[R] plotting a grid with color on a map

[Jim Lemon]

dxc13 wrote:
> Hi all,
> I have posted similar questions regarding this topic, but I just can't seem
> to get over the hump and find a straightforward way to do this.  I have
> attached my file as a reference.
> Basically, the attached file is a 5 degree by 5 degree grid of the the world
> (2592 cells), most of them are NA's.  I just want to be able to plot this
> grid over a world map and color code the cells.  For example, if a cell has
> a temperature less than 20 degrees it will be blue, 21 to 50 green color,
> 51-70 orange, 71+ red colored cells.  For any NAs, they should be colored
> white.
>
> I know how to create a map of the world using map() and add a grid to it
> using map.grid(), but I can't color code the cells the way I need.  Is there
> a way to do this in R?
>   
Hi dxc13,
This might get you started:

temp1<-read.table("time1test.dat",header=TRUE)
mapcol<-color.scale(as.matrix(temp1[36:1,]),c(0.5,1),c(0.5,0),c(1,0))
# have to draw the map to get the user coordinates
map()
# get the limits of the map
maplim<-par("usr")
# transform the temperatures into colors, reversing the row order
color2D.matplot(temp1[36:1,],cellcolors=mapcol,axes=FALSE)
# don't erase the current plot
par(new=TRUE)
# draw an empty plot with the appropriate axes (I think)
plot(0,xlim=maplim[1:2],ylim=maplim[3:4],type="n")
# add the map over the color squares
map(add=TRUE)

This seems a bit wonky, probably because I haven't adjusted the 
coordinates. Also, I'm only getting grayscale colors, even though the 
colors in "mapcol" aren't gray. Don't know why yet.

Jim