Browsing by Author "Ssengendo, Ronald"

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    Computation of the Gravimetric Quasigeoid Model over Uganda Using the KTH Method
    (Sofia Bulgaria, 2015) Ssengendo, Ronald; Sjöberg, Lars E.; Gidudu, Anthony
    The gravimetric quasigeoid can be determined either directly by Stokes formula or indirectly by computing the geoid first and then determining the quasigeoid-to-geoid separation which is then used to determine the quasigeoid. This paper presents the computational results of the gravimetric quasigeoid model over Uganda (UGQ2014) based on the later technique. UGQ2014 was derived from the Uganda Gravimetric Geoid Model (UGG2014) which was computed by the technique of Least Squares Modification of Stokes formula with additive corrections commonly called the KTH Method. UGG2014 was derived from sparse terrestrial gravity data from the International Gravimetric Bureau, the 3 arc second SRTM ver4.1 Digital Elevation Model and the GOCE-only geopotential model GO_CONS_GCF_2_TIM_R5. The quasigeoid-to geoid separation was then computed from the Earth Gravitational Model 2008 (EGM08) complete to degree 2160 of spherical harmonics together with the global topographic model DTM2006.0 also complete to degree 2160. Another aim of this paper is to compare the approximate and strict formulas of computing the quasigeoid-to-geoid separation and evaluate their effects on the final quasigeoid model. Using 10 GNSS/levelling data points distributed over Uganda, the RMS fit of the quasigeoid model based on the approximate formula are 27 cm and 10 cm before and after a 4-parameter fit, respectively. Similarly, the RMS fit of the model based on the strict formula are 15 cm and 6 cm, respectively. The results show the improvement to the final quasigeoid model brought about by using the strict formula to model more effectively the terrain in the vicinity of the computation point. With an accuracy of 6 cm, UGQ2014 represents significant progress towards the computation of a final gravimetric quasigeoid over Uganda which can be used with GNSS/levelling. However, with more data especially terrestrial gravity data and GNSS/levelling we anticipate that the accuracy of gravimetric quasigeoid modelling will improve in future.
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    Geoid Determination In Uganda: Current Status
    (Advances in Engineering and Technology, 2017) Ssengendo, Ronald; Sjöberg, Lars.E.; Gidudu, Anthony
    Many professionals e.g. surveyors, engineers and GIS specialists are increasingly using Global Positioning System (GPS) or some other Global Navigation Satellite Systems (GNSS) for positioning and navigation. One of the greatest advantages of GPS is its ability to provide three-dimensional coordinates (latitude, longitude and height) anywhere in the world, any time irrespective of the weather. The GPS latitude and longitude can easily be transformed from the WGS84 reference system to a local reference (e.g. Arc 1960). However the GPSdetermined heights, i.e. ellipsoidal heights, are geometrical heights which have no physical meaning and therefore cannot be used in surveying and engineering projects. Their conversion to more meaningful orthometric heights require knowledge of the geoidal undulations, which can be determined from high resolution geoid models. Its absence in Uganda means that the full potential of GPS cannot be fully realized. This paper gives an overview of the need for an accurate geoid model in Uganda, the current status of the geodetic network in Uganda and different methods of geoid determination. Pending further investigation, preliminary findings indicate that in Uganda, the EGM2008 is the best geoid modal for GPS/leveling projects.
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    Towards a New Height Datum for Uganda
    (South African Journal of Geomatics, 2020) Ssengendo, Ronald; Gidudu, Anthony
    A new height datum for Uganda is computed using the corrective surface principle. It is based on a combination of the Uganda Gravimetric Quasigeoid model (UGQ) 2014 and GNSS/levelling. UGQ2014 was derived from the Uganda Gravimetric Geoid model (UGG) 2014, which was computed from sparse terrestrial gravity data from the International Gravimetric Bureau, the 3 arc second Shuttle Radar Topography Mission digital elevation model and the GOCE – only global geopotential model GO_CONS_GCF_2_TIM_R5. The corrective surface was constructed based on 21 discrete GNSS/levelling points and then evaluated with 4 independent points. Three interpolation techniques were tested for the creation of the corrective surface with the Kriging method giving the lowest standard deviation and noise level suggesting that it is the best method for interpolation. In absolute terms, the Root Mean Square of the fit between the known and computed normal-orthometric heights based on the new height datum is 11cm, which is about 5cm (31%) better than using UGQ2014 alone. For relative heights an average precision of 29 ppm is computed for all baselines tested. Both the absolute and relative tests show that the new height datum satisfies the precision and accuracy requirements of third order precise levelling. Therefore, UGQ2014C represents a significant step towards the determination of a precise new height datum for Uganda.