[Proj] "Double Ellipsoid" error, reproduction

Noel Zinn ndzinn at comcast.net
Sun Dec 7 13:12:44 EST 2008

Yes, and there is another inappropriate coupling, that of a datum with a
specific transformation to WGS84.  A local datum is not defined by its
transformation to WGS84 (but by some adjustment that probably happened
before WGS84 was defined).  That (derived, empirical) transformation to
WGS84 will depend upon the specific survey monuments occupied (spatial
variance and distortion in the local datum), the instruments used (GPS,
Doppler, how much data and how processed), and the date of derivation (since
the world datums vary over time).  Happily, many software vendors have come
to understand this and separate the transformation from the datum.  -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 11:10 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] "Double Ellipsoid" error, reproduction

On Sunday 07 December 2008 11:30:09 am Noel Zinn wrote:
> Gerald,
> (theoretically) without empirical error.  Except - and this is my
> in this thread heretofore - except when the ellipsoid is changed and a
> datum transformation is implicitly coupled with a map conversion.

I concur and that is where I am also having the most problem.  I have always

viewed map conversions and datums as completely different entities.  This 
goes back to the early days of several of the programs that tightly bound
projections with the datum process and my pleas to more clearly separate the

processes were pretty much ignored.

Alas.  :-(

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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