[Proj] mercator projection to spherical ellipsoid from WGS84

[Scott Ellington]

Hi Frank,

Thanks for responding.  Unfortunately, the issue manifests itself in mapserver so I don't think I can chain two commands together.

As for revisiting this issue, I think this would be helpful.  At the very least, I'd like the ability to avoid the datum transform as it used to be possible to do.  I can file a bug to keep this on the radar if you'd like.

Thanks,
Scott

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
Sent: Wednesday, March 28, 2007 1:59 PM
To: PROJ.4 and general Projections Discussions
Cc: Melita Kennedy
Subject: Re: [Proj] mercator projection to spherical ellipsoid from WGS84

Scott Ellington wrote:
> Hi,
> 
> Thanks for your response.  As I replied earlier, it appears in the latest
> versions of proj that you cannot avoid the datum transformation simply by
> removing the towgs84 parameter.  I am wondering if there is now a way to
> avoid the datum transformation without setting the input ellipsoid to match
> the output...

Scott,

I don't think there is in a single cs2cs command, but if you really just want
to go from one projection to it's corresponding lat/long, and then from that
lat/long to projected coordinates on another datum/ellipsoid without doing
anything to the lat/long values, then you should chain it as two operations.

eg.
cs2cs +proj=utm +zone=11 +datum=WGS84 +to +proj=latlong +datum=WGS84
  | cs2cs +proj=latlong +ellps=sphere +to +proj=merc +ellps=sphere

This accurately represents what you want to do.  It could also be done
with two proj commands instead of using cs2cs.

I do think the whole issue of what cs2cs does to get from one datum/ellipsoid
to another when there is no useful transformation information needs to be
revisited.  It is causing no end of problems.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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