[gdal-dev] gdaltranslate and gdalwarp rotating and resizing image without being asked to, why?

[Jesse McGraw]

Even,
  Thank you for the very detailed responses.  Time to go do some more
googling on GDAL perl bindings (or dig into python)



On Tue, Jul 1, 2014 at 4:37 PM, Even Rouault <even.rouault at mines-paris.org>
wrote:

> Le mardi 01 juillet 2014 15:50:18, Jesse McGraw a écrit :
> > On Mon, Jun 30, 2014 at 3:07 PM, Even Rouault
> > <even.rouault at mines-paris.org>
> >
> > wrote:
> > > That might the problem. If you create the GCPs on the PDF and not on
> the
> > > PNG,
> > > and that the PDF has rotation, pdftoppm I think will apply the
> rotation,
> > > hence
> > > the axis swapping.
> >
> > Remember, I'm using the PNG I've created for all gdal operations, gdal
> > never interacts with the source PDF
> >
> > I'm probably fundamentally misunderstanding some aspect of the command
> > lines I'm using in gdal*.  My end goal is to get the necessary affine
> > transformation information to display the PNG I create in step 3a
> properly
> > in a separate Android moving-map application (eg. your current position
> > displayed on the diagram as you move around the airport)
>
> Jesse,
>
> hum ok, with the images I think I now understand the situation (I've only
> looked at the landscape diagram) and what you want.
>
> You probably don't want to use gdalwarp, which by default will result in a
> north-up image. You likely want to compute the geotransform affine matrix
> for
> your GCPs and set it, since indeed your GCPs define a 1st-order
> transformation.
>
> The GDAL C API has GDALGCPsToGeoTransform() which does exactly this. I see
> it
> is also available in the Python API :  gdal.GCPsToGeoTransform().
> See http://svn.osgeo.org/gdal/trunk/autotest/gcore/gcps2geotransform.py
> for
> how to use it.
>
> You could then just do a CreateCopy() of the PNG to a VRT, and set the
> returned geotransform by the previous step with SetGeoTransform().
>
> Even
>
>
> --
> Geospatial professional services
> http://even.rouault.free.fr/services.html
>
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