GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 311

(extent) you asked for in your zoom or other map navigation action.
When using this mode, the map likely won't fill the display.

Map fills display window
This mode causes the map to fill the entire display area. This mode
is the same as that used by uDig and QGIS.

C.5 Digitizing and Editing

To complete our introduction to GRASS, let's look at how to digitize
and edit a layer. We'll create a layer and digitize some lakes off a world
mosaic raster.

The GRASS Vector Model
Before we start digitizing, we need to go over a few facts about how
GRASS stores and works with vector features. The GRASS vector model
is all new at version 6.x. Rather than go into all the gory the details now,
just be aware of the following:

• Each vector feature has a “category” number that serves as an
identifier.
• Vector features can have 0..n category numbers.
• A GRASS layer consists of link(s) from a set of geographic objects
to attribute table(s).
• Layers don't contain any geographic features but are linked to
them.
• For a feature to be part of a layer, one or more of its category
values must appear in the attribute table.

You're probably wondering what the story is with this confusing sound-
ing category and attribute table thing. Basically, it's pretty powerful in
that it allows you to link multiple attribute tables to a single feature.
Why would we want to do that? Well, in a contrived example, suppose
for our lakes layer we want to store physical characteristics such as
volume and maximum depth, width, and length. We also want to store
information about fish species and their percentage of the total fish
population. As you can see, these two goals don't really translate into
one attribute table. In the first case, it's a one-to-one relationship, and
in the second (fish), it's a one-to-many relationship. Besides that, the
two types of data just don't belong together. In GRASS, we can create
two attribute tables and link them to our vector layer. Then we can
add records to each, using the vector category as an identifier in the
attribute tables. If a lake has no fish, it won't have any records in the