[R] ountour "resolution"

(Ted Harding) Ted.Harding at manchester.ac.uk
Mon Sep 14 00:54:56 CEST 2009

Greetings all!
I'm wanting to plot contours of a function, and I don't
want to evaluate it at a dense grid of points (i.e. I don't
want a huge array of values).

Say I have a vector of x-values such as x <- 0.1*c(1:10),
and the same for y <- 0.1*(0:10).

I then evaluate a 10*10 matrix z of values of z = f(x,y).

so I can then do

  CL <- contourLines(x,y,z)

and get a list of contour-lines, each list containing the
x and y coordinates.

When I look at the plot from contour(x,y,z), I see that the
lines are (as expected) made up of straight-line segments.
But the break-points between the segments seem to either have
x-coords on the x-values in x, or have y-coords on the y-values
in y. In other words, the breaks occur on the sides of the
squares of the grid defined by x and y.

So quite often the straight-line segments are quite long, and
the resulting contours do not look at all smooth.

An example of what I'm talking about can be seen from the
following code:

  n2 <- function(x,y){
    dnorm(x,0.25,0.3)*dnorm(y,0.25,0.3) +

  x <- y <- 0.1*(0:10)
  z <- matrix(numeric(121),nrow=11)
  for(i in (0:11)){ for(j in (1:11)){ z[i,j]<-n2(x[i],y[j]) } }

  for(i in (1:11)) lines(c(x[i],x[i]),c(0,1))
  for(i in (1:11)) lines(c(0,1),c(y[i],y[i]))

I could get whatever minute difference between contour-levels
I like by setting the nlevels or levels parameter. But there
doesn't seem to be any way of achieving finer resolution in
where the break-points occur. It is just as coarse, and always
breaks on the grid-lines.

Looking at the plot, it seems that this is due to an algorithm
which linearly interpolates along the sides of the grid squares,
and joins the resulting points by line segments.

It seems that contourLines() generates the same breakpoints as
occur in the plot from contour().

I would be happy with the relatively few contour lines and the
fairly coarse spacing between contour levels shown in the above
plot, but would like to have smoother contours without having to
use denser grid spacing such as

  x <- y <- 0.005*(0:200)
  z <- matrix(numeric(40401),nrow=201)

Is there a function which, perhaps, produces contours by non-linear
interpolation, and curved smoothing between interpolated points,
to a specified resolution between break-points?

I have had a poke under "functions" in the R-Site Help and Archive
Search at http://finzi.psych.upenn.edu, searching on "contour",
but nothing has leapt obviously to the eye amongst the 494 hits.

With thanks,

E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 13-Sep-09                                       Time: 23:49:06
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