Getting the Most Out of QGIS and GRASS Integration - Desktop GIS: Mapping the Planet with Open Source Tools - page 211

Working with Modules in the Toolbox
In previous sections we've seen how to use the import modules in the
toolbox to import both vector and raster data. In this section, we'll move
beyond that and look at some of the other modules and what you can
do with them. First a word about how the modules in the toolbox work:
unlike working in GRASS, the modules in the toolbox require a layer
loaded in QGIS to use as input to the module. For example, when we
imported the world_mosaic.tif into GRASS, we loaded the TIFF into QGIS
first and then used the import tool. Let's start our exploration of the
toolbox by creating a buffer or two.
Buffering Vector Features
We can use the toolbox to buffer point, line, or polygon features. Buffers
are useful for visualizing “things” that are within a given distance of
other “things.” For example, suppose Harrison has spotted eagles' nests
in an area where a new trail is to be built. He is concerned about the
potential for all the eager hikers disturbing the baby eagles and would
rather keep them at least 500 meters from the nest. Harrison goes off
to help the city planners with a bit of analysis.
With a map of eagles' nests, we can create a polygon layer that buffers
each nest by a distance of 500 meters. Basically, it's like drawing a
circle with a radius of 500 meters around each nest. Once we have the
buffer layer, we can use it to site the trail to avoid the nests. Of course,
this is just a simple example. In practice, buffers are an important part
of GIS analysis in many disciplines. On the flip side, you might also
analyze a proposed trail by buffering it and seeing whether it overlaps
any nests.
When you buffer a feature, you must specify the distance in map units.
In other words, if your map is in latitude and longitude, you would
specify the distance in decimal degrees. This usually isn't very prac-
tical, so in most cases a projection that uses meters or feet for units
of measure is used. Obviously to be successful (and accurate) in your
analysis, you have to know a little bit about your data, its projection,
and units of measure.
In Figure 12.9 , on the following page, we have taken a hypothetical
grouping of eagles' nests and have applied a 500-meter buffer to them.
The nests and the buffers are displayed over a topographic map. In
this case, we are interested in the distance of each nest from the roads
in the area. From looking at the map, we can see that all of the nests