[GRASS-dev] Re: v.krige.py Documentation

[Anne Ghisla]

Il giorno mer, 02/09/2009 alle 07.30 -0400,
grass-dev-request at lists.osgeo.org ha scritto:
> Greetings,
> 
> I am attempting to use the v.krige.py extension with RC 5 by testing  
> it with 20,000 points that it is trying to allocate 3 gigabytes of  
> memory for and failing (which is odd because I have 8 GB on this  
> machine and top reports that I have 4.3 GB free). I was able to  
> successfully krig 5,000 points.

Hello Jeshua,

first of all thanks for testing v.krige. Your case study is very useful
for v.krige improvement.

> Ultimately I need to be able to handle around a million points - not  
> looking like it will be possible based on my test.
> 
> Can anyone tell me what the required memory is based on? Is it based  
> on the region, the points, the block size, etc?

On the number of points and on the region. They both contribute to
create large intermediate and output objects.

R memory usage use deserves some consideration: you can find information
here [0,1]. I know it is a limitation when handling large datasets: a
good improvement for the code will be for example to store objects on
the hard drive instead of RAM. Alternatively, v.krige could work on a
slice of data at once, but I don't know if it is possible - worth to ask
to Roger Bivand and Edzer Pebesma.

> Also, I can find very little documentation for it. For instance I am  
> not sure what the block size is used for, how is it different than the  
> cell size set with g.region?

Block size is a parameter used by block kriging, different from region
resolution - see krige() documentation and examples [2].

> It looks very interesting if I were able to actually use it.
> 
> The only documentation I can find for it is the man page and the wiki  
> page neither of which really address any of these questions.

That's right - as v.krige is just a wrapper, its documentation should
contain all useful pointers to the code it includes. Will improve this
asap.

> Note that I can only run it from the command line, when I try to start  
> the wxpython interface it displays a box for an instant and then quits  
> without any error messages.

I suppose you run Windows: that's what happen when you use head revision
of v.krige and an outdated revision of GRASS. The module is presently
developed with develbranch_6, so needs recent updates (r38834 or
higher). I plan to improve backward compatibility in the near future.

Hope this helps!

best regards,
Anne Ghisla

> Thanks,
> 
> Jeshua Lacock, Owner
> <http://OpenOSX.com>
> phone: 208.462.4171

[0]
http://stat.ethz.ch/R-manual/R-patched/library/base/html/Memory-limits.html 
[1] http://www.matthewckeller.com/html/memory.html
[2] http://sekhon.berkeley.edu/library/gstat/html/krige.html

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