Working with Raster Data - Desktop GIS: Mapping the Planet with Open Source Tools - page 60

Figure 4.2: NASA world mosaic viewed in QGIS
the raster from http://desktopgisbook.com/sample_data or the NASA site.
Loading it into QGIS gives us a world mosaic, as shown in Figure 4.2 .
The NASA raster is a georeferenced image in geographic coordinates,
meaning it can be used in conjunction with our world vector layers.
If you look carefully at Figure 4.2 , you'll notice we've added the cities
on top of the raster, just to prove they line up. Fortunately, both the
raster and vector data have geographic coordinates in the same datum.
In case you're wondering, a datum is a model of the shape of the earth
used to measure positions. In this case, the coordinates are in WGS
84, the same datum commonly used in modern GPS units. We'll take
a further look at datums and projections in Chapter 9 , Projections and
Coordinate Systems, on page 138 .
Observatory team ( http://earthobservatory.nasa.gov )