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