to Mariners are added to the on-demand chart files so that these charts are up to date at the time of printing.

Data sources

Nautical charts are based on

hydrographic survey Hydrographic survey is the science of measurement and description of features which affect maritime navigation, marine construction, dredging, offshore wind farms, offshore oil exploration and drilling and related activities. Surveys may als ...

s and bathymetric surveys. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and are sometimes unreliable. Depths are measured in a variety of ways. Historically the

sounding line Depth sounding, often simply called sounding, is measuring the depth of a body of water. Data taken from soundings are used in bathymetry to make maps of the floor of a body of water, such as the seabed topography. Soundings were traditional ...

was used. In modern times,

echo sounding Echo sounding or depth sounding is the use of sonar for ranging, normally to determine the depth (coordinate), depth of water (bathymetry). It involves transmitting acoustic waves into water and recording the time interval between emission and ...

is used for measuring the seabed in the open sea. When measuring the safe depth of water over an entire obstruction, such as a

shipwreck A shipwreck is the wreckage of a ship that is located either beached on land or sunken to the bottom of a body of water. It results from the event of ''shipwrecking'', which may be intentional or unintentional. There were approximately thre ...

, the minimum depth is checked by sweeping the area with a length of horizontal

wire file:Sample cross-section of high tension power (pylon) line.jpg, Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample d ...

. All depths on charts is measured with respect to a datum/reference level. This ensures that difficult to find projections, such as masts, do not present a danger to vessels navigating over the obstruction.

Publication

Nautical charts are issued by power of the national

hydrographic office A hydrographic office is an organization which is devoted to acquiring and publishing hydrographic information. Historically, the main tasks of hydrographic offices were the conduction of hydrographic surveys and the publication of nautical ch ...

s in many countries. These charts are considered "official" in contrast to those made by commercial publishers. Many hydrographic offices provide regular, sometimes weekly, manual updates of their charts through their sales agents. Individual hydrographic offices produce national chart series and international chart series. Coordinated by the

International Hydrographic Organization The International Hydrographic Organization (IHO) (French: ''Organisation Hydrographique Internationale'') is an intergovernmental organization representing hydrography. the IHO comprised 102 member states. A principal aim of the IHO is to ...

, the international chart series is a worldwide system of charts ("INT" chart series), which is being developed with the goal of unifying as many chart systems as possible. There are also commercially published charts, some of which may carry additional information of particular interest, e.g. for yacht skippers.

Chart correction

The nature of a waterway depicted by a chart may change, and artificial aids to navigation may be altered at short notice. Therefore, old or uncorrected charts should never be used for navigation. Every producer of nautical charts also provides a system to inform mariners of changes that affect the chart. In the United States, chart corrections and notifications of new editions are provided by various governmental agencies by way of Notice to Mariners,

Local Notice to Mariners Local may refer to: Geography and transportation * Local (train), a train serving local traffic demand * Local, Missouri, a community in the United States Arts, entertainment, and media * ''Local'' (comics), a limited series comic book by Bria ...

, Summary of Corrections, and

Broadcast Notice to Mariners Broadcasting is the distribution of audio audiovisual content to dispersed audiences via a electronic mass communications medium, typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began with ...

. In the U.S., NOAA also has a printing partner who prints the "POD" (print on demand) NOAA charts, and they contain the very latest corrections and notifications at the time of printing. To give notice to mariners, radio broadcasts provide advance notice of urgent corrections. A good way to keep track of corrections is with a ''Chart and Publication Correction Record Card'' system. Using this system, the navigator does not immediately update every chart in the portfolio when a new ''Notice to Mariners'' arrives, instead creating a card for every chart and noting the correction on this card. When the time comes to use the chart, he pulls the chart and chart's card, and makes the indicated corrections on the chart. This system ensures that every chart is properly corrected prior to use. A prudent mariner should obtain a new chart if he has not kept track of corrections and his chart is more than several months old. Various Digital Notices to Mariners systems are available on the market such as Digitrace, Voyager, or ChartCo, to correct British

Admiralty chart Admiralty charts are nautical charts issued by the United Kingdom Hydrographic Office (UKHO) and subject to Crown Copyright. Over 3,500 Standard Nautical Charts (SNCs) and 14,000 Electronic Navigational Charts (ENCs) are available with the Admira ...

s as well as NOAA charts. These systems provide only vessel relevant corrections via e-mail or web downloads, reducing the time needed to sort out corrections for each chart. Tracings to assist corrections are provided at the same time. The

Canadian Coast Guard The Canadian Coast Guard (CCG; ) is the coast guard of Canada. Formed in 1962, the coast guard is tasked with marine search and rescue (SAR), communication, navigation, and transportation issues in Canadian waters, such as navigation aids and i ...

produces the Notice to Mariners publication which informs mariners of important navigational safety matters affecting Canadian Waters. This electronic publication is published on a monthly basis and can be downloaded from the Notices to Mariners (NOTMAR) Web site. The information in the Notice to Mariners is formatted to simplify the correction of paper charts and navigational publications. Various and diverse methods exist for the correction of electronic navigational charts.

