[GRASS-dev] Re: [GRASS-user] problems using r.proj with large
dylan.beaudette at gmail.com
Mon Dec 18 20:20:10 CET 2006
On Sunday 17 December 2006 17:14, Glynn Clements wrote:
> Glynn Clements 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:
> > > http://intevation.de/rt/webrt?serial_num=241
> > 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.
> I've added to CVS a modified version of r.proj named r.proj.seg. This
> behaves identically to r.proj, but uses a tile cache rather than
> reading the entire map into memory.
> Currently, each tile is 64 * 64 cells, and the size of the cache is
> fixed at 2 * (nx + ny) tiles (where nx * ny is the size of the source
> region in tiles), which I would expect to suffice for any realistic
> [It will definitely suffice for any affine or "near"-affine
> transformation. It would be useful to know what level of distortion
> needs to be accommodated in practice.]
> Cache replacement is random (i.e. when a new tile is loaded, the tile
> which it replaces is selected at random). In practice, random
> replacement tends to work almost as well as an "optimal" algorithm,
> but without the overhead of maintaining usage statistics, as well as
> not having any degenerate cases.
> Suggestions for improvements and/or performance statistics for
> "real-world" usage are welcome.
Interesting work Gylnn. Not having tried your module yet, I am wondering: does
the segmenting result in any sort of artifacts in the output raster? i.e. I
have seen artifaces related to segmenting with the v.surf.rst module before,
when optimal settings are not used.
i'll give the new r.proj.seg a try later today.
Soils and Biogeochemistry Graduate Group
University of California at Davis
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