[Proj] Some questions

Michael P Finn mfinn at usgs.gov
Tue Apr 7 11:58:04 EST 2009

Noel, thank you for sharing the explanation.   Mike Finn

"Noel Zinn" <ndzinn at comcast.net>
"'PROJ.4 and general Projections Discussions'" <proj at lists.maptools.org>
04/07/2009 06:03 AM
Re: [Proj] Some questions
Sent by:
proj-bounces at lists.maptools.org


Your two objectives, (1) full ellipsoid control and (2) more and more
accuracy, are contradictory and self defeating.  As we learned in the 
Maps debate, allowing full ellipsoid control changes the definition of the
projection that (you think) you are using.  For example, the spherical
Mercator is conformal if the geographicals (lat/lon) come from a sphere of
the correct radius.  But if we feed the spherical Mercator equations
geographicals that come from an ellipsoid, the result is non conformal. 
projection we are actually using is not the spherical Mercator but some
Pseudo Mercator because the equations for distortion (scale factor in this
case) have to be rewritten.  That is not "more and more accuracy".  It's 
opposite.  Who knows what happens to other conformal projections?  Who 
what happens to the equal area property of equal area projections?
Cartography is more than XY to and from LL.  Projections have properties.
Full ellipsoid control destroys those properties.

Noel Zinn

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of 
support.mn at elisanet.fi
Sent: Saturday, April 04, 2009 8:51 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Some questions

I understand. The problem for our part is that we are seeking
all the time more and more accuracy. Could we put it behind
some switch? The standard version would not allow such
parameters, but if one switched some switch on, then one
could use such parameters.

Something like:


The full control of the calculation is important for us since we have
to get accurate results (especially in the future). If it is behind some
switch, that does not bother us? The other people will also not
see any difference, since the switch could be off by default.

Regards: Janne. / MNS Support


> > For example something like:
> > 
> > pellps=.....
> > dellps=.....
> > 
> > where "pellps" would stand for "projection ellipsoid" and
> > "dellps" for "datum ellipsoid".
> > 
> > and when both are the same, just normal
> > 
> > ellps=....
> > 
> > where "ellps" stands for both datum and projection. That
> > would not alter the existing definitions, but allow more
> > accurate control of the ellipsoids involved.

> No, no such capability has been implemented.  I'm still not too keen
> on doing so either for reasons previously stated.

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