Nautical chart
238244
214257392
2008-05-22T19:44:30Z
Mark.murphy
42906
[[Image:NOAA chart 25664 1976.png|right|thumb|A 1976 United States [[National Oceanic and Atmospheric Administration|NOAA]] chart of part of [[Puerto Rico]]]]
A '''nautical chart''' is a graphic representation of a [[Sea|maritime]] area and adjacent [[coast]]al regions. Depending on the [[scale (map)|scale]] of the chart, it may show depths of [[water]] and heights of land ([[topographic map]]), natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids to [[navigation]], information on [[tide]]s and [[Current (water)|current]]s, local details of the [[Earth's magnetic field]], and man-made structures such as [[harbour]]s, buildings and [[bridge]]s. Nautical charts are essential tools for marine navigation; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper or computerised [[electronic navigational chart]]s.
==Sources and publication of nautical charts==
Nautical charts are based on [[hydrographic survey]]s. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and not always reliable. Depths are measured in a variety of ways. Historically the [[sounding line]] was used. In modern times, [[echo sounding]] 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]], the minimum depth is checked by sweeping the area with a length of horizontal [[wire]]. This ensures that difficult to find projections, such as [[Mast (sailing)|masts]], do not present a danger to vessels navigating over the obstruction.
Nautical charts are issued by the national [[hydrographic office]]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 office]]s produce national chart series and international chart series. Coordinated by the [[International Hydrographic Organization]], 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]], [[Summary of Corrections]], and [[Broadcast Notice to Mariners]]. Radio broadcasts give advance notice of urgent corrections.
A convenient 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.
Various and diverse methods exist for the correction of electronic navigational charts.
==Map projection, positions, and bearings==
[[Image:Fernão Vaz Dourado 1571-1.jpg|thumb|right|250px|A pre-Mercator nautical chart of 1571, from Portuguese cartographer Fernão Vaz Dourado (c. 1520-c.1580). It belongs to the so-called ''plane chart'' model, where observed latitudes and magnetic directions are plotted directly into the plane, with a constant scale, as if the Earth were plane (Portuguse National Archives of Torre do Tombo, Lisbon)]]
The [[Mercator projection]] is almost universally used in nautical charts. There are however some exceptions for very large or small [[Scale (measurement)|scales]] where projections such as the [[gnomonic projection]] may be used. Since the Mercator projection is [[conformal map|conformal]], that is, bearings in the chart are identical to the corresponding angles in nature, bearings may be measured from the chart to be used at sea or plotted on the chart from measurements taken at sea.
Positions of places shown on the chart can be measured from the [[longitude]] and [[latitude]] scales on the borders of the chart, relative to a [[map datum]] such as [[WGS 84]].
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 (navigation)|course]] or a compass reading to a landmark. On nautical charts, the top of the chart is always [[true north]], rather than [[magnetic north]], towards which a [[magnetic compass]] points. Most charts include a [[compass rose]] depicting the [[Magnetic declination|variation]] between magnetic and true north.
==Electronic and paper charts==
[[Image:US_NOAA_nautical_chart_of_Bering_Strait.png|right|thumb|Portion of an electronic chart of the [[Bering Strait]]]]
Conventional nautical charts are printed on large sheets of paper at a variety of [[scale (map)|scale]]s. Mariners will generally carry many charts to provide sufficient detail for the areas they might need to visit. [[Electronic navigational chart]]s, which use computer software and electronic databases to provide navigation information, can augment or in some cases replace paper charts, though most mariners carry paper charts as a back up in case the electronic charting system fails.
===Labeling nautical charts===
[[Image:Nautical-chart-1.jpg|left|thumb|Automatically labeled nautical chart]]
Nautical charts must be labeled with navigational and depth information. There are a few software solutions in the market that do label placement automatically for any kind of map or chart. For more information visit the Wikipedia article on [[Automatic label placement]].
==Details on a nautical chart==
===Pilotage information===
[[Image:NOAA_chart_25664_1976_detail.png|right|thumb|Detail of a United States NOAA chart, showing a harbour area]]
The chart uses symbols to provide [[pilotage]] information about the nature and position of features useful to navigators, such as sea bed information, [[seamark]]s and landmarks. Some symbols describe the sea bed with information such as its depth, depth contours, materials as well as possible hazards such as [[shipwreck]]s. Other symbols show the position and characteristics of [[buoy]]s, [[characteristic light|lights]], [[lighthouse]]s, coastal and land features and structures that may be useful for [[position fixing]].
Colour may be used to distinguish between man-made features, dry land, sea bed that dries with the tide and seabed that is permanently [[underwater]] and to indicate water depth.
===Depths===
[[image:British Admiraly chart colours.svg|right|thumb|250px|Use of colour in British Admiralty charts]]
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 [[metre]]s. Older charts, as well as those published by the United States government, may use [[Foot (unit of length)|feet]] or [[fathom]]s. Depth [[contour]] lines show the underwater [[topography]]. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from the [[chart datum]], which will be stated on the chart; this is often the depth of the lowest tide caused by [[gravity]] alone which is known as the "Lowest Astronimical Tide".
===Tidal information===
[[Tidal race]]s and other strong currents have special chart symbols. Tidal flow information may be shown on charts using [[tidal diamond]]s, indicating the speed and bearing of the tidal flow during each hour of the tidal cycle.
==See also==
*[[Aeronautical chart]]
*[[Automatic label placement]]
*[[Nautical publications]]
*[[American Practical Navigator]]
*[[Coast Pilots]]
*[[Light List]]
*[[List of Lights]]
*[[Local Notice to Mariners]]
*[[Notice to Mariners]]
*[[Sailing Directions]]
*[[World Port Index]]
*[[Distances Between Ports]]
*[[Admiralty chart]]
==External links==
* [http://chartmaker.ncd.noaa.gov/mcd/chart1/chart1hr.htm Online version of ''Chart No.1'' with "Symbols, Abbreviations and Terms" used in nautical charts]
* [http://www.irbs.com/bowditch/pdf/chapt03.pdf Nautical Charts] - chapter from the online edition of [[Nathaniel Bowditch]]'s ''American Practical Navigator''
* [http://coastmariner.com/NY/bin/chart12327_1.html Sample Chart] A sample chart of New York Harbor
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[[Category:Cartography]]
[[Category:Navigation]]
[[Category:Hydrography]]
[[Category:Infographics]]
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[[es:Carta náutica]]
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