Analysis of some factors that affect accuracy in Long wavelength geoid determination using GrafLAB

Abstract

In recent times, the advent of computer basedtools for long wavelength geoid computation has popularized the ease of long wavelength geoid and gravity anomaly computation. Evaluating these gravity field functionals however require a proper understanding of the effects of certain computational preferences in the overall geoid computation process as was applied in the study area of Minna. Four computational factors were identified for analysis in this study using the grafLab program. The factors considered are: the Associated Legendre Function (ALF), choice of maximum degree of expansion of harmonic model, choice of geo-potential model (EGM96 and EGM2008) and choice of ellipsoidal surface (WGS 84 and GRS 80). The results obtained confirm the numerical stability of the ALF’s even at ultra high degrees within the study area (Minna metropolis of Niger state). However, the degree of expansion and ellipsoid of computation greatly affect the result of the computed long wavelength geoid. A geoid undulation difference ranging between 0.93m – 0.94m was observed between the WGS84 and GRS80 ellipsoid while haphazard fluctuation in values was observed between different degrees of expansion.Long wavelength geoidmaps of the study area wasproduced from the maximum degree expansion of each harmonic model used.