دیکشنری تخصصی مهندسی نفت - نسخه آنلاین

n: an imaginary line on a map that joins places on the earths surface at which the variation of a magnetic compass needle from true north is the same. This variation, which may range from 0 to 30 or more degrees either east or west of true north, must be compensated for to obtain an accurate reading of direction.