[Proj] Geod: I need a little guide

Clifford J Mugnier cjmce at lsu.edu
Sun Apr 25 23:06:46 EDT 2004

Info directly related to EGM96 is DoD-only.  However, a subset of EGM96 is
"GEOID03" (and its predecessors) for the United States as done by the U.S.
National Geodetic Survey.  At their site, (www.ngs.noaa.gov) you will find
a number of utilities for working with that geoid subset that includes
binary translators, re-gridding facilites, etc.

The Canadians, the Australians, the U.K. and many European nations have
national geoids.  Some of them have variable-mesh grids (Canada &
Australia) that are different from what the Americans use (constant-density
mesh).  The format of those geoids in binary files are of course,
different.  Depending on the nature of the data set you seek to
"customize," you may want to use the American-based techniques or the
Canadian-based techniques.

Please report back to the list what you find of use to you.

Cliff Mugnier
Hello My Prof,
Thanks for the information. After reading the sources in the
Geotrans, I searched the internet and got a lot of information
on this.
I thought proj4 was doing a similar thing with the geod program,
and also, there is a point somewhere in the proj4 docs that if you
pass in a 3-point, there is conversion of the Z and so I thought, it
was geoid conversion being applied there.

Again, thanks for the information and the support.

Any information of the format of the binary file for the geoid-ellipsoid
height conversion in the Geotrans, "egm96.grd"?

Best regards,

----- Original Message -----
From: "Clifford J Mugnier" <cjmce at lsu.edu>
To: <proj at remotesensing.org>
Sent: Monday, April 26, 2004 6:17 AM
Subject: Re: [Proj] Geod: I need a little guide

> EGM96 is "Earth Gravity Model 1996" and represents the relation between
> 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
> unclassified.
> 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
> inertial reference system - centered at the center of mass of the Earth -
> the origin of the WGS84 datum and ellipsoid.  To obtain elevations from
> 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
> from the ellipsoid) to convert ellipsoid heights to elevations.
> I don't think PROJ4 does that.
> Cliff Mugnier
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