Figure 2.41: (A) You need a GPS receiver to use the GPS system. (B) It takes signals from
4 GPS satellites to find your location precisely on the surface ( 15 )
Computer-Generated Maps
Prior to the late 20thand early 21stcenturies, map-makers sent people out in the field to
determinetheboundariesandlocationsforvariousfeaturesformaps. Stateorcountyborders
were used to mark geological features. Today, people in the field use GPS receivers to mark
the locations of features. Map-makers also use various satellite images and computers to
draw maps. Computers are able to break apart the fine details of a satellite image, store
the pieces of information, and put them back together to make a map. In some instances,
computers can make 3-D images of the map and even animate them. For example, scientists
used computers and satellite images from Mars to create a 3-D image of a large Martian
valley called Valles Marineris (Figure2.42). The image makes you feel as if you are on the
surface of Mars and looking into the valley.
Figure 2.42: This three-dimensional image of a large valley on Mars was made from satellite
images and computers. ( 9 )
When you link any type of information to a geographical location, you can put together
incredibly useful maps and images. The information could be numbers of people living in