Working with Vector Data - Desktop GIS: Mapping the Planet with Open Source Tools - page 43

Unique Values
Harrison can now get an idea of the bird counts at each site by using
the continuous color or graduated renderers. But he also wants to view
the information by species. This is where the unique value renderer
comes into play.
The unique value renderer is useful for visualizing things that are the
same. By that I mean rendering features using the same color when
they have the same value for an attribute. Some common examples are
the following:
• Displaying all the polygons for a land type in the same color, such
as state lands vs. city lands
• Displaying volcanoes by their type
• Displaying roads by type: interstate, highway, secondary road,
and primitive road
• And of course displaying birds by name
The common thread in that list is: display “xyz” by type. That's the
purpose of a unique value renderer.
To display the bird sightings by name, we set up the unique value ren-
derer as shown in Figure 3.13 , on the next page. We can adjust the
colors for each bird by clicking its name in the list and changing the
fill color. We can also adjust the style and size of the marker symbols.
Once we are happy with the setting and click OK, we get a nice display
of our locations by bird name, as shown in Figure 3.14 , on page 56 .
Now Harrison is happy, and we have gotten a good look at using render-
ers to help us understand our data. While we are here, let's look at one
more example that has nothing to do with birds. So far in this section
we have been working with point locations. Let's take an example that
is a bit more colorful and is composed of polygons—a geologic map.
A geologic map portrays rocks by type (note that word type again), and
each type should be rendered in the same color. The unique renderer
setup to display our geologic map is shown in Figure 3.15 , on page 57 .
Notice that the rendering is done using the UNIT field. This field con-
tains the abbreviation for the rock types.
As with the graduated renderer, you must set the color and style for
each unique value. In the case of this geologic map (which is for the