Departure (dep)

Departure, abbreviated dep, is the east-west distance a vessel makes good, measured in nautical miles along a parallel rather than in arc of longitude. In plane and parallel sailing it is the easting or westing component of the run, paired with difference of latitude as the north-south component. The defining relation is departure equals difference of longitude times the cosine of the latitude, converting a change of longitude in minutes of arc into a distance in nautical miles. The navigator uses it to resolve a course and distance into d.lat and dep, or combine them back into a course and distance. The cosine term shrinks departure toward the poles.