--- roger koenker <roger at ysidro.econ.uiuc.edu>
wrote:>
>
> On May 6, 2005, at 2:45 PM, Roger Bivand wrote:
>
> > On Fri, 6 May 2005, m p wrote:
> >
> >> Hello,
> >> I'd like to make a z(x,y) plot for irregularly
> spaced
> >> x,y. What are routines are available in R for
> this
> >> purpose?
> >>
> >
> > One possibility is to interpolate a regular grid
> using interp() in the
> > akima package, then use image() or contour().
> Another is to use
> > levelplot() with formula z ~ x + y in the lattice
> package, and the
> > equivalent contourplot(); here, the x,y pairs must
> lie on a grid,
> > but do
> > not need to fill the grid (so are regularly spaced
> with missing grid
> > cells).
> >>
>
> You could also try tripack and rgl.triangles to
> produce piecewise linear
> surfaces on the Delaunay triangulation of the x,y
> points.
>
Or perhaps try gridding first using one of the kriging
packages.
It is just a little thing, but the method you decide
to use is dependent on how you intend to use/represent
the result ;O). Then there is the need for judgement,
and then judgement, and oh, yes, more judgement.
Density of points, clustering, blah, blah,... .
Why do I say this? My experience with data if this
nature is that you really do have to be careful or you
may have yourself and others interpreting artifacts.
If you are at the beginning of getting involved in
some serious application(s) and time investment, you
should explore the 'art and craft' of
estimation....curl up with a nice geostatistics book,
usw. That is, there is more to it than finding a
package in R. If it is a 'once-over' with tame data,
who cares? huh?
Good luck and enjoy...
Michael Grant
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