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1 Calling up a GRASS project

If the start script of the version GRASS 6.0in the path ($PATH), GRASS can be started within an open command interpreter (X-Terminal) via the command grass60 . A more detailed description can be found in chapter 3.

The resulting start screen is shown in which some data must be entered via mouse or keyboard. A graphical or text-based variant can be chosen by setting the parameter -text or -gui when starting GRASS (see Fig. 8 and 7).

grass60 -text   # text-based start screen
grass60 -gui    # graphical start screen

Figure 7: TclTk start screen in GRASS

Here, an existing Location and mapset can be chosen from the menu or a new Location and mapset can be created.

If a new mapset is to be added to an existing Location, first choose the appropriate Location, then enter the name of the new mapset in the window below the text 'Create New mapset' and click the button 'Create ...' (see Fig. 7).

There are three ways to create a new Location. (A) The projection data can be provided directly by the user. The necessary information can usually be found in the metadata associated with the data file. (B) The projection can be assigned automatically by entering the appropriate EPSG code (see http://www.epsg.org/). This assigns standard projection and national grid systems parameters to the Location. The appropriate code number can be found in the menu (see Fig.9) or as metadata. (C) The projection data can be imported directly with the data by using the importmodules r.in.gdal and v.in.ogr.

(A): In order to define the projection of a Location itself click with the mouse on the button 'Create New Location'. GRASS 'changes' to the text-based mode, which can also be your start screen - depending on the start settings (see Fig. 8). The data for creating a new Location are entered as described in chapter 4.5.1.

Figure 8: Screen for defining a new Location in GRASS

(B): If an EPSG code is exists for the project Location, it can be used to create a new Location automatically. Click with the mouse on the button 'Create Location from EPSG' and enter the appropriate code number (see Fig. 9). The button 'EPSG Codes' lists existing codes with their appropriate definitions.

Figure 9: Screen for intergating an EPSG code

(C): In some cases, the data already contain all necessary projection information. It is thus possible to generate a new Location during a running GRASS session using the projection information contained in the data set to be imported. For example, a new Location can be created with a SHAPE file or a GeoTIFF file, if a 'correct' projection file (.prj) is available (unfortunately this is rarely the case).

Example: Content of a SHAPE projection file (.prj)

PROJCS["Transverse Mercator",GEOGCS["bessel",
DATUM["Deutsches_Hauptdreiecksnetz",
SPHEROID["bessel",6377397.155,299.1528128],
TOWGS84[590.5,69.5,411.6,-0.796,-0.052,-3.601,8.30]],
PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",9],PARAMETER["scale_factor",1],
PARAMETER["false_easting",3500000],PARAMETER["false_northing",0],
UNIT["meter",1]]

Example: Projection data of a raster map (GeoTiff)

Driver: GTiff/GeoTIFF Size is 3570, 3753 Coordinate System is:
PROJCS["Transverse Mercator",
    GEOGCS["Deutsches_Hauptdreiecksnetz",
        DATUM["Deutsches_Hauptdreiecksnetz",
            SPHEROID["bessel",6377397.155,299.1528128000033],
            TOWGS84[590.5,69.5,411.6,-0.796,-0.052,-3.601,8.3]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",9],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",3500000],
    PARAMETER["false_northing",0],
    UNIT["meters",1]]
Origin = (3368561.280000,5928333.120000) 
Pixel Size = (0.32000000,-0.32000000) 
Corner Coordinates: 
Upper Left  ( 3368561.280, 5928333.120) (  7d 1'12.86"E, 53d28'18.24"N) 
Lower Left  ( 3368561.280, 5927132.160) (  7d 1'14.67"E, 53d27'39.41"N)
Upper Right ( 3369703.680, 5928333.120) (  7d 2'14.77"E, 53d28'19.26"N) 
Lower Right ( 3369703.680, 5927132.160) (  7d 2'16.56"E, 53d27'40.43"N) 
Center      ( 3369132.480, 5927732.640) (  7d 1'44.71"E, 53d27'59.33"N)

It is important that such information is precisely controlled and scrutinized and that they should be available as completely as possible. Small deviations, an incorrect DATUM or SPHEROID, can produce enormous deviations in data positioning.