[Proj] Re: ECEF

[Strebe at aol.com]

In a message dated 2/28/2005 12:15:00 PM Eastern Standard Time, Frank Warmerdam <fwarmerdam at gmail.com> writes:

...
>It also has support for a geocentric coordinate system 
>(using proj=geocent).  However, my understanding is
>that there are two types of geocentric coordinates.  One
>from the center of the ellipsoid, and one from a height 
>perpendicular to the ellipsoid.  I'm not sure, off hand, 
>which it is supported by PROJ.4 but can investigate if 
>needed. 
...

I can't speak for any software implementions, but as far as datums go, 'geocentric' should refer to those intended for world-wide coverage. That includes any created for satellite measurements since, and including, WGS 72, and excluding any others I know about.

As far as latitude measurement goes, first consider the ray from the center of the ellipsoid through the equator at the meridian of the point in question. Call that the "equatorial ray". Then 'geocentric latitude' refers to measuring the angle subtended by the equatorial ray and the point in question. 'Geodetic latitude', on the other hand, places the vertex at the intersection of the equatorial ray and the ray normal to the ellipsoid at the point in question.

Hence there are at least two distinct usages for 'geocentric', one with respect to measurement of latitude, and the other with respect to the applicability of the ellipsoidal model.

(Geocentric coordinates are used, I hear, for satellite tracking and data gathering, but are always converted to geodetic before general release. )

Regards,

daan Strebe
Geocart author
http://www.mapthematics.com