GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 306

the first textbox. This will display a list of layers in the mapset that you
can choose from. For our example, we'll just add the world_borders layer.
We could save a step and just type in the layer name if we happen to
know it.
Entering the layer name is just the start. GRASS can display a number
of features from the layer, including the following:
• Shapes
• Categories
• Topology
• Line directions
• Points
• Lines
• Boundaries
• Centroids
• Areas
• Faces
This gives us a lot of options. Don't worry if you don't understand what
all of them mean. For now we just want to produce a map showing the
polygons that make up the countries of the world. To do this, all we
really need to select is shapes and areas. To draw the layer now, switch
to the map window, and find the Zoom to button located near the middle
of the toolbar (it looks like a magnifying glass next to a map). Notice the
tool is actually a drop-down tool button. Click and hold the button to
display the options. Click Zoom to Default Region, and the world_borders
map will be drawn to fill the display window. Notice we didn't set any
options other than the layer name and what features to draw so the
countries are drawn in the default color (gray). If we want to symbolize
the map by population or some other attribute, it turns out you can't
do that by adding a vector layer to the map—you have to use a thematic
map layer.
Adding a thematic map layer is done in a similar fashion as a regu-
lar vector layer. In fact, the button to add a thematic layer is just to
the right of the button we just used—if in doubt, hover the mouse.
Clicking the Add thematic map layer button adds a “thematic 1” object
to the manager window. Clicking it brings up the options panel. First
we specify world_borders as the vector map. You will notice the options
for a thematic map are different than for the vector map. The critical
thing is we need a numeric field to classify the map with. Fortunately,