Navigation charts published by both military and civilian agencies responsible for nautical chart publication in the United States had information on magnetic variation, the difference between true and magnetic north. A standard feature was the compass rose with true north in an outer circle and magnetic north in an inner circle with annual change noted so that navigators could make a correction until a new chart or compass rose update was made. Special world charts were published with isometric lines for magnetic variation (isogonic chart) and magnetic dip (isoclinic chart).
