GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 312

fish table. If we draw the “fish” layer, that lake won't show up. Although
this may not be the best example, you get the idea.

In fact, you can combine totally unrelated features (datawise) that are
topologically related in a single map and break it out into layers using
this scheme. The GRASS documentation uses the example of forests
and lakes, but any mapping of land parcels that includes water bod-
ies is a good example. The lakes and parcels are topologically related
(share common boundaries) but are different types of features. In con-
ventional GIS, we might represent these with two separate layers (for
example, shapefiles). In GRASS, they can live in the same map and be
distinguished using categories and attributes.

Digitizing and Editing in GRASS
GRASS digitizing is a bit different from what you may have seen with
other applications. First, of course, we need to create a new map in
which to store our vector features. We can do this and start digitizing
all in one step using v.digit . When you use v.digit from the GRASS shell,
it fires up the digitizing tools and a map display window. But for this
to work, we have to open a map monitor first using d.mon . 1 Instead of
going that route, we'll just start up GRASS in GUI mode and let it do
some of the behind-the-scenes work for us.

Once you have the GRASS GIS Manager up and running using the
world latitude/longitude location, choosing Digitize from the Develop
map item on the Vector main menu brings up the v.digit dialog box. This
allows us to enter the name for the new layer and tick the box to allow
creating it since it doesn't exist yet. In Figure C.10 , on the next page,
you can see the v.digit dialog box with the options we need filled in. You'll
notice that we specified a display command of d.rast map=world_mosaic .
This becomes the background image that we will digitize from. We could
specify more than one background map if needed. The command d.rast
simply tells GRASS to draw the raster specified by the map option, in
this case our world mosaic.

You've perhaps noticed that the entire v.digit command required to ac-
complish the same result is displayed at the bottom of the dialog box,
next to a little “copy” icon. This allows you to copy the command to
the clipboard and use it elsewhere—for example in script. This is also

In GRASS 6.3 the use of d.mon is no longer required.

1.