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

[G. Allegri]

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....

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
>



-- 
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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