[GRASS-dev] Re: [GRASS-user] problems using r.proj with large data
morten at untamo.net
Wed Dec 13 17:44:17 CET 2006
Glynn Clements wrote:
> Maciej Sieczka wrote:
>>> Is this a problem with large files that I will just have to work around or
>>> is it something to do with my setup?
>> Propably the same, very old issue:
> I looked into this a while ago. Unfortunately, you can't use rowio (or
> a home-grown equivalent), as libgis doesn't allow the projection to be
> changed while maps are open. So, you have to read the entire input
> map, close it, change the projection, then write the output map.
> To get around the memory issues, you would first need to copy the
> relevant portion of the input map to a temporary file, then use a
> cache backed by that file.
> The segment library would do the job, although it could add a
> significant performance overhead.
Are you sure it would help for all possible projections? The way r.proj
works -- reverse-projecting cell-by-cell into the input map and looking
up the value of the nearest neighbor cell (default method) -- it seems
difficult to always have the right "relevant part" of the input map in
memory. Between two cylindrical projections perhaps, but for other types
wouldn't there be a lot of swapping in and out of memory of the
"relevant parts". Hmm... I guess this is what you mean with performance
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