If be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip is given by
Let and be the horizonal and vertical components of earth's magnetic field . Since is the angle of dip
Suppose planes 1 and 2 are two mutually perpendicular planes and respectively make angles and 90°- with the magnetic meridian. The vertical components of earth's magnetic field remain same in the two planes but the effective horizontal components in the planes will be