[Qgis-developer] Georeferencer produces wrong (shifted) result

Agustin Lobo alobolistas at gmail.com
Sat Dec 17 15:07:01 EST 2011


Many thanks Manuel.
No hurry for including the patch, once the problem is identified and
you even provide a simple way to circumvent it.
What is needed is to have users know of these potential problems, ie. through
a note of "Known Problems" included in the release mentioning that
"the Georeferencer plugin requires users to check that their files do
not include
an implicit geotransform and use e.g.
"gdal_translate -co "PROFILE=BASELINE" -of GTiff Ilerfly125v2.tif
Ilerfly125v2-nogeotrans.tif"
to remove it in case this is actually needed".
In other words, once the problem is identified and warned, users can
rely on using the software for the rest of cases.
The question is thus that, in addition of the excellent developers
already involved,
we need a team of users and a battery of tests to be regularly performed.
Agus






2011/12/17 Manuel Massing <m.massing at warped-space.de>:
> Hi Agustin,
>
>
>> The points are correct (therefore the errors etc), but the
>
>> georeferenced image is shifted. May be the datum is not taken into
>
>> account
>
>> when the georeferenced image is created.
>
>
> I did a clean checkout, and could finally reproduce the problem:
>
> it stems from the fact that your input file has a geotransform,
>
> which flips the yaxis of the local coordinate system and changes the
>
> origin. As the geotransform plugin does not handle geotransform
>
> info correctly, this results in the shift you have observed.
>
>
> Removing the geotransform (e.g. using "gdal_translate -co "PROFILE=BASELINE"
> -of GTiff Ilerfly125v2.tif Ilerfly125v2-nogeotrans.tif") should give you
>
> a file which the georeferencer can handle correctly (you may have to delete
> the accompanying .aux file, if gdal uses this to keep the geotransform).
>
>
> I had a local patch in the georeferencer which handles geotransform
>
> information, so this is why I couldn't reproduce the problem (talk about a
>
> mixture of good and bad luck :-)). I will see what is required to make my
>
> local changes ready for submission, so we can avoid such subtle problem with
>
> georeferenced/pseudo-georeferenced files in the future. As I already
>
> mentioned, due to an important deadline at work, I have little time to
>
> dedicate to this atm, so it may take a few day... is there a release in the
>
> pipeline?
>
>
> cheers,
>
>
> Manuel
>
>
>
>
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