EGM96

EGM96
Name	EGM96
Type	Geopotential model
Epoch	1996
Degree	360
Order	360
Institution	National Geospatial-Intelligence Agency
Release	1996
Successors	EGM2008

EGM96 EGM96 is a global geopotential model that provides a spherical harmonic representation of Earth's gravity field used for geodesy, oceanography, and satellite navigation. It combines terrestrial, airborne, and satellite gravity data into a unified spherical harmonic expansion to degree and order 360, enabling geoid and gravity anomaly computations for mapping and positioning. The model was released in 1996 and has been widely used alongside global positioning systems and oceanographic missions.

Overview

EGM96 was produced as a collaborative effort to improve global gravity field representation for applications in cartography, National Geospatial-Intelligence Agency, National Oceanic and Atmospheric Administration, and scientific communities such as NASA and European Space Agency. The model's spherical harmonic coefficients permit computation of the geoid, gravity anomalies, and vertical deflections useful to agencies like United States Geological Survey and programs including TOPEX/Poseidon and Jason-1. Its 360×360 harmonic resolution corresponds to spatial wavelengths of about 55 kilometers, informing work by institutions such as International Hydrographic Organization and regional surveying authorities.

Development and data sources

EGM96 was developed by combining data from satellite missions, terrestrial gravity surveys, and altimetric measurements. Satellite contributions included data from LAGEOS and tracking systems used by NOAA and NASA; satellite altimetry from TOPEX/Poseidon and ERS-1 improved marine gravity estimates. Terrestrial and airborne gravity data originated from national agencies like USGS, British Geological Survey, and survey programs in Russia, China, and India. The incorporation of gravimetric datums required coordination with organizations such as International Association of Geodesy and regional mapping agencies like Ordnance Survey.

Mathematical model and formulation

EGM96 represents Earth's gravitational potential as a spherical harmonic series with coefficients C_nm and S_nm up to degree and order 360, following formalism used in geodetic texts and by researchers at Ohio State University and University of Texas at Austin. The potential V(r,θ,λ) is expanded in Legendre functions and associated harmonics analogous to formulations applied in analyses of GRACE and GOCE data. The processing involved least-squares collocation, covariance estimation, and techniques familiar to groups at Jet Propulsion Laboratory and Woods Hole Oceanographic Institution. Normalization conventions align with standards from International Union of Geodesy and Geophysics and allow conversion between ellipsoidal heights used by International Civil Aviation Organization and local vertical datums.

Accuracy and comparisons

At its release, EGM96 improved over prior models such as spherical harmonic solutions derived from early SEASAT and GEOSAT data by reducing geoid height errors to a few meters globally, with regional performance varying according to data density. Comparisons with later models, notably EGM2008 and gravity field estimates from GRACE and GOCE, show EGM96 has coarser resolution and larger residuals at wavelengths below ~100 km compared to successor models. Evaluations by agencies like USACE and academic groups at University of Bern highlighted that terrestrial data gaps, particularly in polar and continental interior regions such as Antarctica and parts of Siberia, limited local accuracy.

Applications and usage

EGM96 has been used extensively for geoid determination in national mapping campaigns by agencies such as Ordnance Survey, National Geospatial-Intelligence Agency, and Geoscience Australia. It supports height datum unification in projects involving Global Positioning System, International GNSS Service, and surveying programs conducted by National Oceanic and Atmospheric Administration. Oceanographers used EGM96 in conjunction with altimetry missions like TOPEX/Poseidon and Jason-2 to infer marine gravity anomalies; aerospace engineers employed it for orbit determination of satellites tracked by Space Surveillance Network. Hydrologists and civil engineers referenced EGM96 outputs for floodplain mapping tied to vertical datums maintained by agencies such as Federal Emergency Management Agency.

Limitations and updates

Limitations of EGM96 include its finite spherical harmonic truncation at degree 360, reliance on unevenly distributed terrestrial gravity data, and reduced fidelity in polar and inland regions such as Antarctica and Greenland. Subsequent updates and successors—most notably EGM2008 and gravity field solutions based on GRACE and GOCE—provide higher-degree expansions and time-variable gravity information addressing mass transport and secular trends observed by NASA and European Union programs. Ongoing work by institutions including University of Texas at Austin and National Geospatial-Intelligence Agency focuses on combining time-variable gravity, satellite altimetry, and dense terrestrial surveys to refine geoid models for modern applications.

Category:Geodesy