Le jeudi 19 novembre 2015 09:24:46, Simon Lyngby Kokkendorff a écrit :
> Basically it seems to be an issue with interpreting "center" directives
> when doing the gdal_translate. Consider:
>
> ncols 2
> nrows 2
> xllcenter 0.0
> yllcenter 0.0
> cellsize 1.0
> nodata_value -999
> 2 2
> 2 2
>
> gdalinfo on this file, will give the expected GeoTransform:
>
> In [2]: ds = gdal.Open("center.asc")
>
> In [3]: ds.GetGeoTransform()
> Out[3]: (-0.5, 1.0, 0.0, 1.5, 0.0, -1.0)
>
> Seems OK to me.
>
> If I then do a gdal_translate: gdal_translate center.asc center.tif, I get
> another interpretation:
>
> In [4]: ds = gdal.Open("center.tif")
>
> In [5]: ds.GetGeoTransform()
> Out[5]: (0.0, 1.0, 0.0, 1.0, 0.0, -1.0)
>
> And, if I do a roundtrip: gdal_translate -of AAIGRID center.tif
> center_back.asc, I get:
> ncols 2
> nrows 2
> xllcorner 0.000000000000
> yllcorner -1.000000000000
> cellsize 1.000000000000
> NODATA_value -999
> 2 2
> 2 2
>
> Which is definitely not the same GeoTransform as my input center.asc, but
> shifted half a cellsize from what I would expect...
>
> In [6]: ds = gdal.Open("center_back.asc")
>
> In [7]: ds.GetGeoTransform()
> Out[7]: (0.0, 1.0, 0.0, 1.0, 0.0, -1.0)
>
>
>
> 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.
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
--
Spatialys - Geospatial professional services
http://www.spatialys.com
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
