Assessing The Reliability Of Dual Frequency Differential GNSS Observations For Orthometric Height Determination In Parts Of Accra, Ghana.

Abstract

Height refers to the vertical distance of a point above or below a reference datum. Ordinary Level offers accurate height values, but its use is time-consuming, labor intensive and limited in sighting distances. The GNSS technology is making such traditional methods of surveying unpopular in Ghana. This study uses simple linear regression analysis, root mean square errors, calculation of residuals and graphs to assess the reliability of differential GPS/GLONASS observations made with dual frequency receivers and compared with MSL heights determined trigonometrically.  The study area covers the major roads in Accra. The road sections with a total chainage of about 13km includes a section of George Walker Bush Motorway (N1), Olusegun Obasanjo Highway and a section of the Liberation road.The Trimble combined GNSS receivers (R4) and the Sokkia 1’’ Total Station (Set 310) were used for the data collection. The Geoidal separations (undulations) for the study area were estimated using Alltrans EGM2008 Calculator 1.2.  The analysis showed that the combined GNSS derived Orthometric heights are reliable and comparable to the MSL heights determined trigonometrically. The arithmetic residuals between the derived Orthometric heights and MSL heights ranged between -0.5025m to +1.4365m with an average residual of +0.5730m. The root mean square error (RMSE) of the observation was estimated as 0.7198m, while the Correlation Coefficient was 0.9981; denoting a measure of higher quality of fitbetween the GNSS derived orthometric heights and those of MSL from Trigonometry. The average least squares residual from the regression analysis was estimated in absolute terms, to be 0.5970m. This accuracy in height can be used to produce contour maps at a smaller scale in Ghana with respect to the national survey and mapping specifications. It is recommended that, surveyors and engineers in Ghana adopt fully, GNSS receivers in height determination. In addition to this, the determination of the National Geoid Model by Ghana Land Administration project should be seriously pursued since it is a major pre-requisite for sustainable height realization in Ghana.

Keywords: Levelling, GNSS, GPS, GLONASS, Geoid, Ellipsoid, Orthometric.