[gdal-dev] Georeference without rotation of output image

[Even Rouault]

Le vendredi 14 août 2015 19:15:44, Ivan Castellucci Vidal a écrit :
> Thanks Even!
> 
> That's exactly what I was looking for and I just have one question about
> it. GDAL documentation states: "Due to imprecision in the calculations the
> fit algorithm will often return non-zero rotational coefficients even if
> given perfectly non-rotated inputs"
> 
> Is there a way to use a more precise algorithm?

The algorithm is mostly a least square regression. There are also a few 
heuristics to force the rotational coefficients to 0 in a few situations, like 
when there are 4 GCPs that form a rectangle. But since you mentionned that 
your images would have rotation, that shouldn't be much of a concern I think.
Anyway in cases where the theory should be perfect 0, the values got in 
practice should be sufficiently close to zero for most practical purposes.


> It's not a requirement by
> now since this program is in development but at some point I'll need to
> count with the maximum possible precision to make it work in a production
> release.
> 
> 
> On Fri, Aug 14, 2015 at 1:22 PM, Even Rouault <even.rouault at spatialys.com>
> 
> wrote:
> > > Instead of warping the image, you could compute a geotransformation
> > 
> > matrix
> > 
> > > that would include rotational terms with GDALGCPsToGeoTransform()
> > > 
> > > http://www.gdal.org/gdal_8h.html#ae44b32f08d61072dddbdd30ac8ca962b
> > > 
> > > and set it to your geotiff with GDALSetGCPs()
> > > 
> > > http://www.gdal.org/gdal_8h.html#acd47aa85ca5751fbe9ea09c1a2341ef8
> > 
> > Arg, sorry I meant with GDALSetGeoTransform() of course :
> > http://www.gdal.org/gdal_8h.html#ae93448112c1a7e69f2764c1aa3c6c8b5()
> > 
> > GDALSetGCPs() is to set the "raw" GCPs.
> > 
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