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?
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 > -- Giovanni Allegri http://about.me/giovanniallegri Twitter: https://twitter.com/_giohappy_ blog: http://blog.spaziogis.it GEO+ geomatica in Italia http://bit.ly/GEOplus
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