Reading and writing spatial data¶
Introduction¶
Reading and writing spatial data is complicated by the fact that there are many different file formats. However, there are a few formats that are most common that we discuss here.
Vector files¶
The shapefile
is the most commonly used file format for vector data.
It is trivial to read and write such files. Here we use a shapefile that
comes with the terra
package.
Reading¶
We use the system.file
function to get the full path name of the
file’s location. We need to do this as the location of this file depends
on where the raster package is installed. You should not use the
system.file
function for your own files. It only serves for creating
examples with data that ship with R.
library(terra)
filename <- system.file("ex/lux.shp", package="terra")
filename
## [1] "C:/soft/R/R-4.0.3/library/terra/ex/lux.shp"
Now we have the filename we need we use the vect
function.
s <- vect(filename)
s
## class : SpatVector
## geometry : polygons
## dimensions : 12, 5 (geometries, attributes)
## extent : 5.74414, 6.528252, 49.44781, 50.18162 (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=longlat +datum=WGS84 +no_defs
## names : ID_1 NAME_1 ID_2 NAME_2 AREA
## type : <num> <chr> <num> <chr> <num>
## values : 1 Diekirch 1 Clervaux 312
## 1 Diekirch 2 Diekirch 218
## 1 Diekirch 3 Redange 259
The vect
function returns SpatVector
objects. It is important to
recognise the difference between this type of R object
(SpatVector
), and the file (“shapefile”) that was used to create it.
Thus, you should never say “I have a shapefile in R”, say “I have a
SpatVector of polygons in R”, (and in some cases you can add “created
from a shapefile”). The shapefile is one of many file formats for vector
data.
Writing¶
You can write new files using the writeVector
method. You need to
add argument overwrite=TRUE
if you want to overwrite an existing
file.
outfile <- "shp_test.shp"
writeVector(s, outfile, overwrite=TRUE)
To remove the file again:
Raster files¶
The terra package can read and write several raster file formats.
Reading raster data¶
Again we need to get a filename for an example file.
f <- system.file("ex/logo.tif", package="terra")
f
## [1] "C:/soft/R/R-4.0.3/library/terra/ex/logo.tif"
Now we can do
r <- rast(f)
r
## class : SpatRaster
## dimensions : 77, 101, 3 (nrow, ncol, nlyr)
## resolution : 1, 1 (x, y)
## extent : 0, 101, 0, 77 (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
## source : logo.tif
## names : red, green, blue
## min values : 0, 0, 0
## max values : 255, 255, 255
Note that x
is a SpatRaster of three layers (“bands”). We can subset
it to get a single layer.
r2 <- r[[2]]
r2
## class : SpatRaster
## dimensions : 77, 101, 1 (nrow, ncol, nlyr)
## resolution : 1, 1 (x, y)
## extent : 0, 101, 0, 77 (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
## source : logo.tif
## name : green
## min value : 0
## max value : 255
The same approach holds for other raster file formats, including GeoTiff, NetCDF, Imagine, and ESRI Grid formats.
Writing raster data¶
Use writeRaster
to write raster data. You must provide a SpatRaster
and a filename. The file format will be guessed from the filename
extension. If that does not work you can provide an argument like
wopts=list(format=GTiff)
. Note the argument overwrite=TRUE
and
see ?writeRaster
for more arguments, such as datatype=
to set
the datatype (e.g., integer, float).
x <- writeRaster(r, "test_output.tif", overwrite=TRUE)
x
## class : SpatRaster
## dimensions : 77, 101, 3 (nrow, ncol, nlyr)
## resolution : 1, 1 (x, y)
## extent : 0, 101, 0, 77 (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
## source : test_output.tif
## names : red, green, blue
## min values : 0, 0, 0
## max values : 255, 255, 255