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3 The GRASS database

Geodata are internally saved by GRASS in a standard subdirectory called a GRASS database. In most cases a new folder (e.g. grassdata/) is created in the users's Home directory before GRASS is installed for the first time:

cd              # Linux changes automatically to the Home directory 
                # of the user
mkdir grassdata # Compiling the subdirectory for the GRASS database

A subdirectory tree (Location) will automatically be created in GRASS for each project region defined in GRASS. All project data are saved in the Location subdirectory. The Location can be further subdivided into map subdirectories called (mapsets). This is how GRASS controls the organization and the access to the data (see chapter 2.3.1 and 2.3.2).

Figure 5: Example structure of a GRASS 6.0

Since different components (geometry, attribute and graphics data) of the individual layers are stored in different subdirectories, all management of the project data should be done using GRASS commands, i.e. all file operations (copy, delete, rename) should be conducted with the appropriate GRASS commands (g.copy, g.remove, g.rename)

1 The PERMANENT mapset

All information about projection, resolution and extent of the project area are stored within the compiled location in the PERMANENT mapset, which is automatically generated by GRASS. If necessary the core data (original maps) of the project can be stored here because only the user who created the new project has write permissions. Due to the internal structure it is guaranteed that the data can not be changed by other users.

Other GRASS users and naturally the person who has write permissions for the PERMANENT mapset should create additional mapsets for creating, saving and changing their own files and analysis results based on the core data in the PERMANENT mapset.

Files in the PERMANENT mapset

The access to a single mapsets can be controlled individually for each project (Location) in GRASS. If no maps (core data) are saved in PERMANENT it will only contains files with information about the project area:

DEFAULT_WIND	Specifications of the edge coordinates, extension and resolution of the environment PERMANENT
MYNAME	Name of the project -> e.g.: hanover
PROJ_INFO	Specifications of the projection -> e.g. tmerc (Transverse Mercator Projection), bessel (ellipsoid), potsdam (date)
PROJ_UNITS	Specifications of the units used e.g.: meter
WIND	Specifications of the current REGION and of the MAPSET projection
VAR	Specifications of the database driver and path

2 Design of further mapsets

Each GRASS user can create one or several mapsets in which he administers his own project data. They can have the extent of the whole project or smaller.

This characteristic of the GRASS database structure makes it possible to work with several users on one project at the same time e.g. in computer networks, without running the risk of changing or destroying another user's data. The mapsets of the other users can be specifically integrated in the users 'own project' by granting read-only permission. The resulting maps of any analysis are saved in the mapset of the users current GRASS session.

Mapset file structure

cats/	Category values (e.g. color or temperature values) and attributes (classes with caption) of the individual raster maps
cell/	Individual raster maps
cellhd/	Header rows of the individual raster maps
cell_misc/	Statistical data of the individual raster maps
colr/	Color information of the individual raster maps
dbf/	Contains the internal vector attributes in DBASE format
fcell/	Raster maps with floating point numbers (f: floating point)
hist/	'Developing history' of the individual raster maps
vector/	Contains the individual vector data (geometry, topology, etc.)
WIND	Data of the current REGION and the MAPSET projection

GRASS 6.0one user to start several GRASS sessions in parallel.