[GRASS-user] Best format for exporting raster data

[Rainer M Krug]

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On 02/09/10 21:21, Glynn Clements wrote:
> 
> Rainer M Krug wrote:
> 
>>> But GRASS doesn't generally read data from files per se, but from
>>> either the GRASS "database" or from GDAL (and the former might
>>> eventually go away if we can get "native" GRASS support into GDAL).
>>
>> I don't understand that statement - I am the database is ultimately a
>> file, so grass is requesting the data from the database which then reads
>> it from the mapset which consists of files.
> 
> Ultimately, it's files, but only at the lowest level of the core
> libraries. Modules get their data from the "GRASS database", but the
> nature of that database is an implementation detail.

OK - that's clear.

> 
>> And gdal is used when data is read which is not in the mapset -
>> right?
> 
> GDAL is used if you create a "linked" map using r.external or
> r.external.out. 

Makes sense.

> 
>>> Similarly, while there exist filesystems which can mount archives, tar
>>> files (and especially compressed tar files) are a poor choice, as they
>>> are designed for sequential access. ZIP/RAR are more suited to such
>>> tasks.
>>
>> but they can actually be mounted as was pointed out by Markus Neteler
>> (http://www.linux.com/archive/feature/132196).
> 
> They can, but the filesystem will have to decompress a substantial
> part of the archive just to access one file. If you use zip/rar, it
> only has to decompress files which are accessed.

That is definitely an interesting detail, I was not aware of. I will
definitely think about using one of those.

> 
>>> Ultimately, I don't think that this situation is common enough to be
>>> worth doing anything about. If you want to access archived data, you
>>> just unpack the archives first (or use an archive format which can be
>>> mounted).
>>
>> I'll folow the following approach: I'll leave the data uncompressed,
>> until the analysis are completed (I have a new 2TB hdd now) and will
>> compress them into tar.gz files as soon as the project is finished. If I
>> need them again, I will use archivemount to access the data.
> 
> If you're planning on using archivemount, use zip/rar rather than
> .tar.gz (and don't use "solid" rar archives, which have the same
> problems with random access as tar files).

Thanks for this tip - I will likely do as suggested.

Cheers,

Rainer

> 


- -- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

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