The Earth magnetic field deduced from many observations on land, sea and atmosphere, is represented by the operation of a compass. The direction of the Earth's magnetic field at a point on the surface of the Earth is the direction taken by a freely sudpend magnetic needle. The Earth main field could be demonstrated to be similar in shape to that of pair of electric charges with opposite signs. The Earth magnetic field is considered to be the sum of two parts; the main geomagnetic field and the crustal field. The main geomagnetic field is believed to be generated by the electric currents in the Earth's fluid core. Crustal field is caused by presence of the magnetic matter in the Earth's crust. The intensity of the main geomagnetic field ranges from a lower bound of less 24,000 nanotesla (nT) to an upper bound of more than 60,000 nanotesla (nT) in the polar regions, but, the intensity of the crustal field is no more than several hundreds of nanotesla. Thus, the Earth magnetic field could essentially represented as the main geomagnetic field. The main field undergoes slow changes with the passage of time such change is called a secular variation of the main field [Peddie 83], Field can be represented by a function of radial distance (r), Colatitude (θ), and Longitude (φ) , in the spherical coordinate system. When we consider field measurements made over a closed sphere that has outside currents (external field) and within currents (Internal field], but no current flowing between the two locations. The external field came from the current system in the ionospheric and magnetospheric regions. Maxwell equation provide a method for distinguishing the direction of the two sources and their dependence. Gauss found a way to represent a special solution for Maxwell's equations, for global magnetic field observations, that separates the field contribution on r, θ and φ, and is composed of two series of terms in ascending powers of r and (1/r). The two series, represented by spherical harmonics, separate the observed surface field into two parts that originated from the external and internal currents. The external and internal terms of the series can be considered to result from individual contributing source currents. The field modeling utilized special functions called Legendrere Polynomials. These polynomials define the spherical harmonic series of polynomial terms that are tabulated as "Gauss Coefficients" g and h wi t h distinguishing n and m indices. A group of geomagnetism field modelers belong to the International Association of Geomagnetism and Aeronomy (IAGA) periodically examined various field representations for accuracy in reproduction of actual field determinations. They select the best "Gauss Coefficients" to represent a particular epoch and entitle this the "Interne-twnal Geomagnetic Reference Field" (IGRF). IGRF are periodically updated and published to represent the fields for each 5-ycars epoch.