Classify integer raster in tmap

Question

I want to plot a raster of the forest cover accross Europe using tmap::tm_raster(). The raster data are available here:
If I plot it, this is what I get:

# Read raster data:
forest <- raster("eea_r_3035_100_m_forest-area-2015_p_2015_v1_r1.tif",      NAvalue = 65535)

# Plot data:
  tm_shape(forest_br) +
    tm_raster(col = "eea_r_3035_100_m_forest-area-2015_p_2015_v1_r1.tif",
              palette = "darkgreen",
              title = "Forest cover")

when I check the raster properties, I see that I have an NA value equal 65535:
> forest
class      : RasterLayer 
dimensions : 46000, 65000, 2.99e+09  (nrow, ncol, ncell)
resolution : 100, 100  (x, y)
extent     : 9e+05, 7400000, 9e+05, 5500000  (xmin, xmax, ymin, ymax)
crs        : +proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs 
source     : C:/input/eea_r_3035_100_m_forest-area-2015_p_2015_v1_r1.tif 
names      : eea_r_3035_100_m_forest.area.2015_p_2015_v1_r1 
values     : 0, 65535  (min, max)

My forest = 1, land = 0, and NA should be transparent. If I set NAvalues(forest)<-65535, my min-max values remains the same.
Please, how can I specify this in tmap??? palette seems to works only for continuous data, but what about in my raster is binary and have many NA values (resp. 65535)?
THis is how it should look like (raster reclassified outside of R. Reclassify the values using classical matrix takes forever on my Win 10 64 b, R version 4.0.2 (2020-06-22)).

Spacedman · Answer

Here's a way to do it using GDAL's virtual raster driver. Here's my test tiff, which, like yours, has 65535 in it (all over the right hand side):

r = raster("r.tif") plot(r)

Now use the gdalUtils package (from CRAN, install if you need to) and create a VRT file version of your TIFF:

> library(gdalUtils)
> gdal_translate("r.tif","r.vrt", of="VRT")

This is a tiny text file which you can edit in a text editor. It starts like this:

<VRTDataset rasterXSize="10" rasterYSize="10">
  <GeoTransform>  0.0000000000000000e+00,  1.0000000000000001e-01,  0.0000000000000000e+00,  1.0000000000000000e+00,  0.0000000000000000e+00, -1.0000000000000001e-01</GeoTransform>
  <VRTRasterBand dataType="Float32" band="1">
 ....

find the line where the "NoDataValue" is set, and change it to 65535:

 <NoDataValue>65535</NoDataValue>

Now read the VRT (instead of the TIFF) and plot it:

> r = raster("r.vrt")
> plot(r)

bingo - the 65535 are treated as missing and the small numbers on the LHS are shown.

Note that the statistics on the layer (such as min/max) are taken from the VRT file so are wrong if 65535 is NA:

> r
class      : RasterLayer 
dimensions : 10, 10, 100  (nrow, ncol, ncell)
resolution : 0.1, 0.1  (x, y)
extent     : 0, 1, 0, 1  (xmin, xmax, ymin, ymax)
crs        : NA 
source     : /tmp/r.vrt 
names      : r 
values     : 1, 65535  (min, max)

You can delete the block if having wrong min/max messes anything up but R will recalculate it on load which might be slow.

But in short this is a neat way to add a missing data value flag to a raster. It does not change the original raster.

Classify integer raster in tmap

One Answer

Add your own answers!

Ask a Question