[R] akima interpolation and triangulation question

[steven wilson]

thanks.

one more thing w/r this particular data set:

tri.mesh doesn't work in R but triangulation (S+SpatialStats) does work
interp.old (diff ncp) doesn't work in R but it does in Splus for the
same ncp values???

so,....R or SPlus? who's right?


On Thu, May 29, 2008 at 4:34 AM, Prof Brian Ripley
<ripley at stats.ox.ac.uk> wrote:
> 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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>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> --
> 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)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>