Getting the Most Out of QGIS and GRASS Integration - Desktop GIS: Mapping the Planet with Open Source Tools - page 214

Vector Overlays
Now we turn our attention to a group of modules that have to do
with vector overlays. These modules are found in the toolbox under
the heading “Vector overlay.” The modules available to us include the
following:
Vector Union
This module overlays two vector maps to create a new map con-
taining the union or combination of the features. The attribute
tables from the two maps are merged, and a prefix is added to
column names so you can tell from which map they originated.
The boundaries are not dissolved so all original boundaries of the
features are still visible.
Vector Intersection
This creates a new map containing the portions of features com-
mon to features on the input maps. Where two polygons overlap,
only the common portion will be included in the resulting map.
Vector Subtraction
The second polygon map is subtracted from the first. The result
for each feature is that portion of the feature in the first map not
overlapping a polygon feature in the second map.
Vector Non-intersection
This removes the common portion between polygon features. If two
polygons overlap, the overlapping part will be removed, resulting
in what looks like a hole in the combined polygons.
In Figure 12.11 , on the following page, you can see a graphic illustra-
tion of the results of each vector overlay operation. In the center are
two polygons, and surrounding them is the result. The graphics in the
GRASS toolbox also provide a visual cue for each overlay operation in
case you need reminding. You can start thinking about how you would
use these modules for something real. We'll take a simple example to
illustrate the use of the Vector Subtraction command. This will be suf-
ficient to illustrate the use of the modules, and then you can go crazy
with the others.
Let's take a semireal but mostly manufactured example using logging. 2
Suppose the logging company is allowed to cut certain stands of trees,
2. Before you take this example seriously, I confess I know little about timber industry
practices—it's just an example.