Le samedi 05 juillet 2014 20:35:45, G. Allegri a écrit : > Given that an ASCII Grid doesn't provide a data type, maybe the driver > choose the larger one, while the utilities guess it trying to fit the > values into a smaller type?
No, GDAL does not do that sort of magic. Are you sure the GDAL version used by Python is the same as by the utilities ? > > giovanni > > 2014-07-05 20:31 GMT+02:00 G. Allegri <[email protected]>: > > I've debugged it, and it gets Float64 directly from > > GDALGetRasterDataType( GDALRasterBandH ). > > I've also tried opening it with Python GDAL and I get Float64. > > gdalinfo tells me Float32. > > I will try to send you a subset of it. It's not easy, as it is a heavy > > ASCII Grid and I don't want to alter it. > > > > giovanni > > > > > > 2014-07-05 20:22 GMT+02:00 Even Rouault <[email protected]>: > > > > Le samedi 05 juillet 2014 20:18:50, G. Allegri a écrit : > >> > Finally I've reached the point: GDAL's driver is opening the ASCII as > >> > Float64, consequently QGIS treats it this way. > >> > I wonder why gdalinfo and gdal_translate treat it as Float32 > >> > instead.... > >> > >> If gdalinfo opens it as Float32, then it is Float32 for everybody. I > >> guess QGIS probably promotes Float32 to Float64. > >> > >> > giovanni > >> > > >> > 2014-07-05 18:53 GMT+02:00 G. Allegri <[email protected]>: > >> > > The problem was simpler then it appeared: I didn't realize that QGIS > >> > > output is Float64. > >> > > Yet I don't know why QGIS chosed to use this data type... > >> > > > >> > > giovanni > >> > > > >> > > > >> > > 2014-07-05 17:56 GMT+02:00 Even Rouault > >> > > <[email protected] > >> > > > >> > > Le samedi 05 juillet 2014 17:25:48, G. Allegri a écrit : > >> > >> > > QGIS is usually just calls to gdal, which makes this even more > >> > >> > > mysterious. > >> > >> > > >> > >> > Yes, in the end it uses gdal, but it chooses the way blocks are > >> > >> read > >> > >> > >> > and written. > >> > >> > If I remember correctly geotiff final sizes may depend on block > >> > >> > ordering and memory alignment. > >> > >> > >> > >> In that instance, the target geotiff has a natural block dimension > >> > >> which > >> > >> > >> is a > >> > >> raster line. > >> > >> If QGIS writes the geotiff by 256x256 blocks (this is just a guess. > >> > >> I've > >> > >> > >> not > >> > >> verified), the same raster line will be written several times. As > >> > >> it > >> > >> is > >> > >> > >> a compressed geotiff, the resulting raster line will be each time > >> > >> being > >> > >> > >> bigger > >> > >> since the initial zeros will be replaced by actual values. And if > >> > >> the new size > >> > >> of the line is bigger than its previous size, the new line will be > >> > >> rewritten > >> > >> at the end of the file, losing the space previously occupied. > >> > >> > >> > >> > Maybe this is the case, QGIS raster provider not doing the best > >> > >> > at this level? Don't know, but this discussion is for the QGIS > >> > >> > ml ;) > >> > >> > > >> > >> > giovanni > >> > >> > > >> > >> > > On 7/5/2014 9:09 AM, G. Allegri wrote: > >> > >> > > > I agree with you David, I'm surprised too. > >> > >> > > > Anyway, gdal_translate is run without compression options. > >> > >> > > > I've written to the QGIS devs (it was the software) to verify > >> > >> > > > what's happening with its raster file writer code... > >> > >> > >> > >> -- > >> > >> Geospatial professional services > >> > >> http://even.rouault.free.fr/services.html > >> > > > >> > > -- > >> > > Giovanni Allegri > >> > > http://about.me/giovanniallegri > >> > > Twitter: https://twitter.com/_giohappy_ > >> > > blog: http://blog.spaziogis.it > >> > > GEO+ geomatica in Italia http://bit.ly/GEOplus > >> > >> -- > >> Geospatial professional services > >> http://even.rouault.free.fr/services.html > > > > -- > > Giovanni Allegri > > http://about.me/giovanniallegri > > Twitter: https://twitter.com/_giohappy_ > > blog: http://blog.spaziogis.it > > GEO+ geomatica in Italia http://bit.ly/GEOplus -- Geospatial professional services http://even.rouault.free.fr/services.html _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
