truncate colour pallete with ggplot2 and geom_spatraster() in different layers

Question

I want to be able truncate the colour pallete for certain values of a raster using ggplot, which I am able to do using basic plotting of the terra object, but not with ggplot2.

This is what I have been able to produce

library(terra)
library(tidyterra)
library(ggplot2)

rast1 <- rast(ncol=100, nrow=100, nlyr=1)
rast1 <- init(rast1, "cell")
rast1 <- spatSample(rast1, ncell(rast1), "random", as.raster=TRUE)

#full raster with basic terra plotting
plot(rast1)


#try to truncate values for better viz
mean.val <- mean(terra::values(rast1), na.rm = TRUE)
low.val<- min(terra::values(rast1), na.rm = TRUE)
zmin <- max(mean.val, 0.001, na.rm = TRUE)
zmin <- max(zmin, 0.01, na.rm = TRUE)
zmax <- max(terra::values(rast1), na.rm = TRUE)
q99 <- quantile(terra::values(rast1), probs=c(0.999), na.rm=TRUE)

#Truncated raster using terra base plotting
plot(rast1, col = whitebox.colors(100), range=c(zmin,q99), axes=FALSE)

#plotting high values
plot(rast1, col="#255668", range=c(q99,zmax), axes=FALSE, legend=FALSE, add=TRUE)

#plotting low values
plot(rast1, col="#F9FFAF", range=c(low.val,zmin), axes=FALSE, legend=FALSE, add=TRUE)

Which produces this output desired ouput

#So far I can only set limits with ggplot
#mid values
ggplot() +
  geom_spatraster(data = rast1) +
  scale_fill_whitebox_c(limits = c(zmin,q99))

incomplete ggplot2 output

I would need to incorporate the remaining colors for the highest and lowest range of values. But I staill haven't figure out how to do it, without "reclassifying" the data, and I still think it may be a bit tricky because I only want to truncate to a high and low value, but retain the full range of mid values. Which I believe reclassifying wont really do.

Any input would be greatly appreciated!

Hello @stefan, quick question, why remove the raster tag on my post? It is about plotting a raster (not about the raster package, which tag is r-raster, correct? thanks for any comment on this, trying to learn more about posting on SO! Cheers! — Francis van Oordt
– Francis van Oordt, Commented Nov 23, 2023 at 14:29
I had to decide which was one drop to make room for the R tag (only 5 tags are allowed). The reason is that your question is first a question about R programming then about ... And many questions are unnoticed because people forget about the R tag. — stefan
– stefan, Commented Nov 23, 2023 at 15:30
That is a very good point! I didn't think of that! its interesting that the R tag makes questions more noticeable. thanks for letting me know! — Francis van Oordt
– Francis van Oordt, Commented Nov 23, 2023 at 16:19

stefan · Accepted Answer · 2023-11-22 23:20:19Z

0

One possible option would be to create a custom color scale using scale_fill_gradientn (which under the hood is also used by scale_fill_whitebox_c). The vectors to be passed to the colors= and values= argument are pretty straightforward with the exception that the values= argument requires a vector in the range c(0, 1) and hence require some rescaling. Finally, set oob = scales::oob_squish which in contrast to the default oob_censor "squishes" values which fall outside of the limits into the range of the scale. This way the low values are assigned the low color and the high values the high color. There is still a small drawback though as the low and (especially) the high color are visible in the legend if you take a close look.

library(terra)
library(tidyterra)
library(ggplot2)
library(scales)

ggplot() +
  geom_spatraster(data = rast1) +
  scale_fill_gradientn(
    colors = c(
      "#F9FFAF",
      whitebox.colors(100),
      "#255668"
    ),
    values = scales::rescale(
      sort(c(range(terra::values(rast1)), c(zmin, q99))),
      to = c(0, 1)
    ),
    oob = scales::squish,
    limits = c(zmin, q99)
  ) +
  theme_void()

answered Nov 22, 2023 at 23:20

stefan

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1 Comment

Francis van Oordt Over a year ago

This seems to work nicely! will try playing around with it! Thanks!

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