Alternate Definitions for geomagnetic elements

The components of the geomagnetic field at the surface of the Earth. These elements are usually denoted thus in the literature:

X-the geographic northward component
Y -the geographic eastward component
Z-the vertical component, reckoned positive downward
H-the horizontal intensity, of magnitude (X2 + y2 )1/2
F-the total intensity (H2 + Z2 )1/2
I-the inclination (or dip) angle, tan -1 (Z/H)
D-the declination angle, measured from the geographic north direction to the H component direction, positive in an eastward direction.
D = tan-l (Y/X)

However, in NOAA use, the geomagnetic northward and geomagnetic eastward components are called the H and D components. The H axis direction is defined by the mean direction of the horizontal component relative to the geomagnetic north by using the small-angle approximation. Thus the D component = H (the horizontal intensity) multiplied by delta D (the declination angle relative to geomagnetic north, expressed in radians).