[gdal-dev] GeoTransform confusion

[Simon Lyngby Kokkendorff]

Done: https://trac.osgeo.org/gdal/ticket/6225#ticket

Thanks once again!!
Simon


On Thu, Nov 19, 2015 at 10:59 AM, Even Rouault <even.rouault at spatialys.com>
wrote:

> Le jeudi 19 novembre 2015 10:47:50, Simon Lyngby Kokkendorff a écrit :
> > I can confirm also that it works when I set a target srs.
> > But must admit that the current behaviour was confusing. I knew how to
> > interpret the input data - under the assumption that a geotransform is
> not
> > changed when I do no transformations or interpolations:
> > If I know that my data are "points" rather than "pixels" - the first
> > "point" is lying half a cellsize downwards and rightwards from the upper
> > left corner that GDAL reports. That's what I always assume - not relying
> on
> > other metadata items than the GeoTransform.
> >
> > So I agree that it might be worth a ticket.
>
> Please file one.
>
> >
> > Cheers,
> > Simon
> >
> > On Thu, Nov 19, 2015 at 10:29 AM, Ari Jolma <ari.jolma at gmail.com> wrote:
> > > On 19.11.2015 11:26, Even Rouault wrote:
> > >> GDAL version:
> > >> GDAL 1.11.2, released 2015/02/10
> > >> Simon,
> > >>
> > >> Regarding the fact that the geotiff file looses the half-pixel shift,
> it
> > >> seems
> > >> to be due to the absence of a SRS. If I do "gdal_translate test.asc
> > >> test.tif -
> > >> a_srs EPSG:32630" for example, then the geotransform I get back is
> > >> correct.
> > >> This is a defect of the GeoTIFF writer that writes the GeoTIFF
> Tiepoints
> > >> with
> > >> the PixelIsPoint convention (since your original data is flagged as
> > >> such), but
> > >> fails to write GTRasterTypeGeoKey=RasterPixelIsPoint as a geotiff key
> > >> when there's no SRS. Could be worth a ticket.
> > >
> > > My conclusion is the same. I think the key should be set regardless of
> > > the existence of a SRS.
> > >
> > > Ari
> > >
> > >> Some background:
> > >> https://trac.osgeo.org/gdal/wiki/rfc33_gtiff_pixelispoint
> > >>
> > >> A workaround :
> > >> gdal_translate test.asc test.tif -mo AREA_OR_POINT=AREA
> > >>
> > >> This will force the geotiff writer to use the default convention, and
> > >> thus avoiding going back to the centerofpixel convention when writing
> > >> the GeoTIFF
> > >> tiepoints.
> > >>
> > >> Even
> > >
> > > _______________________________________________
> > > gdal-dev mailing list
> > > gdal-dev at lists.osgeo.org
> > > http://lists.osgeo.org/mailman/listinfo/gdal-dev
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>
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