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

[G. Allegri]

I've extracted a subset from the original ASCII:
https://dl.dropboxusercontent.com/u/13861666/FI_clipped.asc
I'm obtaining the same behaviour, so it can be a test case.

giovanni


2014-07-05 21:43 GMT+02:00 G. Allegri <giohappy at gmail.com>:

> 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
>



-- 
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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