[Proj] "Double Ellipsoid" error, reproduction

[Noel Zinn]

Gerald,

The input and output of datum transformations are geographical coordinates.
Some methods of datum transformation (e.g. NADCON, NTv2, multiple regression
equations) work in geographicals and ellipsoid parameters are not required.
Other methods of datum transformations (e.g. 3 through 7 parameter
similarity, Helmert, Bursa-Wolf, 10-parameter Molodensky-Badekas) require a
transit through geocentric Cartesian, Earth-Centered Earth-Fixed (ECEF)
coordinates for which ellipsoid parameters are required.  Datum
transformation parameters are always empirically derived and spatially
variant and, therefore, always contain error.  Map conversions (geographical
to grid) are merely mathematical mappings that are (theoretically) without
empirical error.  Except - and this is my objection in this thread
heretofore - except when the ellipsoid is changed and a datum transformation
is implicitly coupled with a map conversion.

So, the answers to your questions are (1) true and (2) false (in some
cases).

You are absolutely correct that there is no LAW about what a cartographer
does, limited by his or her own imagination, but there is a physical reality
for those who have an interest in conforming to it as closely as possible.
The best fit between physical reality and geographical coordinates is
defined by the ellipsoid in which the datum's least-squares adjustment of
survey data (collected in the real world, not in pixel space) was done.
Change the ellipsoid and the quality of the fit deteriorates (even in a
datum transformation and especially in a map conversion).  I already
demonstrated in this thread that Google Maps Projection grid coordinates
bear an undocumented relationship with physical reality for the casual user.


Regards,
Noel

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Gerald I. Evenden
Sent: Sunday, December 07, 2008 9:25 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] "Double Ellipsoid" error, reproduction

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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