# [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
```