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A map primarily intended for navigation, one of the earliest being the plane chart, hence plain sailing. In very general terms, two types of nautical chart are used at sea, the straightforward Mercator projection chart on which rhumb lines appear as straight lines; and gnomonic charts on which great circles appear as straight lines. Normally ships sailing from one place to another steer rhumb line courses and use Mercator projection charts. Gnomonic charts are used to display great circles when that is necessary while special lattice charts are used for plotting fixes from hyperbolic navigation fixing aids. The nautical chart is essentially a map of a sea area, showing coastlines and soundings with all hazards such as rocks as well as the position of buoys, lighthouses, and other visual aids to navigation. Charts are kept up to date by the regular issue of notices to mariners by the charting authority. A compass rose on the navigational chart enables the mariner to plot courses and bearings and to find the local variation and its rate of change. See also chartmaking; isobath; isogonic lines; portulan chart.

Subjects: Maritime History.

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