chart, term referring to maps prepared for marine navigation and for air navigation. All charts show, in some convenient scale, geographic features useful to the navigator, as well as indications of direction, e.g., true north (the direction of the geographic North Pole), magnetic north (the direction indicated by the north-seeking end of a magnetic compass needle), and magnetic declination (the difference between these two directions). Data shown on marine charts include the outline and nature of coasts, with landmarks; currents and undercurrents (both direction and force); winds; tides; location and type of lighthouses, buoys, beacons, and lightships; position of rocks, bars, reefs, shoals, wrecks, or other dangers; contour and nature of bottom (mud, sand, rock, or gravel); and depth. Depth is indicated in great detail in harbors and shallow and intricate waterways; the value indicated is usually that at mean low water. Most national governments publish charts of their coasts and harbors; the British admiralty has done the most work along these lines. In the United States the Coast and Geodetic Survey and the Hydrographic Office of the Dept. of the Navy issue charts; these are drawn using the gnomonic or Mercator map projections. Aeronautical charts show natural or man-made surface features by the use of various symbols. These charts give locations of radio-navigation stations and graphic representations of the directional information they broadcast; radio communication channels of airports and spacecraft centers; standard flight paths; and dangerous or forbidden areas (e.g., certain military installations). Elevations on the earth's surface are indicated by contour lines. The U.S. Coast and Geodetic Survey issues many kinds of aeronautical charts.