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helptext - GRASS GIS Quickstart

GRASS GIS Quickstart GRASS GIS Quickstart

Geographic Resources Analysis Support System¶

GRASS GIS, commonly referred to as GRASS (Geographic Resources Analysis Support System), is a free Geographic Information System (GIS) software used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization. GRASS GIS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. GRASS GIS is an official project of the Open Source Geospatial Foundation (OSGeo).

1. Startup of GRASS GIS¶

After launching GRASS GIS, the startup screen will open:

GRASS data are stored in a directory referred to as DATABASE¶

(also called "GISDBASE"). This directory has to be created with a file manager or the mkdir command, before starting to work with GRASS. Within this DATABASE, the projects are organized by project areas stored in subdirectories called LOCATIONs.

A LOCATION is defined by its coordinate system, map projection and¶

geographical boundaries. The subdirectories and files defining a LOCATION are created automatically when GRASS is started the first time with a new LOCATION. It is important to understand that each projection stays in its own LOCATION.

See the "Location Wizard" to easily create a new LOCATION from scratch from a geocoded file, by defining the parameters or by selecting the corresponding EPSG projection code.

Each LOCATION can have many MAPSETs. Each MAPSET is a LOCATION's¶

subdirectory. New MAPSET can be added at GRASS startup (see related button).

The "Location Wizard" let's you easily create a new LOCATION. You¶

will be guided through a series of dialogues to browse and select predefined projections (also via EPSG code) or to define individual projections. Find below some rules to define the default raster resolution for a new LOCATION.

Once you have selected an existing LOCATION/MAPSET or defined a new¶

one, you can enter GRASS. The graphical user interface wxGUI will open and provide you with a menu system, map visualization tool, digitizer, and more.

2. Background: GRASS GIS Location structure¶

A LOCATION is simply a set of directories which contains the GRASS data of a project. Within each LOCATION, a mandatory "PERMANENT" MAPSET exists which contains projection information and some more definitions. It can be used to store the base cartography in it as "PERMANENT" is visible to all users accessing a LOCATION.

Fig. 1: GRASS GIS location structure

Creating and maintaining MAPSETs¶

One motivation to maintain different MAPSETs is to store maps related to project issues or subregions. Another motivation is to support simultaneous access of several users to the map layers stored within the same LOCATION, i.e. teams working on the same project. For teams a centralized GRASS DATABASE would be defined in a network file system (e.g. NFS). Besides access to his/her own MAPSET, each user can also read map layers in other users' MAPSETs, but s/he can modify or remove only the map layers in his/her own MAPSET.

You can learn more about mapsets and how to seamlessly access maps found in another MAPSET of the same LOCATION in the g.mapsets documentation.

The role of the PERMANENT LOCATION¶

When creating a new LOCATION, GRASS automatically creates a special MAPSET called PERMANENT where the core data for the project can be stored. Data in the PERMANENT MAPSET can only be added, modified or removed by the owner of the PERMANENT MAPSET; however, they can be accessed, analyzed, and copied into their own MAPSET by the other users. The PERMANENT MAPSET is useful for providing general spatial data (e.g. an elevation model), accessible but write-protected to all users who are working in the same LOCATION as the database owner. To manipulate or add data to PERMANENT, the owner would start GRASS and choose the relevant LOCATION and the PERMANENT MAPSET. This mapset also contains the DEFAULT_WIND file, which holds the default region boundary coordinate values for the location (which all users will inherit when they start using the database). Additionally, in all mapsets a WIND file is kept, for storing the current boundary coordinate values and the currently selected raster resolution. Users have the option of switching back to the default region at any time.

3. Creating a GRASS Database with Sample Data¶

To create the GRASS database:

1: Find a place on your disk where you have write access and that has enough disk space to hold your spatial data.

2: Create a subdirectory that will hold the general GRASS database (e.g. using a file manager or with mkdir /data/grassdata or mkdir /home/yourlogin/grassdata).

Sample data such as the "Spearfish" or the "North Carolina" sample datasets may be downloaded from http://grass.osgeo.org/download/data.php and the compressed data package(s) extracted into this new database directory.

Now you are ready to select a sample dataset in the GRASS GIS startup screen (see above) and start the session.

4. Creating A New Location with the Location Wizard¶

The wxGUI graphical user interface provides a graphical "Location Wizard" which let's you easily create a new LOCATION for own data. You will be guided through a series of dialogues to browse and select predefined projections (also via EPSG code) or to define individual projections. The rules to define the resolution as described above also apply here.

Hint: You can create new location easily from a georeferenced data file. In this case you are asked whether the data itself should be imported into the new location. The default region is then set to match imported map and the GRASS GIS session is opened.

After defining new location, wxGUI starts automatically. If data were already imported, you can add them into the Layer Manager now and display them. If your location is empty you can import your data from the menu: Go to "File" -> "Import raster/vector data" (see also the related Wiki page on Importing data).

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