PRACTICAL OPTIONS FOR THE RE-DEFINITION OF THE NIGERIAN VERTICAL REFERENCE SYSTEM: A CASE STUDY OF LAGOS STATE
DOI:
https://doi.org/10.5281/zenodo.20191896Keywords:
Orthometric Height System, Ellipsoidal Height System, Gravity Field, Geopotential Number, Spheropotential NumberAbstract
Adopting a height system, especially in areas with a spatially-vast land mass, is a complicated choice between physically and geometrically meaningful systems, given the level of computational and observational rigor involved in either case. For Nigeria, there is no clear legislation as to the height system that should be used by practitioners, and this uncertainty about the adopted height system for the country has resulted in an untenable situation in which multiple height systems are used across the country. The aim of this study is to evaluate practical options for redefining and harmonizing the Nigerian Vertical Reference System (VRS), using the lowlands of Lagos State as a case study. The approach involved the acquisition of spirit-leveled elevation differences, GNSS ellipsoidal heights, and absolute gravity observations on 218 benchmarks across the state, and the comparative computation of three height candidates: (1) Helmert Orthometric Heights (HOH) obtained by applying the orthometric correction to spirit-leveled elevation differences using measured gravity; (2) Normal Orthometric Heights (NOH) obtained by applying the normal orthometric correction to spirit-leveled elevation differences using normal gravity; and (3) HOH derived from GNSS ellipsoidal heights using the South-Western Gravimetric Geoid 2018 (SWGG’018). Statistical analysis of the three options using a student’s t-test of significant difference at the 95% confidence level indicates that there is no significant difference in the results obtained by all three options within the study area. However, considering the observational convenience and computational simplicity, option 3 is recommended for the redefinition of the Nigerian VRS. The study further recommends that a national height system be legislated for use by all practitioners in the production of topographic plans.
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