contours of a SpatialPixelsDataFrame?

I am unsure on how to deal with this... (this is usually easy on  
grass), however cuz all me figs have been done in R I need to be  
consistent.

The question is how to to get a contour plot from a  
SpatialPixelsDataFrame.

So far I have generated I seem to be succesfully generating the plot  
using :

 >    contour(seq(-2423411.1,271933.9,length=674),seq 

(248129.5,3239402.6,length=748),matrix(grPC at data$PC1,ncol=748))

but the plot seem to be upside down. It looks ok, but wen adding the  
map lines with :

 > plot(usaca,add=T)

I notice that only the 'y' axis is shifted.... a transformation of  
the matrix won't do the trick.  So, I wonder is what the best  
approach is for this. I tried making an image of it, but thinking  
that  as.image.SpatialGridDataFrame will create a usable matrix don't  
seem to be quite right

anyhow, I hope that some more experienced user will be able to tell  
me the correct way to do this,

thanks,

PD: here is the definition of my sp object:

 > str(grPC)

Formal class 'SpatialPixelsDataFrame' [package "sp"] with 7 slots
   ..@ data       :Formal class 'AttributeList' [package "sp"] with 1  
slots
   .. .. ..@ att:List of 6
   .. .. .. ..$ PC1: num [1:504152] 0.299 0.301 0.302 0.303 0.305 ...
   .. .. .. ..$ PC2: num [1:504152] -0.218 -0.219 -0.220 -0.220  
-0.221 ...
   .. .. .. ..$ PC3: num [1:504152] -0.0222 -0.0226 -0.0230 -0.0233  
-0.0237 ...
   .. .. .. ..$ PC4: num [1:504152] -0.0290 -0.0288 -0.0286 -0.0285  
-0.0283 ...
   .. .. .. ..$ PC5: num [1:504152] 0.0607 0.0608 0.0609 0.0610  
0.0611 ...
   .. .. .. ..$ PC6: num [1:504152] 0.0681 0.0683 0.0685 0.0687  
0.0688 ...
   ..@ coords.nrs : num(0)
   ..@ grid       :Formal class 'GridTopology' [package "sp"] with 3  
slots
   .. .. ..@ cellcentre.offset: Named num [1:2] -2421412   250129
   .. .. .. ..- attr(*, "names")= chr [1:2] "s1" "s2"
   .. .. ..@ cellsize         : Named num [1:2] 3999 3999
   .. .. .. ..- attr(*, "names")= chr [1:2] "s1" "s2"
   .. .. ..@ cells.dim        : Named int [1:2] 674 748
   .. .. .. ..- attr(*, "names")= chr [1:2] "s1" "s2"
   ..@ grid.index : int [1:504152] 1 2 3 4 5 6 7 8 9 10 ...
   ..@ coords     : num [1:504152, 1:2] -2421412 -2417413 -2413414  
-2409414 -2405415 ...
   .. ..- attr(*, "dimnames")=List of 2
   .. .. ..$ : NULL
   .. .. ..$ : chr [1:2] "s1" "s2"
   ..@ bbox       : num [1:2, 1:2] -2423411   248130   271934  3239403
   .. ..- attr(*, "dimnames")=List of 2
   .. .. ..$ : chr [1:2] "s1" "s2"
   .. .. ..$ : chr [1:2] "min" "max"
   ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
   .. .. ..@ projargs: chr NA

--
Horacio Samaniego
Dept. Biology
University of New Mexico
Albuquerque 87106, NM

http://www.unm.edu/~horacio




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