[R] Colorramp in Maptools, how to choose min and max values for the fg= argument

[Barry Rowlingson]

On Wed, Sep 22, 2010 at 9:56 AM, schaber <kschaber at ipp.mpg.de> wrote:
> Hi Jannis,
>
> thanks a lot for your reply. Unfortunately the solution you proposed does
> not work.
> Maybe the reason is, that plot.Map only accepts hsv colours and I do not
> know to convert the rgb colours to the right colour space.
>
> Isn't there a possibility to specify minimal an maximal values when using
> colorRampPalette to only choose a certain part of the colorRamp(Palette)? I
> could than define a new colorset  jet.colors2, which is picks out the
> colours between min(Vector)/10 and max(Vector)/10?
>
> jet.colors = colorRampPalette(c("#00007F", "blue", "#007FFF",
>                                "cyan", "#7FFF7F", "yellow", "#FF7F00",
> "red", "#7F0000"))
> Intervalls <- cut(as.numeric(Matrix[,nc]),n, na.rm=TRUE)  }
> fgs <- jet.colors(n)[Intervalls]
>
> Maybe this is a bit confusing...

If you want to control precisely the mapping of numeric values to
colours, then you could try my colourschemes package from R-forge:

install.packages("colourschemes", repos="http://R-Forge.R-project.org")

then you create a function that maps values to colours by, for
example, interpolating colours at fixed values, eg:

> rs = rampInterpolate ( limits =c(-2 , 2),
               ramp = c("red", " yellow ", " blue "))

 and then:

 > rs(2)
[1] "#0000FFFF"  # gives blue
 > rs(-2)
[1] "#FF0000FF"  # gives red
 > rs(0.5)
[1] "#BFBF40FF"  # nearer yellow than blue

 so now whatever the range of your data you can just use the rs()
function to get the same colours for the same values.


 > d=data.frame(x=1:10,y=1:10,V1=runif(10,0,2),V2=runif(10,-2,0),V3=runif(10,-2,2))
 > plot(d$x,d$y,col=rs(d$V1))
 > plot(d$x,d$y,col=rs(d$V2))
 > plot(d$x,d$y,col=rs(d$V3))

Barry