GRASS Quickstart

Geographic Resources Analysis Support System

Commonly referred to as GRASS, this is a Geographic Information System (GIS) used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization. GRASS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies.

Introduction

GRASS data are stored in a directory referred to as DATABASE (also called ``GISDBASE''). This directory has to be created with mkdir or a filemanager, 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.

Each LOCATION can have several 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.

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 such as elevation model write-protected to other users who are working in the same LOCATION. To import data into PERMANENT, just start GRASS with the relevant LOCATION and the PERMANENT MAPSET. This mapset also contains the DEFAULTWIND file which holds the default region boundary coordinate values. In all mapsets additionally a WIND file is kept for storing the current boundary coordinate values and the currently selected raster resolution.

Creating a GRASS database

To create the GRASS database:

Find a place on your disk where you have write access and that has enough diskspace to hold your decompresses data.
Create a subdirectory that will hold the general GRASS database (e.g. mkdir /data/GRASSDATA or mkdir /home/yourlogin/GRASSDATA).

Sample data such as the Spearfish sample dataset may be downloaded from http://grass.itc.it/download/data.php.

Click on the button "Create New Location" which will get to the above text screen on which you can enter a non-existing location name and continue with ``ESC''-``RETURN''.

Now, you need to attribute different parameters to the location, such as the coordinate system you want to use, including the ellipsoid, the coordinates of the boundaries of the project area and the default resolution for raster data:

Start out by chosing between, x,y, Latitude-Longitude, UTM or another coordinate system. This choice depends on your data and the use will make of it.
Now one line describing the project area, for example "Topo Map of the Alps".

Now some more information about the projection follows. Note that the prompts vary from projection to projection:

(if you chose "D - Other Projection") "specify projection name": "list" gives you the list of all available projections, examples are "tmerc" for Transverse Mercator, "lcc" for Lambert Conformal Conic, "moll" for Mollweide, etc.
specify ellipsoid name}: again use "list" to get a list of available ellipsoids, examples are "sphere", "clark80", "wgs84", etc.
if you want, you can specify a datum, again use "list" for available options
Enter Central Parallel: 0 if you want the Equator as the central parallel
Enter Central Meridian: 0 if you want the Greenwich meridian as central meridian
Enter Scale Factor at the Central Meridian}
Enter plural form of units: for example, meters

Next step is the description of the boundary coordinates of the project area and the definition of the default raster resolution:

The default raster resolution (GRID RESOLUTION) has to be chosen according to the needs. Generally, it is advisable to work in steps of 0.25 (0.25, 0.5, 1.75, 2.00, 12.25 etc.). This resolution does not concern vector and site data since these are stored with their exact coordinate values. Note that every raster map may have its own resolution. You can leave this screen with ``ESC''-``RETURN'' and then accept the list of parameters that appears, if everything is correct.

You will get back to the startup screen again to enter the mapset's name (if not already entered). Another ``ESC''-``RETURN'' will finally let you leave this screen. This mapset is created within the new location by answering ``yes'' to the next question. The mapset will use the parameters of the location (such as the region and resolution definitions) as its default parameters.

Now the project area, i.e. the location including a mapset has been created. You have "arrived" in the GRASS system and can start working within this new location.

GRASS Quickstart

Geographic Resources Analysis Support System

Introduction

Creating a GRASS database

Further Reading