Computations of Geoid Undulation from Comparison of GNSS/Levelling with EGM 2008 for Geodetic Applications

Abstract

Consistency is an important characteristic in height systems which the mean sea level (msl) surface cannot guarantee. Only a geoid surface can provide height consistency. The quality of geoid undulation (N) will obviously affect the resulting orthometric height (H) determined from GNSS. The geoid undulation may be global, regional/national and local. Online software CSRS-PPP was used for post processing rinex data. 𝑁^{𝐸𝐺𝑀2008} was computed from AllTrans EGM2008 geoid calculator while h was used to compute 𝑁^𝐺𝑃𝑆 from the relationship N= h-H. H is the existing orthometric height. Twenty-four controls with FCT 260 P as base reference station were used for this study. The computed standard deviation of differences in 𝑁^𝐺𝑃𝑆 − 𝑁^{𝐸𝐺𝑀2008} (σ) is used as accuracy indicator and σ =0.419m .The root mean square error (RMSE) is 0.934m. This indicates the quality and reliability of the geoid undulation from the EGM2008 model. Comparing the observed 𝑁^{𝐺𝑁𝑆𝑆} and 𝑁^{𝐸𝐺𝑀2008}, the use of global models may not satisfy the accuracy level of orthometric height desired for local applications in the FCT, Abuja. GNSS (GPS) may be used along with local geoid model as a way to acquire acceptable orthometric height. The smaller the 𝑁^𝐺𝑃𝑆-𝑁^{𝐸𝐺𝑀2008} makes it better model. The range of 1.585m from (𝑁^𝐺𝑃𝑆-𝑁^{𝐸𝐺𝑀2008}) in this study is a strong indication that global models should be avoided as much as possible in local applications.

Keywords: Geoid undulation, Ellipsoidal height, Orthometric height, EGM2008, Accuracy