Limitations

In 1973 the cargo ship ''MV Muirfield'' (a merchant vessel named after

Muirfield Muirfield is a privately owned golf links which is the home of The Honourable Company of Edinburgh Golfers. Located in Gullane, East Lothian, Scotland, overlooking the Firth of Forth, Muirfield is one of the golf courses used in rotation for T ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

) struck an unknown object in the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

in waters charted at a depth of greater than 5,000 metres (16,404 ft), resulting in extensive damage to her

keel The keel is the bottom-most longitudinal structural element of a watercraft, important for stability. On some sailboats, it may have a fluid dynamics, hydrodynamic and counterbalancing purpose as well. The keel laying, laying of the keel is often ...

. In 1983, HMAS ''Moresby'', a

Royal Australian Navy The Royal Australian Navy (RAN) is the navy, naval branch of the Australian Defence Force (ADF). The professional head of the RAN is Chief of Navy (Australia), Chief of Navy (CN) Vice admiral (Australia), Vice Admiral Mark Hammond (admiral), Ma ...

survey ship A survey vessel is any type of ship or boat that is used for underwater surveys, usually to collect data for mapping or planning underwater construction or mineral extraction. It is a type of research vessel, and may be designed for the pu ...

, surveyed the area where ''Muirfield'' was damaged, and charted in detail a previously unsuspected hazard to navigation, the

Muirfield Seamount The Muirfield Seamount is a submarine mountain located in the Indian Ocean approximately 130 kilometres (70 nautical miles) southwest of the Cocos (Keeling) Islands. The Cocos Islands are an Australian territory, and therefore the Muirfield Sea ...

. The dramatic accidental discovery of the Muirfield Seamount is often cited as an example of limitations in the vertical

geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame, or terrestrial reference frame) is a global datum reference or reference frame for unambiguously representing the positi ...

accuracy of some offshore areas as represented on nautical charts, especially on small- scale charts. A similar incident involving a passenger ship occurred in 1992 when the

Cunard The Cunard Line ( ) is a British shipping and an international cruise line based at Carnival House at Southampton, England, operated by Carnival UK and owned by Carnival Corporation & plc. Since 2011, Cunard and its four ships have been r ...

liner ''

Queen Elizabeth 2 ''Queen Elizabeth 2'' (''QE2'') is a retired British ocean liner. Built for the Cunard Line, the ship was operated as a transatlantic liner and cruise ship from 1969 to 2008. She was laid up until converted into a floating hotel, operating sin ...

'' struck a submerged rock off

Block Island Block Island is an island of the Outer Lands coastal archipelago in New England, located approximately south of mainland Rhode Island and east of Long Island's Montauk Point. The island is coterminous with the town of New Shoreham, Rhode Isl ...

in the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

. In November 1999, the

semi-submersible Semi-submersible may refer to a self-propelled vessel, such as: *Heavy-lift ship, which partially submerge to allow their cargo (another ship) to float into place for transport *Narco-submarine, some of which remained partially on the surface *S ...

heavy-lift ship A heavy-lift ship is a vessel designed to move very large loads that cannot be transported by normal ships. They are of two types: *''Semi-submersible'' ships that take on water ballast to allow the load—usually another vessel—to be fl ...

Mighty Servant 2 ''Mighty Servant 2'' was a 29,000-ton semi-submersible, heavy-lift ship operated by Dockwise. The ship drew worldwide attention in 1988 for transporting the mine-damaged from Dubai to Newport, Rhode Island. The ship was built in 1983 by Oshim ...

'' capsized and sank after hitting an uncharted single underwater isolated pinnacle of granite off Indonesia. Five crew members died and ''Mighty Servant 2'' was declared a total loss. More recently, in 2005 the submarine USS ''San Francisco'' ran into an uncharted

seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...

(sea mountain) about 560 kilometres (350 statute miles) south of Guam at a speed of , sustaining serious damage and killing one seaman. In September 2006 the

jack-up barge A jackup rig or a self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit ...

''Octopus'' ran aground on an uncharted sea mount within the

Orkney Islands Orkney (), also known as the Orkney Islands, is an archipelago off the north coast of mainland Scotland. The plural name the Orkneys is also sometimes used, but locals now consider it outdated. Part of the Northern Isles along with Shetland ...

(United Kingdom) while being towed by the tug ''Harold''. £1M worth of damage was caused to the barge and delayed work on the installation of a tidal energy generator prototype. As stated in the Mariners Handbook and subsequent accident report: "No chart is infallible. Every chart is liable to be incomplete".

