Definition
Difference of latitude is the arc between two parallels, expressed in degrees and minutes, where one minute equals one nautical mile everywhere. Difference of longitude is the arc between two meridians, whose linear value diminishes with latitude. Both are signed by direction, north and east conventionally positive. They are the fundamental quantities in plane, parallel and Mercator sailing calculations, and are the output of any dead reckoning arithmetic performed numerically rather than by plotting.
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