Working with Raster Data - Desktop GIS: Mapping the Planet with Open Source Tools - page 56

Proverbs

Chapter 4

Working with Raster Data

Raster data is everywhere in the GIS world. You can use it as a back-
ground layer for your vector data or do full-blown analysis with it. In
this chapter, our goal is to get you up and running with raster data. In
later chapters, we'll delve into some analysis and manipulation.

Nearly every OSGIS desktop application can display at least some raster
formats—and some more than others. In particular, those applications
that are based on the GDAL library can support an impressive range of
raster data. You can find a partial list of formats that GDAL supports
in Section A.2 , GDAL/OGR, on page 283 . Both QGIS and GRASS use
the GDAL library for reading and writing raster data.

4.1 Viewing Raster Data

We'll start with something simple in our endeavor and load a TIFF
image. A fairly common thing you might want to do is view a topo-
graphic map of your area. You might recall that this is what Harrison
used as a background for his bird data. In the United States, many of
these rasters can be downloaded from the U.S. Geological Survey web-
site. Using a topographic map as a base is useful when you want to
view your vector data (for example, GPS tracks and waypoints) over it.

Let's download a TIFF from the Internet Archive of USGS Maps. 1 You
can pick any state you like—for our example, we'll grab a random image
from Montana. 2 You can pick one for your area by browsing the archive
by state. We'll need both the . tif and . tfw files. Once you have your raster
and the world file, you can view it in QGIS.

1. http://www.archive.org/details/maps_usgs