A
nautical chart is a graphic representation of a
maritime area and adjacent
coastal regions. Depending on the
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
tides and
currents, local details of the
Earth's magnetic field, and man-made structures such as
harbours, buildings and
bridges. 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 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. 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 masts, do not present a danger to vessels navigating over the obstruction.
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. Individual hydrographic offices 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.
