Drawing circular contours

Gabor, sp provides a contour method for objects of class 
SpatialGridDataFrame. I don't know how you should create a grid such 
that contours are circles -- simplest to generate distances from 
specific points. 

I don't know if there is easy way to do this right now; 
a rather indirect way is to use the kriging variance from a linear 
variogram and a 1-NN neighbourhood:

library(gstat) # requires sp
data(meuse)
coordinates(meuse) = ~x+y
data(meuse.grid)
gridded(meuse.grid) = ~x+y
# interpolate over the complete rectangular area, not just the study area:
meuse.grid = as(meuse.grid, "SpatialGrid")
# krige from 10 random points with linear variogram and 1-NN neighbourhood:
out = krige(log(zinc)~1,meuse[sample(1:155,10),], meuse.grid, vgm(1, 
"Lin", 0), nmax=1)
# contour kriging variance:
contour(out[2], asp=1)

Note that the contour method does not set the aspect ratio by default -- 
an omission.
--
Edzer


Gabor Grothendieck wrote:

I am currently doing the following to draw circles at the
indicated distances from the center.  The center is given
by utm coordinates: utm.e, utm.n .  It does work:

class(im2) # "SpatialGridDataFrame"
image(im2, "ind", col = col$ct)
library(plotrix)
dd <- c(25, 50, 100, 150)
for(i in 1:4) draw.circle(utm.e, utm.n, dd[i], border = "yellow")

However, I would like to do this in a style better suited to sp
and rgdal by defining a function whose contours are circles and
plotting the indicated contours of it.  Also I assume I will get
labelled contours by doing that which I don't currently have.

How do I do that?  I would appreciate specific code, if possible,

Thanks.