Map projection, positions, and bearings

Historically the first projection, invented by

Marinus of Tyre Marinus of Tyre (, ''Marînos ho Týrios''; 70–130) was a List of Graeco-Roman geographers, geographer, Cartography, cartographer and mathematician, who founded mathematical geography and provided the underpinnings of Claudius Ptolemy's i ...

ca. AD 100 according to

Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

, was what is now called

equirectangular projection The equirectangular projection (also called the equidistant cylindrical projection or la carte parallélogrammatique projection), and which includes the special case of the plate carrée projection (also called the geographic projection, lat/l ...

(historically called plane chart, ''plate carrée'', ). While it is very convenient for small seas like the Aegean, it is unsuitable for seas larger than Mediterranean or an open ocean, even though early explorers had to use it for want of a better. The Mercator projection is now used on the vast majority of nautical charts. Since the Mercator projection is conformal, that is, bearings in the chart are identical to the corresponding angles in nature, courses plotted on the chart may be used directly as the course-to-steer at the helm. The

gnomonic projection A gnomonic projection, also known as a central projection or rectilinear projection, is a perspective projection of a sphere, with center of projection at the sphere's center, onto any plane not passing through the center, most commonly a tan ...

is used for charts intended for plotting of great circle routes.

NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

uses the polyconic projection for some of its charts of the

Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes spanning the Canada–United States border. The five lakes are Lake Superior, Superior, Lake Michigan, Michigan, Lake Huron, H ...

, at both large and small scales. Positions of places shown on the chart can be measured from the

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

and

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

scales on the borders of the chart, relative to a

such as

WGS 84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descri ...

. A bearing is the angle between the line joining the two points of interest and the line from one of the points to the north, such as a ship's

course Course may refer to: Directions or navigation * Course (navigation), the path of travel * Course (orienteering), a series of control points visited by orienteers during a competition, marked with red/white flags in the terrain, and corresponding ...

or a compass reading to a landmark. On nautical charts, the top of the chart is always

true north True north is the direction along Earth's surface towards the place where the imaginary rotational axis of the Earth intersects the surface of the Earth on its Northern Hemisphere, northern half, the True North Pole. True south is the direction ...

, rather than

magnetic north The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...

, towards which a compass points. Most charts include a

compass rose A compass rose or compass star, sometimes called a wind rose or rose of the winds, is a polar coordinates, polar diagram displaying the orientation of the cardinal directions (north, east, south, and west) and their points of the compass, inter ...

depicting the variation between magnetic and true north. However, the use of the Mercator projection has drawbacks. This projection shows the lines of longitude as parallel. On the real globe, the lines of longitude converge as they approach the north or south pole. This means that east–west distances are exaggerated at high latitudes. To keep the projection conformal, the projection increases the displayed distance between lines of latitude (north–south distances) in proportion; thus a square is shown as a square everywhere on the chart, but a square on the Arctic Circle appears much bigger than a square of the same size at the equator. In practical use, this is less of a problem than it sounds. One minute of latitude is, for practical purposes, a nautical mile. Distances in nautical miles can therefore be measured on the latitude gradations printed on the side of the chart.

Electronic and paper charts

Conventional nautical charts are printed on large sheets of paper at a variety of scales. Mariners will generally carry many charts to provide sufficient detail for the areas they might need to visit.

Electronic navigational chart An electronic navigational chart (ENC) is an official database created by a national hydrographic office for use with an Electronic Chart Display and Information System (ECDIS). ECDIS and ENCs are the primary means of electronic navigation on c ...

s, which use computer software and electronic databases to provide navigation information, can augment or in some cases replace paper charts, though many mariners carry paper charts as a backup in case the electronic charting system fails.

Details on a nautical chart

Many countries' hydrographic agencies publish a "Chart 1", which explains all of the symbols, terms and abbreviations used on charts that they produce for both domestic and international use. Each country starts with the base symbology specified in IHO standard INT 1, and is then permitted to add its own supplemental symbologies to its domestic charts, which are also explained in its version of Chart 1. Ships are typically required to carry copies of Chart 1 with their paper charts.

Labels

Nautical charts must be labeled with navigational and depth information. There are a few commercial software packages that do

automatic label placement Automatic label placement, sometimes called text placement or name placement, comprises the computer methods of placing labels automatically on a map or chart. This is related to the typographic design of such labels. The typical features depicted ...

for any kind of map or chart. Modern systems render electronic charts consistent with the IHO S-52 specification, issued by the

(IHO).

Pilotage information

The chart uses symbols to provide

pilotage Piloting or pilotage is the process of navigating on water or in the air using fixed points of reference on the sea or on land, usually with reference to a nautical chart or aeronautical chart to obtain a fix of the position of the vessel or air ...

information about the nature and position of features useful to navigators, such as sea bed information,

sea mark A sea mark, also seamark and navigation mark, is a form of aid to navigation and pilotage that identifies the approximate position of a maritime channel, hazard, or administrative area to allow boats, ships, and seaplanes to navigate safely. ...

