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

[Even Rouault]

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...
> >> 
> >> --
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> > 
> > --
> > Giovanni Allegri
> > http://about.me/giovanniallegri
> > Twitter: https://twitter.com/_giohappy_
> > blog: http://blog.spaziogis.it
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