[gdal-dev] how can a Geotiff occupy three times the disk space of an ASCII Grid?

Sat Jul 5 12:49:59 PDT 2014

This question is better addressed to the QGis-dev list
as QGis is promoting the Float32 type to be Float64 type

https://github.com/qgis/QGIS/blob/master/src/providers/gdal/qgsgdalprovider.cpp#L1103


 Jul 5, 2014, at 3:43 PM, "G. Allegri" <giohappy at gmail.com> wrote:

> ldd tells me that both QGIS and gdalinfo point to the same gdal libs
> 
> ldd /usr/bin/qgis.bin | grep gdal
> libgdal.so.1 => /usr/lib/libgdal.so.1 (0x0284b000)
> 
> ldd /usr/bin/gdalinfo | grep gdal
> libgdal.so.1 => /usr/lib/libgdal.so.1 (0x00c5f000)
> 
> But I must correct about Python. Running it against the right modules it reports raster band data type as GDT_Float32.
> 
> In QGIS Float64 comes out here: https://github.com/qgis/QGIS/blob/master/src/providers/gdal/qgsgdalprovider.cpp#L1076
> 
> giovanni
> 
> 
> 
> 
> 2014-07-05 21:02 GMT+02:00 Even Rouault <even.rouault at mines-paris.org>:
> 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 <giohappy at gmail.com>:
> > > 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 <even.rouault at mines-paris.org>:
> > >
> > > 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 <giohappy at gmail.com>:
> > >> > > 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
> > >> > > <even.rouault at mines-paris.org
> > >> > >
> > >> > > 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
> 
> 
> 
> -- 
> 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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