, and landmarks. Some symbols describe the sea bed with information such as its depth, materials as well as possible

navigational hazards Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

such as

s. Other symbols show the position and characteristics of

navigational aid A navigational aid (NAVAID), also known as aid to navigation (ATON), is any sort of signal, markers or guidance equipment which aids the traveler in navigation, usually nautical or aviation travel. Common types of such aids include lighthouses, ...

s such as

buoy A buoy (; ) is a buoyancy, floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. History The ultimate origin of buoys is unknown, but by 1295 a seaman's manual referred to navig ...

lights Light is an electromagnetic radiation, part of which stimulates the sense of vision. Light or Lights may also refer to: Illumination * Lighting * Light bulb * Traffic light Arts and entertainment Music * Lights (musician) (born 1987), Canadian s ...

lighthouse A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lens (optics), lenses and to serve as a beacon for navigational aid for maritime pilots at sea or on inland waterways. Ligh ...

s, coastal and land features and structures that are useful for

position fixing Geopositioning is the process of determining or estimating the geographic position of an object or a person. Geopositioning yields a set of geographic coordinates (such as latitude and longitude) in a given map datum. Geographic positions may a ...

. The abbreviation "ED" is commonly used to label geographic locations whose existence is doubtful. Colours distinguish between human-made features, dry land, sea bed that dries with the tide, and seabed that is permanently

underwater An underwater environment is a environment of, and immersed in, liquid water in a natural or artificial feature (called a Water, body of water), such as an ocean, sea, lake, pond, reservoir, river, canal, or aquifer. Some characteristics of the ...

and indicate water depth.

Depths and heights

Depths which have been measured are indicated by the numbers shown on the chart. Depths on charts published in most parts of the world use metres. Older charts, as well as those published by the United States government, may use

feet The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up of ...

fathom A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. The fathom is neither an international standard (SI) unit, nor an internationally accepted non-SI unit. H ...

s. Depth

contour line A contour line (also isoline, isopleth, isoquant or isarithm) of a Function of several real variables, function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a ...

s show the shape of underwater

relief Relief is a sculpture, sculptural method in which the sculpted pieces remain attached to a solid background of the same material. The term ''wikt:relief, relief'' is from the Latin verb , to raise (). To create a sculpture in relief is to give ...

. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from the

chart datum A chart datum is the water surface serving as origin (or coordinate surface) of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is ...

, which is related to the local sea level. The chart datum varies according to the standard used by each national

. In general, the trend is towards using

lowest astronomical tide A chart datum is the water surface serving as origin (or coordinate surface) of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is ...

(LAT), the lowest tide predicted in the full tidal cycle, but in non-tidal areas and some tidal areas

Mean Sea Level A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

(MSL) is used. Heights, e.g. a lighthouse, are generally given relative to

mean high water spring A chart datum is the water surface serving as origin (or coordinate surface) of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is ...

(MHWS). Vertical clearances, e.g. below a bridge or cable, are given relative to

highest astronomical tide A chart datum is the water surface serving as origin (or coordinate surface) of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is al ...

(HAT). The chart will indicate what datum is in use. The use of HAT for heights and LAT for depths, means that the mariner can quickly look at the chart to ensure that they have sufficient clearance to pass any obstruction, though they may have to calculate height of tide to ensure their safety.

Tidal information

Tidal race Tidal race or tidal rapid is a natural occurrence whereby a fast-moving tide passes through a constriction, resulting in the formation of waves, eddies and hazardous currents. The constriction can be a passage where the sides narrow, for example ...

s and strong currents have special chart symbols. Tidal flow information may be shown on charts using

tidal diamond Tidal diamonds are symbols on British admiralty charts and others that indicate the direction and speed of tidal streams. The symbols consist of a letter of the ISO basic Latin alphabet in a rhombus, printed in purple ink. On any particular cha ...

s, indicating the speed and bearing of the tidal flow during each hour of the tidal cycle.

References

External links

The Medieval and Early Modern Nautical Chart: Birth, Evolution and Use
Lisbon-based ERC-funded academic project. They develop and maintain th
MEDEA-CHART Database
a sophisticated search engine and aggregator of early nautical charts data.

* [https://web.archive.org/web/20090223175546/http://www.lumenartis.net/index.php?main_page=product_info&cPath=68&products_id=183&language=en&zenid=3e6dc909566a0f8e3fd73aa98aa30530 Portolan Chart of Gabriel de Vallseca, 1439] *
Nautical charts available online (Nautical Free)

Online Nautical Charts Viewer
{{DEFAULTSORT:Nautical Chart