GEOMETRIC AND GRAVIMETRIC EVALUATION OF LOCAL GEOID MODELLING APPROACHES USING GNSS/LEVELLING DATA IN HONG LOCAL GOVERNMENT AREA, ADAMAWA STATE, NIGERIA

Abstract

Precise geoid modelling is essential for transforming Global Navigation Satellite System
(GNSS) derived ellipsoidal heights into orthometric heights referenced to the Earth’s
gravity field. This study evaluates geometric and gravimetric approaches for local geoid
determination within Hong Local Government Area of Adamawa State, Nigeria, using
integrated GNSS/levelling and gravity datasets. Field observations were acquired with a
Hi-Target V90+ differential GNSS receiver operating in Real Time Kinematic (RTK) mode
to obtain ellipsoidal heights, while precise spirit levelling provided orthometric heights at
163 observation points referenced to three primary control stations (CSAD75S, CSAD76S,
and CSAD77). Corresponding geoid heights were extracted from three Global Gravitational
Models (GGMs): EGM2008, SGG-UGM-2, and XGM2019e_2156. The geometric geoid
undulations were computed as the difference between GNSS and levelling heights, and
statistical analyses including correlation coefficient, mean bias, standard deviation, and root
mean square error (RMSE) were employed to assess model accuracy. Results show that
EGM2008 exhibited the highest correlation (R = 0.97) and lowest RMSE (0.16m) with the
geometric model, while SGG-UGM-2 and XGM2019e_2156 displayed higher deviations
(0.23m and 0.26m, respectively). The findings confirm that EGM2008, when locally
validated, provides the most reliable representation of the regional geoid over Hong LGA.
This study contributes to local height system modernization and supports future geodetic
reference frame development in north-eastern Nigeria.