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
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).