International Journal Of Environment And Geoinformatics

 Journal

 Determination Of Orthometric Heights Of Points Using Gravimetric/GPS And Geodetic Levelling Approaches

 Herbert Tata And Eteje Sylvester Okiemute

 2022

The importance of practical, as well as orthometric heights in engineering cannot be underestimated as it is required for the determination of proposed construction levels and to direct the flow of water. This study presents the determination of ortho metric heights of points using gravimetric/GPS and geodetic levelling approaches and compares the resolution of the two approaches to determine which of the methods is better for orthometric height determination in the study area. A total of 59 stations were occupy for gravity observation using Lacoste and Romberg (G-512 series) gravimeter to obtain the absolute gravity values of the points. GNSS observation was carried out in static mode using South GNSS receivers to obtain the positions and ellipsoidal heights of the points. The modified Stokes’ integral was applied to obtain the geoid heights of the points. Similarly, levelling was carried out using the geodetic level to obtain the level heights of the points. The orthometric correction was applied to the geodetic levellin g results to obtain precise level heights of the points. The RMSE index was applied to compute the accuracy of the geoid models. The computed result shows that orthometric heights can be obtained in the study area using the two models with an accuracy of 0.3536m. Z -test was carried out to determine if there is any significant difference between the two methods. The test results show that statistically, there is no significant difference between the two methods. Hence, the two methods can be applied for orthometric heights determinatio n in the study area. 

 Journal Of Geospatial Information Science And Engineering

 Journal

 Comparison Of Two Corrector Surface Models Of Orthometric Heights From GPS/Levelling Observations And Global Gravity Model

 Ibrahim Olatunji Raufu And Herbert Tata

 2022

The advent of space-based measurement systems such as the Global Positioning System (GPS) offers a new alternative in orthometric height determination over conventional spirit levelling. The ellipsoidal height (h) obtained from GPS observations can be transformed into orthometric height if the geoidal height (N) is known from a national gravimetric geoid model. However, the lack of a national geoid model in Nigeria hinders the use of the method. This study compares two corrector surface models of orthometric heights from GPS/levelling observations and the Global Gravity Model. Model A (7-parameter) and Model B (8-parameter) are based on the general 7-parameter similarity datum shift transformation. A network of twenty-one (21) GPS/levelling benchmarks within the study area were used and their geoidal heights were computed using GeoidEval utility software with reference to Global Gravitational Model (EGM08). Least squares adjustment was used to compute the coefficients of the models. Root mean square error (RMSE) was used to assess the accuracy of the models with model A having RMSE=0.171m and model B having RMSE=0.169m. Model B with the lowest RMSE is hence the better of the two models. The t-test and hypothesis tests conducted at a 95% confidence level, however, revealed that the two models did not differ significantly. The study shows that the use of a corrective surface to c ombine the gravity field model EGM08 with GPS/levelling significantly improves the determination of heights as observed from GPS in the study area. 

 Geodetski Glasnik,

 Journal

 ACCURACY ASSESSMENT OF DIFFERENT POLYNOMIAL GEOID MODELS IN ORTHOMETRIC HEIGHT DETERMINATION FOR AKURE, NIGERIA

 Ibrahim Olatunji Raufu, Herbert Tata

 2021

Height is an important component in three dimensional coordinates and determination of the position of points for any meaningful development. Ellipsoidal heights from GNSS require geoid model which could be global, regional and local for transformation to orthometric height. The absence of a national geoid model in Nigeria remains a great drawback to develop local geoid for local application in place of global geoid models. The study aims to assess the accuracy of polynomial geoid models in orthometric height determination. Differential Global Positioning System (DGPS) observations were carried out to determine ellipsoidal heights of the point while nine and eleven coefficients were used for the geoid and orthometric height modelling. Model A and Model C used 2-D (x, y) positions with nine and eleven parameters while model B used 3-D (x, y, ?h) positions with nine parameters. The least-squares method was adopted in computing the parameters of the models. Root Mean Square Error (RMSE) was used to assess the accuracy of the models with the RMSE of model A is 14.3 cm, model B is 15.7 cm and model C is 14.5 cm, respectively. The inclusion of height term (?h) in model B does not improve the accuracy over model A and model C. Model A with the lowest RMSE is hence the better of the three models. One-way ANOVA test conducted at 95% confidence level, however, revealed that the three models did not differ significantly. Model A having lower RMSE is recommended with GPS determined ellipsoidal heights as an alternative to conventional spirit levelling for orthometric height determination within Akure for engineering and environmental applications. 

 J. Appl. Sci. Environ. Manage.

 Journal

 Integration Of GPS And GIS In Mapping Of Secondary Schools In Akure South Local Government Area Of Ondo State Nigeria

 TATA, H; OJO, E. P

 2021

Tropospheric delay is a major error caused by atmospheric refraction in Global Navigation Satellite System (GNSS) positioning. The study evaluates the potential of the European Centre for Medium-range Weather Forecast (ECMWF) Reanalysis 5 (ERA5) atmospheric variables in estimating the Zenith Tropospheric Delay (ZTD). Linear regression models (LRM) are applied to estimate ZTD with the ERA5 atmospheric variables. The ZTD are also estimated using standard ZTD models based on ERA5 and Global Pressure and Temperature 3 (GPT3) atmospheric variables. These ZTD estimates are evaluated using the data collected from the permanent GNSS continuously operating reference stations in the Nigerian region. The results reveal that the Zenith Hydrostatic Delay (ZHD) from the LRM and the Saastamoinien model using ERA5 surface pressure are of identical accuracy, having a Root Mean Square (RMS) error of 2.3 mm while the GPT3-ZHD has an RMS of 3.4 mm. For the Zenith Wet Delay (ZWD) component, the best estimates are derived using ERA5 Precipitable Water Vapour (PWV). These include the ZWD derived by the LRM having an average RMS of 20.9 mm and Bevis equation having RMS of 21.1 mm and 21.0 mm for global and local weighted mean temperatures, respectively. The evaluation of GPT3-ZWD estimates gives RMS of 45.8 mm. This study has provided a valuable insight into the application of ERA5 data for ZTD estimation. In line with the fndings of the study, the ERA5 atmospheric variables are recommended for improving the accuracy in ZTD estimation, required for GNSS positioning