Author: Agaje Atebije Bruno
Department: Surveying and Geoinformation
Affiliation: Nnamdi Azikiwe University Awka
The aim of this project is to develop a gravimetric local geoid model for interpolating geoid undulation values to be used for transformation of ellipsoidal heights to Orthometric heights and vice-versa within the study area.. Ellipsoidal height is one of the three dimensional components of the GPS. The wide spread use of satellite- based positioning techniques, especially the GPS has lead to greater attention being placed on the precise determination of geoid undulations at the local, regional or global levels. Geoid models are being evaluated with the aim of directly determining the elevations of points within the confines of the model without necessary carrying out the much tedious spirit leveling techniques as done during geodetic works. In this project, the geoid undulations of the study area were evaluated. In implementing this work, terrestrial point gravity data was obtained using the absolute and relative methods of gravity data acquisition .Gravity data was obtained randomly within the confines of the study area as defined by the coordinate framework of: latitude from (070 06’ 55” to 070 09’ 55”) and longitude from (060 46’ 01” to 060 49’ 01”), gridded into thirty-six compartments at 30” interval. Gravity data reductions were carried out via the free air and bouguer plate corrections. A gravimetric geoid undulation model was thus computed from the available 295 gridded gravity anomalies using the modified stokes integral .Statistical analysis showed that the mean geoid undulation was (20.895m).Further analysis showed that the standard deviation and the variance as computed were found to be (0.7534357 and 0.5676653) respectively The evaluated geoid undulation profile was validated and interpolated using required data sets. In this respect, the standard deviation of geoid undulation as computed from stokes integral as compared to that obtained from the NAVCO Geoid height software were found to be (0.3157965) and 0.313212401) respectively. Similarly, their respective variances of geoid undulation were found to be (1.349578061) and (0.098102024) respectively. The average covariance function of the free air gravity anomalies was evaluated to a value of (11811.059) milligals and had an estimated root mean square (r.m.s) free air gravity anomaly value of 108.679 milligals. This determined local geoid model including the data sets will in no small measure contribute to the robustness of the national geodetic data infrastructure of the state in particular and the nation at large. Conclusion was drawn to the effect that The Gravimetric Geoid undulation and hence the geoid depicting Okenya has been successfully computed from 295 gridded gravity Anomalies with the mean Geoid Undulation value found to be 20.285m. It was recommended that all the required data sets such as Point gravity values, gravity anomaly data, Digital Terrain Models (DTMs), and Geo-potential models be made available in a viable and user friendly Database. In the same vein, efforts should be intensified for the realization of the completion of the gravity network of Nigeria, which had long been abandoned. It was recommended that all militating factors against the realization of the Nigerian geoid be addressed by the government so as to enjoy the derivable benefits associated with an effective and workable geoid model in varied surveying applications.
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