Plotting terra raster with white color set for 0 values-CodePudding

I have a raster in terra that has values that ranges around 0. I want my negative values to be blue and positive values to be red. This question has already been ask

You can see that the white color was not fixed to the zero value. Thats is probably because terra::plot calculate it's own range which is different from mine.

Reading around

and:

coltab(rr) <- data.frame(
  val=values(rr),
  col=the_palette_fc(values(rr)))

plot(rr)

But the result do not seem to give the wanted results. Also, there is no legend automatically added which is also necessary. Maybe the coltab argument should be only used for categorical values?

So is there a way in terra to fined tune the colors palette so white is fix for 0 values?

CodePudding user response：

To set 0 as the midpoint, you can set the domain of your palette to c(-max(rr[]), max(rr[])):

the_palette_fc <- leaflet::colorNumeric(palette = "RdBu", 
                                        domain = c(-max(rr[]), max(rr[])),
                                        reverse = TRUE)

the_colors <- the_palette_fc(seq(min(rr[]), max(rr[]), length.out = 50))

plot(rr, col = the_colors)

If you want the scale to go from darkest blue to darkest red with a transition of white at 0, it is a bit more involved, but you could do:

the_palette_fc <- leaflet::colorNumeric(palette = "RdBu", 
                                        domain = c(-max(rr[]), max(rr[])),
                                        reverse = TRUE)

breakpoints <- seq(min(rr[]), max(rr[]), length.out = 50)

the_colors <- the_palette_fc(c(seq(-max(rr[]), 0, length = sum(breakpoints < 0)),
                              seq(0, max(rr[]), length = sum(breakpoints > 0))))

plot(rr, col = the_colors)

CodePudding user response：

terra::plot actually does have a "breaks" argument; but it is only documented in the examples so it was easy to overlook. You can do

library(terra)
set.seed(1234)
rr <- rast(matrix(rnorm(400, 1.5, 1), nrow=20, ncol=20))
ceil <- values(rr) |> abs() |> max() |> ceiling() 
pal <- leaflet::colorNumeric(palette = "RdBu", domain=c(-ceil, ceil), reverse = T)

b <- seq(-ceil, ceil, 1)
plot(rr, type="interval", breaks=b, col=pal(b))

In response to your comment:

For your purposes, you can also use the range argument, I think.

plot(rr, range=c(-ceil, ceil), col=pal(seq(-ceil,ceil,.1)))

And if you use "breaks", with terra 1.6-14 (currently the development version) you can now overwrite the default legend type.

plot(rr, type="cont", breaks=b, col=pal(b))