[R-sig-Geo] spplot handling of overlapping points?

[MacQueen, Don]

I have a SpatialPointsDataFrame object in which many points are very close
together, such that the markers (plotting characters) tend to overlap. (Of
course, this depends on marker size and the scale at which I plot; if I
"zoom in" there is less overlap.)

It appears that when markers overlap, spplot() places a marker associated
with the larger value after, and therefore on top of, a marker associated
with a smaller value.

Am I correct? Or more generally, what is the algorithm that determines the
order in which markers are added?


I have searched ?spplot and related help pages and haven't found an
explanation (at least, not yet).

I can add that I don't believe markers are placed in the order in which
they appear in the SpatialPointsDataFrame. I say this because when I
attempt to reproduce an spplot using base graphics plot() I have to sort
from smallest to largest value to succeed.

I can probably provide a small reproducible example if necessary, but I'm
hoping it's not necessary.

Here are my actual commands.

  tmps is the SpatialPointsDataFrame.
  tmp is coordinates(tmps)
(they have the same number of rows in the same order)

Note that I'm using the cuts argument to break a continuous variable
('cpm2') into bins. I have carefully matched the colors in tmps$col2 with
the cbin.cols object passed to spplot(), and the break points for the bin
boundaries, so I believe that everything else that could affect the final
appearance, other than the order in which the markers are placed, is
controlled.

#1 using spplot
spplot(tmps,c('cpm2'),
   key.space='right',
   legendEntries=cbin.lbls,
   cuts=cbin.brks,
   col.regions=cbin.cols,
   cex=0.4)

#2 using base graphics
plot(tmp[,1],tmp[,2], asp=1, cex=0.6, pch=16, col=tmps$col2)



Thanks
-Don

-- 
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062