[R-sig-Geo] Jenks classification for raster representation

[Tara Bridwell]

Thank you Robert.  That worked perfectly.  I had been missing the
proper structure in using the rasterize function.

Tara

On Sat, Sep 17, 2011 at 5:42 PM, Robert J. Hijmans <r.hijmans at gmail.com> wrote:
> Arnaud,
>
> You can (directly) do the same with Raster* objects what Roger did with sp
> objects:
>
> library(raster)
> t<-raster(ncol = 10, nrow = 10)
> t[1:2,] <- seq(1,10,1)
> t[3:5,1:5] <- seq(1,5,1)
> t[6:8,6:10] <-  seq(6,10,1)
> t[9:10,] <- seq(1,10,1)
> plot(t)
>
> library(classInt)
> zClass <- classIntervals(values(t), n=5,style="jenks")
>
> # image
> image(t, breaks=zClass$brks, col=colorRampPalette(c("red", "yellow"))(5) )
>
> # or spplot
> spplot(t, "values", at=zClass$brks, col.regions=colorRampPalette(c("red",
> "yellow"))(5))
>
> # or plot, after an adjustment to zClass that won't be needed in the future
> (versions > 1.9-13)
> zClass$brks[1] <- zClass$brks[1] - 1
> plot(t, breaks=zClass$brks, col=colorRampPalette(c("red", "yellow"))(5) )
>
>
> # The  only additional consideration is that classIntervals will fail, or be
> very slow, for very large rasters.
> # To avoid that, you can take a sample:
>
> zClass <- classIntervals(na.omit(sampleRegular(t, 100000)),
> n=5,style="jenks")
> zClass$brks[1] <- zClass$brks[1] - 1
>
> plot(t, breaks=zClass$brks, col=colorRampPalette(c("red", "yellow"))(5))
>
>
> Robert
>
>
> On Fri, Sep 16, 2011 at 12:58 PM, Arnaud Mosnier <a.mosnier at gmail.com>wrote:
>
>> Dear list,
>>
>> In order to allow others benefiting from my errors, see below a
>> presentation
>> of a problem and it's solution by Roger Bivand (Thanks !).
>>
>> ######################################
>>
>> I want to use natural breaks (jenks) method to find class intervals into
>> raster's values in order to plot it with an adapted color range.
>> I know that the package "classInt" allows to do exactly what I want but I
>> do
>> not achieve to use it correctly.
>>
>> Here is a small script that represent my case
>>
>> # here a raster with some NAs
>>
>> library(raster)
>> t<-raster(ncol = 10, nrow = 10)
>> t[1:2,] <- seq(1,10,1)
>> t[3:5,1:5] <- seq(1,5,1)
>> t[6:8,6:10] <-  seq(6,10,1)
>> t[9:10,] <- seq(1,10,1)
>> plot(t)
>>
>> # Then I tried to use classIntervals and findColours function
>>
>> library(classInt)
>> zClass <- classIntervals(as.data.frame(t)[!is.na(as.data.frame(t))], n=5,
>> style="jenks")  # I have to remove NAs
>> plot(t, col = findColours(zClass, pal = c("red", "yellow"))) # I do not
>> understand the result both in the plot and in the legend !
>>
>> I would be pleased if you can give me tips about this.
>>
>> ########################################
>>
>> I do not think that the plot method for RasterLayer objects lets you pass a
>> vector of breaks, nor does the image.plot from fields that maybe is the
>> source of the legend. If you do things simply, it works:
>>
>> tSG <- as(t, "SpatialGridDataFrame")
>> spplot(tSG, "values", at=zClass$brks,
>>  col.regions=colorRampPalette(
>> c("red", "yellow"))(5))
>>
>> or
>>
>> image(tSG, "values", breaks=zClass$brks,
>>  col=colorRampPalette(c("red", "yellow"))(5))
>>
>> or using graphics::image
>>
>> image(as.image.SpatialGridDataFrame(tSG), breaks=zClass$brks,
>>  col=colorRampPalette(c("red", "yellow"))(5))
>>
>> I'm a bit unsure about whether the itervals are closed which way.
>>
>> It's up to the raster developers to answer.
>>
>> Roger
>>
>> --
>> Roger Bivand
>> Department of Economics, NHH Norwegian School of Economics,
>> Helleveien 30, N-5045 Bergen, Norway.
>> voice: +47 55 95 93 55; fax +47 55 95 95 43
>> e-mail: Roger.Bivand at nhh.no
>>
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>>
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>
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