Post-projection affine transformations?

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Post-projection affine transformations?

Michal Migurski
Hello,

I'm trying to determine whether Proj parameters can express an arbitrary post-projection affine transformation, e.g. shear or rotation. I've looked at the +towgs84 parameter but this doesn't seem to be a good fit. Is it possible to transform a projection, for example to flip a mercator map upside-down? I'm rendering images with Mapnik which seems to use Proj internally.

-mike.

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Re: Post-projection affine transformations?

Frank Warmerdam
Michal Migurski wrote:
> Hello,
>
> I'm trying to determine whether Proj parameters can express an arbitrary
> post-projection affine transformation, e.g. shear or rotation. I've looked
> at the +towgs84 parameter but this doesn't seem to be a good fit. Is it
> possible to transform a projection, for example to flip a mercator map
> upside-down? I'm rendering images with Mapnik which seems to use Proj
> internally.


Mike,

Generally speaking I'm not aware of a way of doing arbitrary post
projection affine transformations as part of the projection process
via proj commandline switches.

However, "proj trunk" does add new commandline arguments for switching
around the axes that might be sufficient for your mirroring needs.  If
you can build with trunk, you could try adding something like:

   +axis=esu

which means that the X axis is "easting" the Y axis is "southing"
and the Z axis is "up".

Best regards,
--
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I set the clouds in motion - turn up   | Frank Warmerdam, [hidden email]
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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