[R] akima interpolation and triangulation question

[Prof Brian Ripley]

You have repeated (u,v) values in your data frame.  It is not possible to 
interpolate such data, and I get an error from 'interp' (not NAs).

*** SDTRAN Error 2: The first three data points are collinear.

There is no problem with interpolating from a regular grid, but be aware 
that the triangulation-based methods are not isotropic in such a case (as 
the triangulation is not unique).

R-sig-geo would be a more appropriate list for such questions.

On Thu, 29 May 2008, steven wilson wrote:

> Dear all;
>
> First of all, this is probably a more conceptual question than a
> R-related one, but still want to give it a try. When working with the
> interpolation function "interp" from the package akima and the
> triangulation function "tri.mesh" from package tripack I've got NA's
> for the interpolation and "error" for the triangulation. The data is
> arranged in a regular grid as opposed to what the help page for interp
> says but I think this is an interesting problem because I found this
> code on the web as part of an stats course (and it is not the only
> example I found). Because I'm not really into the details of the
> functions I would like to hear comments from people who have used
> these functions before.
>
> This is the code
>
> library(akima)
> library(fields)
> library(tripack)
>
> soil <- read.table("http://www.unc.edu/~zhuz/teaching/Stat890/Data/soil.txt",header=TRUE)
> attach(soil)
> soil.interp <- interp(u, v, moist) # linear interpolation works
> plot.surface(soil.interp)
>
> # spline interpolation
> soil.interp2 <- interp(u, v, moist, linear = F) # spline interpolation
> returns only NA's
>
> # triangulation
> tri.mesh(soil$u, soil$v)  # got error
>
> Thanks
>
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>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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