How to legend a raster using directly the raster attribute table and displaying the legend only for class displayed in the raster?

Question

I would like to use the raster attribute table information to create the legend of a raster such as the raster 1 and display the legend only for the class displayed in the raster. I build an example to explain what I would like to get.

1/ Build the raster

r <- raster(ncol=10, nrow=10)
values(r) <-sample(1:3,ncell(r),replace=T)

2/ Add the Raster Attribute Table

r <- ratify(r) # build the Raster Attibute table
rat <- levels(r)[[1]]#get the values of the unique cell frot the attribute table
rat$legend <- c('Class A', 'Class B', 'Class C')
levels(r) <- rat

3/ Plot the raster Fig 1

my_col=c('blue','red','green')
plot(r,col=my_col,legend=F,box=F,axes=F)
legend(x='top', legend =rat$legend,fill = my_col)

I would like to replace the legend =rat$legend parammeter by a properties of the raster linked to the ratser attribute table. I have tried different combination using levels() such as c(levels(r)[[1]][1]) but I generate a list and not a character not usable in the legend parameter.

4/ Crop and plot the raster to a part with only 2 classes (here the 4 pixels at the down right extent) Fig 2

rcrop<-crop(r,extent(r,9,10,9,10))
plot(rcrop,col=my_col,legend=F,box=F,axes=F)

For this second fig, I thus would like to diplay automatically only the legend of the class displayed on the raster 2.

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Here is the solution proposed by Roman 4.

Answer 1

If you are willing to use lattice graphics, the levelplot method defined in the rasterVis package is able to display categorical data with a legend based on the RAT:

library(raster)
library(rasterVis)

r <- raster(ncol=10, nrow=10)
values(r) <- rep(1:4, each=25)

r <- ratify(r) 
rat <- levels(r)[[1]]
rat$legend <- c('Class A', 'Class B', 'Class C', 'Class D')
levels(r) <- rat

levelplot(r)

Besides, I have commited some changes in the development version available at GitHub to manage a Raster* whose RAT levels are not all present in the data:

rcrop <- crop(r,extent(r,6,10,1,10))
levelplot(rcrop)

rcrop <- crop(r,extent(r,1,5,1,10))
levelplot(rcrop)

Answer 2

Maybe query the rcrop raster to find out which are the levels in the cropped raster and pass that to the legend?

legend(x = 'top', legend = unique(getValues(rcrop)), fill = my_col)

Answer 3

Thanks. Based on what you proposed I found a solution:

plot(rcrop,col=my_col[unique(getValues(rcrop))],legend=F,box=F,axes=F)
legend_full<-data.frame(levels(rcrop))
legend(x='top', legend =as.character(unlist(legend_full[unique(getValues(rcrop)),2])),fill = my_col[unique(getValues(rcrop))])

It works and allows to be independent of the values displayed in the raster.