interpolation, rast.to.sites

[Brian Culpepper]

> 
> 
> I have been told that s.surf.tps is the "best" interpolater available
> in grass.  Why is this?  My little experience with it has shown
> that it is certainly not quick!  What are its relative merits,
> in comparison with idw and idw2?
> 
> What is the easiest way to convert a raster to a site list, so
> that I can use the s.surf.tps interpolater?  Is it necessary to
> go via an ascii intermediate step?
> 
> Thanks for your advice.
> 
> Simon Cox
>---------------------------------

Simon,

Have you tried the r.random?  It will extract a percentage of points from a
raster map, which could then be read into s.menu to create a sites file. 

As for s.surf.tps, not knowing what you are going to interpolate creates a
problem.  s.surf.tps uses splining under tension to create a surface and 
assumes all input sites are of equal importance (ie. contour site list),     
whereas idw (inverse distance weighting) would be useful for interpolating
migratory bird densities between sampling sites.  The man pages might give
a better example, but s.surf.tps is only the "best" interpolator if it is
the correct tool for the job.

s.surf.tps has been known to "blow-up" if interpolating between many sites.
This is usually because of the extremely large .tmp files created during
the processing.  My advice to you would be to interpolate a sample area 
that offers an honest example of your study area.  The parameters are what
caused the most problem. Once the parameters are set for your sample
area, meaning that your results are acceptable, then run
the entire study area. Otherwise a lot of time/money could be wasted. 

I have been satisfied with the results s.surf.tps has given us for DEM
production, but the correct parameter settings are the key to success.

I hope this helps,


R. Brian Culpepper                                 Research Assistant
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Center for Advanced Spatial Technologies           (fax) 501 575-3846
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University of Arkansas                            brian at cast.uark.edu
Fayetteville, AR 72701
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All opinions are my own, and do not necessarily reflect those of the
Center for Advanced Spatial Technologies or the University of Arkansas.