The terra package¶
This vignette describes the R package
terra. A raster is a spatial
(geographic) data structure that divides a region into rectangles called
“cells” (or “pixels”) that can store one or more values for each of
these cells. Such a data structure is also referred to as a “grid” and
is often contrasted with “vector” data that is used to represent points,
lines, and polygons.
terra package has functions for creating, reading, manipulating,
and writing raster data. The package provides, among other things,
general raster data manipulation functions that can easily be used to
develop more specific functions. For example, there are functions to
read a chunk of raster values from a file or to convert cell numbers to
coordinates and back. The package also implements raster algebra and
most functions for raster data manipulation.
A notable feature of the
terra package is that it can work with
raster datasets that are stored on disk and are too large to be loaded
into memory (RAM). The package can work with large files because the
objects it creates from these files only contain information about the
structure of the data, such as the number of rows and columns, the
spatial extent, and the filename, but it does not attempt to read all
the cell values in memory. In computations with these objects, data is
processed in chunks. If no output filename is specified to a function,
and the output raster is too large to keep in memory, the results are
written to a temporary file.
To understand what is covered in this vignette, you must understand the basics of the R language. There is a multitude of on-line and other resources that can help you to get acquainted with it.
In the next section, some general aspects of the design of the
package are discussed, notably the structure of the main classes, and
what they represent. The use of the package is illustrated in subsequent
terra has a large number of functions, not all of them are
discussed here, and those that are discussed are mentioned only briefly.
See the help files of the package for more information on individual
help("terra") for an index of functions by topic.