Getting the Most Out of QGIS and GRASS Integration - Desktop GIS: Mapping the Planet with Open Source Tools - page 215

Figure 12.11: Result of each type of vector overlay operation
based on species and age. We have a polygon map outlining the stands
eligible for cutting. A stream runs through the logging area. To protect
both the stream and the fish populations, there is a 100-meter setback
requirement from any activity. We need to identify the portions of the
eligible stands that are “legal” for harvest.
First we prepare our data and make sure we have a good and accurate
polygon map of the eligible stands. Next we need to buffer the stream to
100 meters to create the second polygon map needed for the analysis.
Once we have those, we can proceed with the subtraction operation. In
Figure 12.12 , on the next page, you can see the stands map and the
digitized streams along with the buffer. You can guess by looking at it