[Proj] "Double Ellipsoid" error, reproduction

Gerald I. Evenden geraldi.evenden at gmail.com
Sun Dec 7 10:25:09 EST 2008

I find this thread so incredible confusing I have tried to stay out of it 
entirely but I have one, maybe two questions:

I thought the basic detail operations of datum conversion were done in 
geographic data space (latitude-longitude) or perhaps x-y-z.  That is, you 
have to have the data in geographic space to do the datum conversion 
calculation.   True or False?

Also, ellipsoid factors are not part of the datum conversion as long as the 
data is in geographic coordinates:  True or False?

If the above is true, what does the ellipsoid values have to do with anything?  
That is a rhetorical question and the answer is obviously: nothing.

The question of the ellipsoid parameters only comes up when dealing with data 
in Cartesian coordinates which need to be transformed back to geographic 
coordinates for datum transformation.  The ellipsoid values are only a factor 
in de-projecting the points.

The ellipsoid chosen for the Cartesian projection is probably a capricious and 
arbitrary choice of the people who created the projected data in the first 
place.  I know of no LAW that requires someone to choose a particular 
ellipsoid purely because of the datum of the data.  The plotter may not have 
any idea as to the datum of the data.

If the issue is simply two completely separate operations--- datum conversion 
and ellipsoid-projection--- then what is all the discussion about???

What I am arguing is that the use of the ellipsoid parameters and datum 
conversion are two completely separate issues.  Thus the problem discussed in 
this thread is merely an issue finding the correct ellipsoid to get data back 
into geographic coordinates, doing a datum conversion, and selecting an 
arbitrary a new ellipsoid for the target display map.

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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