2 Creating a new location

1 Defining a new location interactively

Sometimes considered a tricky process, we'll next learn to create our own GRASS locations from scratch. Remember, you can generate new locations from existing datasets automatically (see above). But it can be useful to know how to do it interactively. A major difference between GRASS and other GIS is that GRASS wants the projection definitions before the user can work in a location. The advantage is that things are well defined and a mess of projection mixtures is avoided.

First you start GRASS:

     grass60

The welcome screen shows a couple of buttons. Click on the Create New Location button, which will take you to a text screen (someday there will be a graphical interface for this). In this screen you enter a new location name (not containing white space), and then continue by pressing ``ESC''-``RETURN'' - i.e. press (NOT hold) the ESC key, and then press the RETURN key on your keyboard.

Below we outline the general procedure:

You will need to assign parameters to the location such as the coordinate system and datum you want to use, the project area's boundary coordinates, and the default resolution for raster data:

• Projection: Start by chosing between, X,Y, Latitude-Longitude, UTM, or "other" coordinate system. This choice depends on your data and the use you will make of it.

• Description: You are then prompted for a single line of text describing the project area, for example ``Topo Map of the Alps''.

• Projection details: Next you are asked for some more information about the projection. Note that the prompts vary from projection to projection; an example follows:
  • (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 datum name: again use ``list'' to get a list of available datums, examples are ``wgs84'', ``nad27'', ``eur79'', etc.

  • 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: 1.0 or 0.9996 or ...

  • Enter plural form of map units: for example, meters

• Area boundary coordinates: The next step is the description of the project area's boundary coordinates and the definition of the default raster resolution:
  • The default raster resolution (GRID RESOLUTION) has to be chosen according to your 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 point 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 if everything is correct accept the list of parameters that appears.

• You will then be taken back to the startup screen 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. Now you can verify the projection settings with:

     g.proj -w

If you read this section without actually defining a new location, you may try this command in the Spearfish location.

2 Creating your own location from EPSG code

Figure 13: GRASS startup screen: Creating locations from EPSG code

As an alternative, to create your own location in an quick and easy way, you can use the EPSG projection codes. Projections and national grid systems have been standardized by the European Petroleum Survey Group (EPSG, http://www.epsg.org), giving an unique ID code to each reference system. In GRASS, they are based on the PROJ4 installation which provides an EPSG code table. Clicking on the button ``Create location from EPSG'' brings you to a new window (see figure 13). You will enter a new location name and the EPSG code number. If you don't know the EPSG code, you can make use of the button that lists the PROJ4-EPSG file. Be warned that sometimes geodetic datum definitions are missing here. They can be added later within the GRASS system, or you are asked in an additional window to select a datum from the available list. After entering the necessary information, click on ``OK''. This will generate a new GRASS location. After that GRASS closes itself, and you have to restart it to select the newly created location. Select the location and mapset, then click the ``Enter GRASS'' button to launch the system.