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

[G. Allegri]

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
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