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

[G. Allegri]

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 <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
>



-- 
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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