Since geomagnetic field is a vector field, at least three
elements (components) are necessary to represent the field.
The elements describing the direction of the field are
declination (**D**), inclination (**I**).
**D** and **I** are measured in units of degrees.
**D** is the angle between magnetic north and true north and
positive when the angle measured is east of true north and
negative when west. **I** is the angle between
the horizontal plane and the total field vector.
Elements describing the field intensity is the total intensity (**F**),
horizontal component (**H**), vertical component (**Z**),
and the north (**X**) and east (**Y**)
components of the horizontal intensity.
These elements are generally expressed in units of
in nanoTesla (10^{-9} Tesla /
10^{-5} Gauss or 1 Gamma in CGS).
Combinations of the three elements frequently used in geomagnetism are
HDZ, XYZ and FDI.

Principal equations relating the values of the elements are as follows:

F=(X^{2}+Y^{2}+Z^{2})^{1/2}
=(H^{2}+Z^{2})^{1/2}

H=F*cos(I), Z=F*sin(I), X=H*cos(D), Y=H*sin(D)