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, informations on tides and currentss, local details of the Earth's magnetic field, and man-made structures such as harbours and bridges. It is an indispensable tool of navigation.
The Mercator projection is universally used in nautical charts (there are however some exceptions for very large or small scales). The reason for this is that in navigation, directions are treated as angles, e.g. a ship´s course is described as the angle between the ship´s heading and a reference, usually true (geographic) north. Since the Mercator projection is conformal, that is, angles in the chart are identical to the corresponding angles in nature, directions may be entered into and taken out of a chart with a minimum of conversion.
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.