How to write a raster with RAT factors in R raster package

Question

I want writeRaster to write the RAT (raster attribute table) that I've built in R.

I'm running R 3.0.1, raster 2.1-49, and rgdal 0.8-10.

My input raster looks like this:

r <-raster("F:/test.img")

class       : RasterLayer 
dimensions  : 3, 3, 9  (nrow, ncol, ncell)
resolution  : 30, 30  (x, y)
extent      : 347325, 347415, 4301655, 4301745  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
data source : F:\test.img 
names       : test 
values      : 1, 19  (min, max)

I then build my attribute table:

r <- ratify(r)
rat <- levels(r)[[1]]
rat$Pixel_Values <- c(1, 7, 8, 9, 19)
rat$Class_Names <- c("A", "B", "C", "D", "E")
levels(r) <- rat

Which results in a raster with attributes:

r

# class       : RasterLayer 
# dimensions  : 3, 3, 9  (nrow, ncol, ncell)
# resolution  : 30, 30  (x, y)
# extent      : 347325, 347415, 4301655, 4301745  (xmin, xmax, ymin, ymax)
# coord. ref. : +proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
# data source : F:\test.img 
# names       : test 
# values      : 1, 19  (min, max)
# attributes  :
#  ID Pixel_Values Class_Names
#   1            1           A
#   7            7           B
#   8            8           C
#   9            9           D
#  19           19           E

I then attempt to write my raster together with its RAT:

ratRaster <- "F:/testRat.img"
writeRaster(r, filename=ratRaster, datatype="INT1U", RAT=TRUE, progress="window", overwrite=TRUE)

But when I read it back into R, it becomes apparent that the attributes did not persist:

r2 <- raster(ratRaster)

r2
# class       : RasterLayer 
# dimensions  : 3, 3, 9  (nrow, ncol, ncell)
# resolution  : 30, 30  (x, y)
# extent      : 347325, 347415, 4301655, 4301745  (xmin, xmax, ymin, ymax)
# coord. ref. : +proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
# data source : F:\testRat.img 
# names       : testRat 
# values      : 1, 19  (min, max)

It would be quick and awesome to build RATs in R. How can I create and export the raster and keep the RAT?

Answer 1

Note that if you use raster's native .grd format (see doc, section 3.3) , the RAT table will be saved:

library(raster)
r <- raster(nrows=5, ncols=5)
r[] <- rep(1:5, 5)
r <- ratify(r)
rat <- levels(r)[[1]]
rat$Pixel_Values <- 1:5
rat$Class_Names <- c("A", "B", "C", "D", "E")
levels(r) <- rat

r
writeRaster(r, filename="raster_rat.grd")

Now re-open:

r2 <- raster("raster_rat.grd")
r2

Answer 2

You could always write the RAT as a csv file, then join that data later.

Write your raster as you specified:

writeRaster(r, filename=ratRaster, datatype="INT1U", RAT=TRUE, progress="window", overwrite=TRUE)

Write the attribute data/table/RAT as a .csv file:

write.csv(rat, file="C:\\merp\\rat.csv", row.names = F)

Then you can join this data in another program later. For example, if exporting from R to ArcMap, write the raster to disk, write the attribute data as csv file, then join your RAT to the raster using the Add Join tool in ArcMap.