[R-sig-Geo] akima interpolation + Delaunay triangulation

[Frede Aakmann Tøgersen]

In answering Steven I forgot to address this list also, so here is a repost of my answer (see below). At closing the R session in Emacs/ESS I got this: 


> Save workspace image? [y/n/c]: n
 
*** SDTRAN Error 2: The first three data points are collinear.

    Error detected in SDTRAN called by SDSF3P

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

    Error detected in SDTRAN called by SDSF3P

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

    Error detected in SDTRAN called by SDSF3P


Process R finished at Wed Jun 04 11:05:28 2008

For this reason I have included the maintainer in the list. It support my answer.



My answer to Steven:

I think that it is because you data allready is gridded.

See ?interp where it is mentioned in the section on arguments:

          'x', 'y', and 'z' must be the same length and may contain no
          fewer than four points. The points of 'x' and 'y' cannot be
          collinear, i.e, they cannot fall on the same line (two
          vectors 'x' and 'y' such that 'y = ax + b' for some 'a', 'b'
          will not be accepted). 'interp' is meant for cases in which
          you have 'x', 'y' values scattered over a plane and a 'z'
          value for each.  If, instead, you are trying to evaluate a
          mathematical function, or get a graphical interpretation of
          relationships that can be described by a polynomial, try
          'outer()'. 

Try:

par(mfrow=c(2,1))
plot(u,v)
plot(jitter(u),jitter(v))

soil.interp3 <- interp(jitter(u),jitter(v),moist, linear=FALSE)
plot.surface(soil.interp3)


Best regards

Frede Aakmann Tøgersen
Scientist


UNIVERSITY OF AARHUS
Faculty of Agricultural Sciences
Dept. of Genetics and Biotechnology
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> -----Oprindelig meddelelse-----
> Fra: r-sig-geo-bounces at stat.math.ethz.ch 
> [mailto:r-sig-geo-bounces at stat.math.ethz.ch] På vegne af steven wilson
> Sendt: 3. juni 2008 5:55
> Til: r-sig-geo at stat.math.ethz.ch
> Emne: [R-sig-Geo] akima interpolation + Delaunay triangulation
> 
> Dear all;
> 
> I would like to have some input from anybody who have used 
> the interpolation function (interp) in the akima package and 
> the triangulation function (tri.mesh) from the tripack 
> pakage. My problem is that for a given data set these 
> functions return errors in R while in S-Plus they don't. The 
> versions of these functions in S-Plus
> (S+SpatialStats) can be slightly different to those in R but 
> the output shoould be the same or approx the same because 
> they're based on the same algorithm and fortran code.
> 
> Here is the code in R:
> 
> library(akima)
> library(fields)
> library(tripack)
> 
> soil <- 
> read.table("http://www.unc.edu/~zhuz/teaching/Stat890/Data/soi
> l.txt",header=TRUE)
> attach(soil)
> soil.interp <- interp(u, v, moist) # linear interpolation 
> works and the results are similar to those in S-Plus
> plot.surface(soil.interp)
> 
> # spline interpolation
> soil.interp2 <- interp(u, v, moist, linear = F) # spline 
> interpolation returns only NA's in R while using S-Plus it works fine
> 
> # triangulation
> tri.mesh(soil$u, soil$v)  # error in R, works in S-Plus
> 
> Any comments will be appreciated
> 
> Thanks
> 
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