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A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. It shows depths of water and heights of land, natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids to navigation, information on tides and currents, local details of the Earth's magnetic field, and man-made structures such as harbours and bridges. It is an indispensable tool of navigation.
Projection and chart datum
The Mercator projection is almost universally used in nautical charts. There are however some exceptions for very large or small scales where projections such as the gnomonic projection may be used. Since the Mercator projection is 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.
As angles are widely used for measurement in navigation, two types of angle are shown on charts; bearings and the longitude and latitude of positions. Positions of places shown on the chart can be measured from scales on the borders of the chart, using longitude and latitude relative to a map datum such as WGS 84. An example of a bearing is a ship’s course, being the angle between the ship’s heading and north. Another example is the bearing from one place to another, being the angle between the line joining the two points and the line from one of the points to the north.
Charts always express bearings relative to true north, at the north pole, rather than magnetic north, in northern Canada, which a magnetic compass points towards. To convert between "magnetic" and "true", the magnetic variation must be applied to the bearing. Often a compass rose on the chart provides the variation, allowing conversion of bearings between true and magnetic.
Sources and Publication of nautical charts
Nautical charts are based on hydrographic surveys. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and not always reliable.
Nautical charts are issued by the national hydrographic offices 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.
Coordinated by the International Hydrographic Organization, a worldwide system of charts ("INT" chart series) is being developed with the goal of unifying as many chart systems as possible. Currently, the Admiralty Charts series issued by the United Kingdom Hydrographic Office covers almost every navigable area on earth.
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