Getting the Most Out of QGIS and GRASS Integration - Desktop GIS: Mapping the Planet with Open Source Tools - page 217

Figure 12.13: Eligible logging areas after vector subtraction
of spatial relationships. OK, enough pretending that I know anything
about the timber industry.
Creating a Contour Map
To further illustrate the power of the GRASS modules in the toolbox,
we'll create a contour map from the Anchorage DEM we used in the
LOS analysis in Section 10.2 , Line-of-Sight Analysis, on page 153 . Cre-
ating a contour map is quite simple, but you need to be aware of the
limitations. When using a DEM, the map will be only as good as the
original data. If the cell size is 50 meters, you can't expect to create
contours at 20 meters. The same holds true for any raster source we
might want to use.
To create a contour map, we first fire up QGIS and open the GRASS
mapset that contains the DEM. To make a contour map, we first have
to add the DEM to the map canvas. Once that's done, we can open the
GRASS toolbox and locate the r.contour module. It is under the “Gener-
ate vector contour lines” heading in the list of modules.