plotting image/contour on irregular grid

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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