[Proj] Geod: I need a little guide

Clifford J Mugnier cjmce at lsu.edu
Sun Apr 25 17:17:27 EDT 2004

EGM96 is "Earth Gravity Model 1996" and represents the relation between the
WGS84 ellipsoid AND the WGS84 datum with respect to the distance of the
surface of equipotential energy (mean sea level) and the ellipsoid.  The
equation is a 360-degree spherical harmonic, and the coefficients are

The surface of equipotential energy is called the "GEOID."

When working with remotely-sensed data for topography, such as LIDAR (or
GPS-controlled photogrammetry), the heights obtained are with respect to an
inertial reference system - centered at the center of mass of the Earth -
the origin of the WGS84 datum and ellipsoid.  To obtain elevations from the
data, one must apply a Geoid model, commonly EGM96 - the most reliable
geoid available.

The query was with regard to utilizing a table of values (geoid separation
from the ellipsoid) to convert ellipsoid heights to elevations.

I don't think PROJ4 does that.

Cliff Mugnier

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