The GRASS database makes use of the UNIX hierarchical directory structure. The setup and arrangement of GRASS data is similar to a filing cabinet with several drawers. The entire filing cabinet contains data about a particular geographic area. In GRASS this area is called the LOCATION, and covers a specific geographic area of the Earth, and contains map layers in a variety of themes. Like the filing cabinet that has several drawers, a LOCATION can contain several or N number of MAPSETS. MAPSETS are subdirectories under a LOCATION. Each MAPSET has a unique owner associated with it, controlled by UNIX permission settings. Each owner or GRASS user can view and use data in any MAPSET, but can only alter data owned by the current user in the current MAPSET. MAPSETS contain files and subdirectories that are associated with the data for the particular geographic area of interest. GRASS MAPSETS contain all data types _ raster, vector, site, and image files. GRASS file names do not adhere to the DOS 8.3 file naming standard (ISO 9660). Filenames can be of any length, and do not require a 3-character extension separated by a dot (.).
Raster data types are used to depict information that has an aerial extent, such as a soils map containing soil type, elevation, satellite imagery, or land use. GRASS raster data are stored as a matrix of grid cells. Each grid cell covers a known, square area that is typically geographically referenced to the Earth. The entire area of the GRASS LOCATION is made up of a grid of rows and columns of raster cells, though data may not always exist for the entire area. The size of the grid cell, or pixel, is user specified, depending on the scale and resolution of the data. A low-resolution map might use a cell with sides representing 100 meters on the ground. Each raster cell is assigned a single attribute value called the category number. The following is a list of the directories that may be found in a GRASS MAPSET that relate to GRASS raster data:
Directory Name Contents
cell raster data files
cellhd header files for raster data files
cats category information for raster data files
colr color tables for raster data files
colr2 secondary color tables for raster data files
cell_misc histogram range for raster data files
hist history information for raster data files
Vector data types can be used to depict linear features such as roads, transmission lines, boundaries, or stream networks. GRASS vector data, known generically as lines, can represent linear features or area edges. Area edges are lines that form closed polygons, or areas, that can be further processed into raster polygons. Lines are linear features that do not close and create areas. All vector data is stored as a series of x,y geographic coordinate pairs. Each vector map feature (either line or area edge) is assigned a single integer attribute value called the category number. In previous vector conversion routines, it was often the category information that was lost during the data conversion process. There are multiple files associated with a single vector theme, each being stored in different directories associated with vector data. The following is a list of the directories that may be found in a GRASS MAPSET that relate to GRASS vector data:
Directory Name Contents
dig binary vector data files
dig_ascii ASCII vector data files
dig_att vector category attribute file
dig_plus vector topology file
dig_cats optional vector category file
reg digitizer point registration
GRASS site data depict single point locations that have neither area nor length. Helicopter pads, firing points, nesting sites, or single tree locations can be examples of GRASS site data, depending on the scale of the data. This data type is stored as the single coordinate pair describing the location of the feature, and can be followed by a category value and a category label for the feature. GRASS site data is easily exported for use with other programs. GRASS site files are located in a directory called site_lists in the user's GRASS MAPSET.