[Proj] Geocentric vs. Geodetic latitude

[Frank Warmerdam]

Folks,

I was recently suprised to encounter locations identified as being
geocentric that were give as a latitude and longitude.  I had expected them
to be x/y/z values in meters.

A client of mine, better versed in such things explained that there is in
fact a distinction between geocentric and geodetic lat/long.  He wrote:

 >"The angle L' is called "geocentric latitude" and is defined as the
 > angle between the equatorial plane and the radius from the geocenter.
 >
 > The angle L is called "geodetic latitude" and is defined as the angle
 > between the equatorial plane and the normal to the surface of the
 > ellipsoid.  The word "latitude" usually means geodetic latitude.  This
 > is the basis for most of the maps and charts we use.  The normal to the
 > surface is the direction that a plumb bob would hang were it not for
 > local anomalies in the earth's gravitational field."

I need to implement some code to convert geocentric latitude to geodetic
latitude.  I think I can do from the above description, but I am wondering
if anyone can confirm the above description, provide a forumla or most
importantly provide some sample latitudes in both systems I can check my
work against.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent