Geoprocessing - Desktop GIS: Mapping the Planet with Open Source Tools - page 156

Figure 10.6: Overlapping collars on DRGs
Clipping Rasters with GRASS
If you are wondering why would we want to clip rasters in the first
place, let me give you an example. When you download a DRG from the
USGS or other source, more than likely it will have collars around the
image. A collar is that nice white paper border (well, it would be paper
if it wasn't digital) that contains information about the map, including
the quadrangle, scale, date published, and other tidbits of information.
This is all good information, except when we want to display more than
one of these rasters side by side. In that case, we end up with the
situation shown in Figure 10.6 . The collar of the DRG on top of the
map stack blots out information from the DRG below it. To make a
seamless data display, we need to remove the collars.
In its original form, a DRG looks just like the paper map you could
buy from your local map store. That's because the USGS DRGs are
scanned from those original maps and include not only the good stuff