ETH-GQS: An estimation of geoid-to-quasigeoid separation over Ethiopia

The determination of accurate orthometric or normal heights remains one of the main challenges for the geodetic community in Ethiopia. These heights are required for geodetic and geodynamic scientific research as well as for extensive engineering applications. The main objective of this study is to estimate the geoid-to-quasi geoid separation (GQS) in Ethiopia (ETH-GQS). Such separation would be required for the conversion between geoid and quasigeoid models, which is mandatory for the determination of accurate geodetic heights in mountain regions. The airborne free-air gravity anomalies and the topographic information retrieved from the SRTM3 (Shuttle Radar Topography Mission of a spatial resolution 3 arc-second) digital elevation model were used to compute the ETH-GQS model according to the Sjoeuroberg's strict formula for the geoid-to-quasigeoid separation. The ETH-GQS was then validated using GNSS-levelling data as well as geoid heights determined from different Global Geopotential Models (GGMs), namely the EGM2008, EIGEN-6C4 and GECO. The results reveal that the standard deviation of differences between the geoid heights obtained from the EIGEN-6C4 model and the geometric geoid heights obtained from GNSS-levelling data were improved by-75% (i.e. from-24 to-6 cm) when considering GQS values obtained from the ETH-GQS. (c) 2021 Editorial office of Geodesy and Geodynamics. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).