# For developers: Locations/projects and mapsets

#### Adam B. Smith

#### 2025-02-25

GRASS GIS uses “projects” (which used to be called “locations” before GRASS 8.4), and “mapsets” to store files (rasters, vectors, etc.). fasterRaster uses projects and mapsets, too, but in a manner that is invisibly to most users. Thus, this tutorial is mostly of interest to developers and other curious people.

## GRASS locations/projects

Upon starting, GRASS creates (or loads) a project, which corresponds to a folder on the user’s system. Importantly, all rasters and vectors in a project must have the same coordinate reference system (CRS). Confusingly, rasters and vectors in the same GRASS location do not necessarily have to represent the same place on Earth (this the renaming to “project”). In general, rasters and vectors can only interact with one another if they are in the same project and mapset. GRASS can only have a connection to a single project at a time.

fasterRaster handles projects and mapsets automatically, so users typically do not need to manage them. Projects are created on an as-needed basis. Within a given R session, if no projects have already been made, the first call to `fast()` to create a `GRaster` or `GVector` will 1) make a connection to GRASS and 2) create a location with a CRS the same as the raster or vector. The raster or vector is then stored in this location. `fast()` starts the connection and creates the location using the exported by hidden function `.locationCreate()`.

If `fast()` is called and a project already exists that has the same CRS as the raster or vector, one of two things will happen. First, if GRASS is already connected to that project, the raster or vector is simply imported. Second, if GRASS is not connected to the project that has the appropriate CRS, it will use the hidden function `.locationRestore()` to connect to the proper one, then import the raster or vector.

Other functions might also cause GRASS to connect to a pre-existing project Generally, if a function is applied to a `GRaster` or `GVector`, it will first check to see that GRASS is connected to the project in which the raster or vector is stored. If not, it will use `.locationRestore()` to do so first.

Creating a new project or switching connections to pre-existing project adds a few seconds to processing time of rasters and vectors. To avoid this, users can work as much as possible on sets of rasters and vectors with the same CRS (i.e., the same location).

## fasterRaster mapsets

GRASS “mapsets” are sub-folders within a location. Every location must have a mapset (and thus, a sub-folder) named “PERMANENT”. Users of mapsets are intended to store sub-projects that use rasters and vectors with the same CRS. Users of GRASS can switch between mapsets. However, for ease-of-use and development, fasterRaster always uses the “PERMANENT” mapset within a given location.

## Functions that manage locations and mapsets

The `crs()` function (wth no arguments) can be used to get the coordinate reference system of the current project/location.

The followoinng functions functions are hidden, but documented. All functions take “`x`” as an argument.

- `.location(x)`: Name of the location of `x` (a fasterRaster object), or the current location (if `x` is missing).
- `.locations()`: Names and CRSs of all available locations (takes no arguments).
- `.locationCreate(x)`: Create a location with the same CRS as `x`.
- `.locationRestore(x)`: Connect to a pre-existing location. `x` can be a fasterRaster object, a `SpatRaster`, `SpatVector`, or `sf` object, or the name of a location.
- `.locationFind(x, return = <option>)`: Given a `GSpatial` object `x`, find the name, index, or CRS of a location that matches it. If `x` is missing, return a list of all locations. `<option>` can be `"name"` (name of the location), `"index"` (index), or “`crs`” (coordinate reference string in WKT format).
- `.ls()`: List all file names of rasters and/or vectors in the current location.
- `.mapset(x)`: Name of the mapset that contains the object `x`.

## More rabbit-holing

Upon being loaded or attached, fasterRaster creates a package-specific environment named `.fasterRaster`, where it keeps a list of projects at `.fasterRaster$locations`. This is a named list of projects, where each element is has the name of the location, and the value of the element is the location’s CRS. The current location is tracked at `.fasterRaster$activeLocation`.

~ FINIS ~