The Earth's magnetic field is a vector quantity; at each point in space it has a strength and a direction. To completely describe it we need three quantities. These may be:
- three orthogonal strength components (X, Y, and Z);
- the total field strength and two angles (F, D, I); or
- two strength components and an angle (H, Z, D)
The relationship between these 7 elements is shown in the diagram.
|the total intensity of the magnetic field vector
|the horizontal intensity of the magnetic field vector
|the vertical component of the magnetic field vector; by convention Z is positive downward
|the north component of the magnetic field; X is positive northward
|the east component of the magnetic field; Y is positive eastward
|magnetic declination, defined as the angle between true north (geographic north) and the magnetic north (the horizontal component of the field). D is positive eastward of true North.
|magnetic inclination, defined as the angle measured from the horizontal plane to the magnetic field vector; downward is positive
D and I are measured in degrees. All other elements are measured in nanotesla (nT; 1 nT = 10-9 Tesla).
The seven elements are related through the following simple expressions.